Testing Lens Test Chart Download
There are a lot of right ways to do optical testing. The gold standard is to put the lens on a $150,000 optical bench or run it through a well-equipped Imatest lab. But unless you have an optical bench or Imatest lab handy, that’s not practical.We have to optically test around 400 lenses a day, which is more than our Imatest lab can possibly handle. So over the years we’ve learned a lot about practical ways to test lenses. We’ve constantly double-checked our methods using Imatest and an optical bench, refining our optical testing.We’ve developed a simple set up that is about 98% accurate in identifying lenses that are decentered or optically misaligned. A lot of people could do this at home themselves.
Certainly any camera club could make an identical setup.Judging from the emails I get, a lot of people want to be able to test their lenses optically and few know how to do it, so this should be useful for them. Post-rental testing bays at LensrentalsBut let me be clear about this is not, so I don’t waste your time if you wanted something else.
Free Eye Chart - Download, Print, and Test. Searching for a free eye chart to check your vision at home? Simply click on the image to the right, and your download will begin. You can use this eye chart to check your entire family's vision. Remember: This is not a substitute for a complete medical eye exam by a licensed optometrist.
This is not going to let you test 6 copies of a lens and determine which is the absolute sharpest copy on your camera. It’s doesn’t let you compare two different lens models and decide which is sharper.
For that you do need Imatest, a bench, or just a lot of photographs.But if you have a lens that isn’t performing like you think it should this will let you determine if the lens is optically aligned and centered properly. It can be helpful to know if the problem is with the optics of that copy, rather than the focus system, technique, or simply that the lens isn’t designed to perform any better than that.Nobody needs to do optical testing, of course.
You can just take pictures and if you don’t like the lens send it back. But that’s not always convenient or possible.
Testing is a lot faster than trying out 4 copies before you decide a lens just isn’t for you. A Few Things Before We Start You will need a tripod.Some of my most experienced techs can test hand held. It usually takes about 6 months of testing 75 lenses a day before they can, though. So I recommend testing your first 9,000 lenses on a tripod.
After that you can try it hand-held if you like, although you will lose accuracy. Being Perfectly Square to the Test Chart is RequiredThis is the main reason a tripod is needed, although not the only reason. If you are tilted at a slight angle to the chart, you’ll get a lot of the same results you would get if the lens were decentered. Center Sharpness is the Least Sensitive Test of a LensMost lenses can be pretty badly decentered before center sharpness is affected very much. Eight out of every 10 lenses we fail as ‘optically decentered’ would pass fine if center resolution was the only thing we checked. Those that do fail in the center always also fail off center. You Can Learn a Lot About a Lens by Observing it Slightly Out of FocusOf course, they’re blurry when slightly out of focus, but the way that they blur can be quite revealing.
Equipment You’ll NeedHere’s all you have to have:. Tripod.
Test chart. Camera output to a computer monitorI don’t think I need to say anything about tripods, but I’ll go into a bit more detail on the other two requirements. Test ChartWe’ve tried basically every type of optical testing chart that exists and by far the most useful is the ISO 12233 chart. A lot of people use charts stuck in multiple places for testing. These aren’t completely useless, but they are very insensitive. Some fairly bad lenses look fine when you just use a bunch of AF 1951 charts to test them. Remember, they were designed to test things back in 1951.
Hopefully our standards are a bit higher today.(As an aside, I know of several service centers that use only a modified version of AF 1951 bars to test lenses optically. Which explains why they send lenses like back, saying they are optically fine.). ISO 12233 test chartThe variable targets in each corner provide great corner-to-corner comparison, which is the most sensitive indicator of an optically decentered lens. The long gradual change in line spacing shows a lot more than the short bars of an AF 1951 chart.
It also provides identical horizontal and vertical test areas in each corner. While not a true test of astigmatism, differences in the horizontal and vertical targets strongly correlate with astigmatism and other aberrations. The thick, slant-edge boxes and bars show chromatic aberration and blurring in a way an AF1951 chart doesn’t do.can be pricey. A good quality, 40″ wide ISO 12233 chart costs about $250. A top quality (photo paper) 60″ chart like we use in our test lab is about $700.
But you can download a courtesy of Stephen Westin, enlarge it, clean it up in Photoshop, and print it yourself. It isn’t quite as sensitive as a professional quality chart, but it is good enough.One thing you will want, though, is a thick black border, perfectly square, around the edge of the test chart. I’ll show you why later. If you’re printing your own, just add a rectangular border in Photoshop. If you’re buying one, it will almost certainly come with a border.You’ll also want the biggest chart you can get. You need the chart to fill the photo frame completely when testing, so using a small chart to test a wide angle lens gets too close to minimum focusing distance for accuracy. I recommend testing at least twice the minimum focusing distance, and further away is even better.There is one other test chart we find exceedingly useful, the Zeiss version of Siemens Star Chart, which you can.
(Or download and print courtesy of John Williams, who has posted a on Wikipedia Commons.) We only use the very center part: the black dot, surrounding white circle, and a bit of the surrounding star.In order to keep things simple, we cut the center 2″ out of the Star chart and glue it over the existing center of an ISO 12233 chart. Sometimes we’ll add stars in the corner, too, just outside the vertical/horizontal line cross. LightingLighting isn’t critical for this kind of testing; you simply want to light the chart with reasonably bright continuous light. It doesn’t have to be extremely bright. Too much light can actually be a problem. We do a lot of this testing in live view mode and too much light will cause the camera to close down the aperture. Try your ambient room lighting before you set anything else up; it will probably be fine.Industrial quality fluorescent fixtures will cause the usual banding problems and should be avoided if possible. If your testing area has standard fluorescent fixtures (like ours does) then you’ll need some additional lights to overwhelm the room lighting. In this case a couple of incandescent or photo-quality fluorescent softboxes are inexpensive and perfectly adequate.
A MonitorThe goal here is simply to be able to see the camera’s live view output on a decent sized, good quality monitor. For most cameras we simply run an HDMI cable from the camera’s HDMI out port to a freestanding monitor. No computer necessary. We prefer a large monitor for the comfortable working distance, but that’s just a preference, not a necessity. You could use a laptop or tablet with a high-resolution monitor, or a video monitor if you have those handy.The HDMI output isn’t going to have resolution as high as an actual photographic image, but it’s good enough for this testing. We always confirm with an actual photographic image, of course, just to be certain. But it’s really rare that the photo shows something we missed in live view.If your camera doesn’t have video output, or you don’t have a monitor around, there are lots of workarounds.
Any tethered shooting setup will give you the same capabilities and can be run to a laptop or tablet. Built-in camera Wi-Fi or a Wi-Fi card may do the same thing, as long as it gives you live-view focusing capabilities. If your camera just isn’t capable of putting out live view to a monitor, you can try doing this using a series of photographs, but you lose a lot of information and it’s much, much slower.Here’s a picture of one of our ‘portable’ setups, like the one I’ll bring to. (The chart is mounted in a non-portable light housing that we use for multiple purposes.
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We need the lighting to wash out the industrial fluorescent lighting in our building, but you probably won’t). Chart, camera on tripod, and HDMI monitor are all that is necessary.Someone will ask if they can do this simply using the camera’s LCD. Yes, you can. But it will take a lot more time, be more prone to error, and you’ll go blind after a while. Setting UpSetting up square to the monitor is the final step, and it’s actually pretty quick to do. The first step is centering the camera.1.
Place the tripod and camera close to the test target, raise the camera so it’s the exact height as the center of the target and lock the column (or legs) in place.2. Gaffer tape or draw a line on the floor at right angles from the center of the test target. Now as long as your tripod is centered over your line, your lens will be centered on the test target.The second step is eliminating any tilt and rotation of the camera, which is also simple.1. Center the tripod over the line you made at right angles to the test chart, at the spot where the test target fills the image on the monitor. (You should just see the black edges around the chart.)2.
Tilt the camera right or left, up or down so that the rectangle is squared. If the camera is tilted the rectangle will look like a trapezoid. (Obviously if the lens has pincushion or barrel distortion it won’t quite be a rectangle, but you get the idea).3. Rotate the camera clockwise or counterclockwise so the top and bottom lines are perfectly horizontalIf you want to double check your final setup take a picture, download it in Photoshop and measure the length of opposite lines (they’ll be equal if the image is square) and their angle.
A degree or two of tilt and rotation is fine for this work (unlike Imatest, where 0.5 degrees is critical).If you look back at the image above you can clearly see from the picture on the monitor that we haven’t squared things yet (look at the white band outside the top and bottom of the test chart’s black box). The camera will need a couple of degrees of clockwise rotation and a bit of turning. We’re using a geared head here, but a ballhead or pan-tilt head works just fine. Basic TestingI’ll give a lot more examples of testing in the third part of this series, but the basics are quite simple. The first point is that we test with the aperture wide open. Even a badly decentered lens may look reasonably good when stopped down.For this reason you don’t want too much light on the target when testing in live-view mode: the camera will stop the lens down and override your aperture setting, making the lens look better than it really is when looking at it live. Then when you take a photograph of your test image, you’ll wonder why it looks so much worse. (If you have a question if this is happening, just walk around to the front of your setup and look into the lens barrel.
You’ll be able to see if the aperture is stopping down.) Focusing and the Center Star ChartThe star chart in the center of the test target makes it very easy to focus in live view, but first examine the pattern of the chart just out of focus at maximum magnification. On a well aligned lens the star should blur into a fuzzy oval like three of the images below do. A badly decentered lens will blur with a flare going in one direction or another, like the image on the lower left below.If you look carefully, you’ll notice the image on the upper left has just a bit of flare going upward, toward 12 o’clock. This amount of flare can occur on some normal lenses, but the amount on the lower left is never normal.When you get the star sharply focused, the pattern should be circular, like the image below. In some decentered lenses the gray circle, where the individual star lines blur together will be oval. In other types of decentering, you may notice the more vertical stars are much sharper than the horizontal stars, or vice-versa.I suggest taking a photograph with the center both in and slightly out of focus.
Once in a while, examining the photograph will show an abnormality of the star pattern or a flare that wasn’t obvious in live view. Checking the CornersOnce the lens is perfectly focused in the center, it’s time to examine the corners. If you have a really good, large monitor and a camera that outputs good live view images you can get a pretty good idea about the corners looking at the entire image in unmagnified view. Most of the time, though, you’ll be much happier doing this at 5X magnification and moving the cursor around the screen to look at the corners individually.The idea is NOT trying to measure how well the lines resolve, especially in live view.
It’s whether the lines all appear equally sharp in each corner. If all four corners look like the close-up of the ISO chart I posted above, you’ve got a great lens.
No further testing is needed, go take pictures. It probably won’t, though.
Camera Lens Testing Chart
So let’s talk a bit about what you’re more likely to see.Normal Field CurvatureMost lenses have some field curvature, so the corners aren’t at their sharpest when the center is in best focus. When you examine them, the corners all look a bit soft, but equally so. The second step is to zoom in on one corner and manually focus the lens to bring the corner into best focus. If you look at how far you have to turn the focus ring to get the corner in best sharpness you’ve learned something about the lenses field curvature: was the corner in best focus closer than the center, or further away?
And by how much? That can be useful to know when you’re out taking pictures later.Once any corner is in sharpest focus, move the cursor around to check the other three corners and see if they are all equally sharp again. Field curvature, which is a normal part of lens design, means the corners will have a different focus point than the center.
But since our alignment is square and centered to the chart, the 4 corners should all focus at the same point. As long as they do, this part of the test is normal.Abnormal Field Curvature (Tilt)If some corners have sharpest focus at a different place than others, that’s not field curvature, it’s tilt. Until now I’ve been using the generic term ‘decentered’ for any lens that is optically out of sorts.
In reality, there are three types of misalignment: decentering, tilt, and spacing errors. If you’re interested in the difference, I describe them. A badly aligned lens often has more than one problem and the symptoms overlap somewhat, but we can make some generalizations.A mildly tilted element often causes two corners to be different than the other two corners. (It can be side-to-side, top-to-bottom, or opposite corners.).
A lens with mild tilt. Even at 25% of actual size, the top corners are obviously softer than the bottom.
In some cases, changing focus slightly you could make the top sharp, but the bottom soft. Most of the time that is not the case; even at best focus the top remains softer. Either case is abnormal. The center of the lens usually remains quite sharp in this situation.How do we tell this is simply tilt, rather than another form of decentering? With tilt, changing focus often makes the top two corners sharp, but the bottom corners will get fuzzy (or left and right, etc). With other problems, the top two corners usually never get sharp, they always remain softer than the bottom corners.
But that’s a generalization that isn’t always true and really doesn’t matter unless you’re optically adjusting lenses.More Fun with CornersA mildly decentered lens may only have one bad corner rather than two like the example above. In that case center resolution is always still excellent, although on an optical bench or in an Imatest lab you might notice the sharpest point of the lens has moved slightly off center, away from the bad corner.In other cases the problem affects only the horizontal or vertical line pairs, like the image below. It is subtler than the example above, but it’s still significant enough to cause a loss of sharpness on that side of a photograph. A less severe problem may only affect either horizontal or vertical bars. In this image the left side vertical lines are softer than the right, but the horizontal lines are equal in all 4 corners.
This lens would not be as bad as the one above, but you’d notice the difference in some images.One other interesting finding that we sometimes see in this situation: by slightly changing focus you may be able to make either the horizontal or vertical lines sharp, but not both at the same time in the affected area. There are some lenses that will show this astigmatism-like behavior in all 4 corners. That’s just the way that particular lens is designed. But if only 1 or 2 corners show it, the lens has issues. Changing focus slightly may change the blur from the vertical to the horizontal lines.When All 4 Corners Look BadA badly tilted element, especially if near the rear of the lens, can make the entire image soft, even in the center. Spacing errors, and significant decentering of an element can have the same effect.
Sometimes a decentered element leaves the center of the lens quite sharp, but all 4 corners very soft.The question becomes how do you tell if a lens has a significant optical abnormality, or it’s just a bad design that is soft no matter what copy you have? Usually when this happens, the lens is so bad you don’t need any kind of testing to see the problem. There are halos, you can’t identify anything in the image, the camera won’t even autofocus with the lens, etc.It’s rare, but there are cases where all 4 corners are bad, or the entire lens is somewhat softer than it should be, but not so bad as to be immediately obvious.
A little common sense helps in this situation. You should expect 10X consumer grade zoom to be rather soft at the long end. An f/1.2 lens is probably going to have slightly soft corners at widest aperture.
Even good telephoto zooms often have soft corners at the extreme telephoto end.There are several things that help identify the ‘optically bad copy’ from ‘badly designed lens’ in this situation. First and foremost is the center star target. A lens decentered this badly will almost always have a significant flare, like we demonstrated above, when just slightly off focus. Some will even show flare at their best focus. If you examine the corners carefully they may all be soft, but one is usually quite a bit better than the others, and one quite a bit worse. Examining the thick black boxes will often show chromatic aberration at the edges and it will be of odd pattern: It might be away from center on one side and toward center at the other, etc.If you’re still not certain, retest the lens with the aperture closed down 1 stop.
Obviously all lenses will be a bit sharper stopped down. But with an optically decentered lens you’ll usually see some corners get much sharper stopped down than others, and the pattern will look like the ones we’ve described above.I want to emphasize that this ‘all over’ or ‘all corners’ softer decentering is really rare.
Lens Testing Chart
Out of every 100 decentered lenses, we find 2 or 3 of them have this kind of pattern. LimitationsZoom lenses have to be checked at 2 or 3 spots. Most optically challenged zooms are bad throughout the range, but there are definitely some that only have a problem at the long or short end. Obviously you have to move the tripod to reframe the image at a different zoom range and realign the camera, but that shouldn’t take more than a few minutes.Because of the limitations of chart size, testing at a given focal length is only done at one focusing distance. There are lenses, although they are rare, that have problems at certain focusing distances but not others and this test might miss that problem. But optical benches and Imatest have the same limitation; they only test at certain distances. Y ou can overcome that to some degree by making charts of different sizes, but that’s a lot of trouble to go to for something you’ll probably never see.One thing I should note: this test may over read a bit, particularly on wide-angle zoom lenses and large aperture, wide-angle prime lenses.
Most of these have a little bit of flare in the center even when perfectly adjusted and all 4 corners will rarely be completely identical, although they should be close. If you look carefully enough, for example, you’ll see slightly softer horizontal or vertical lines in almost every copy of a 16-35 f/2.8 zoom or a 35mm f/1.4 prime shot wide open.That’s one reason I suggest this testing setup for a camera club. If a friend brings another copy of the lens, you’ll feel a lot better comparing yours to his when you have a question, and you should be able to do that in 10 minutes. SummaryA lens that’s optically awful, bad enough to affect sharpness everywhere, will generally show a lot of flare of the Siemens Star in the center when slightly defocused.
All four corners may be blurry, but some corners will be worse than others, or the horizontal and vertical resolution will be different.Less severely decentered lenses may look fine in the center and may, or may not, exhibit center flare. They will show differences in the corners that are readily apparent, however. If a lens has no flare in the center and the corners of the ISO 12233 chart all look identical, it’s optically fine.We’ve been testing exactly this way for several years, identifying 60 to 70 lenses a month as decentered. We retest all of those on an optical bench and rarely find one that was actually OK. In other words, the test is nearly 100% specific. If it says a lens is decentered, it’s almost certainly decentered.We also run QA checks using Imatest or the optical bench on lenses that have passed this optical testing. When we recheck these lenses, we do find that about 0.5% of are actually decentered.
However, if we repeat this optical chart test on those same lenses, they almost always fail the repeat test. In other words human error, not the test itself, let the bad lenses pass.There are lots of other ways to test lenses, of course. But we’ve tried most of them and this is by far the most accurate in our experience, short of setting up an lab. And testing 8,000 to 9,000 lenses a month, experience is one thing we have a lot of.The next article will be a few days in the making, but we’ll have a lot more examples of problem lenses, and will probably do a simple optical adjustment using nothing but this test system.Roger CicalaLensrentals.comFebruary, 2012Addendum. If you read this post and thought, “Yeah, I know that,” Midwest Camera Repair is looking for an experienced Lens and D-SLR tech, mainly to service Nikon Lenses; knowledge of Canon lens would be helpful but not required. Someone who has experience servicing high-end lens would be best.
Most of our repairs are middle to high-end equipment to Professional customers. We will train on use of Nikon & Canon test and adjustment equipment. Our Nikon lens volume has more then doubled in the last year. Knowledge of D-SLR repair would also be helpful.Midwest is a Nikon Authorized Service Center and one of only four authorized, trained and equipped to service the VR-Series lens.
They are also authorized on Canon IS-Series lenses.Send confidential resume to. If I understand you correctly, I would split the pdf into four quadrants and print each one of those at 60 x90, then attach all four to the wall together as if it was one giant version of the original. That should, also, if I’m not mistaken also minimize the effects of not being perfectly square, setting up against a ten foot by fifteen foot target. That should also minimize the effect of the (slightly) reduced resolution of an inkjet as opposed to an imagesetter output.I’ll probably start with a single 60 x 90 print and see where that goes. I’ve actually got only one lens that I feel is not all it could be (Canon 24-70 2.8 II) and this will hopefully provide some good data for Canon to work with.When I do get to taking this to Canon, do you have any idea if it’s better to drop it at the Hollywood CPS center or drive it down to Irvine.
Hell, Hollywood might send them to Irvine anyway, but I don’t know.Thanks again for all you do to educate us out here, wherever we are.Peter.
Camera Lens TestingPart 6 - Lens Testing for Resolution, Chromatic Aberration and DistortionProbably the most frequent complaint or cause for concern about a lens is that it's not sharp, but what does that mean?Well, unless you're shooting a static subject with the camera and lens on a tripod, it probably doesn't mean much. A lot ofphotographers don't realize that hand holding a camera (even if it or the lens has image stabilization), isn't the way to get the maximum possible sharpness. If the shutter speed is high enough and your hands are steady enough, you may get a critically sharp image, but don't bet on it happening every time.So you have the camera on a tripod, what's next. Well, you have to be sure that the lens is focusing properly. To do that you have to do some focus testing as I described in the article. Assuming you have a camera and lens in good calibration and you are getting an accurate indication of focus by the AF system, then what?At this point you have to decide just how much effort you want to put into measuring the optical characteristics of your lens.
If you want to go all the way, I'd recommend you look at thepackage. It's currently (2013) $299 for the 'lite' version and $2200 for the pro version (though a limited free trial edition is available).
I won't go into details of it here since you can find everything you'd ever need to know on their website. It can give you a lot of technical information on an lens (if you do the tests right) and it's an excellent package, but even the 'lite' version is pretty complex and more than most people want to deal with. If you want something to do lens testing that's a little quicker and easier (and much cheaper) then you can do some simpler tests using a test chart as described below.The lens testing chart I'm going to use can be printed on 4x6 card. Now a 4x6 target isn't very big, so the idea is to print a number of them and place them in the center and at the corners of a larger chart. For APS-C sensor cameras I usually place them on a 15'x22' background and if that fills the frame, the magnification of the system will be around 1:25 which for a number of reasons is a good number to show the typical performance of a lens.
If you were doing lens testing using a full frame DSLR, you'd put the targets at the center and corners of a 24' x 36' rectangle to get the same magnification ratio. The target itself is shown below:You can download a high resolution printable version of this image (as a Zipped JPEG) via.With the lens testing target printed on glossy paper, most printers should be capable of printing a good quality target when set to their maximum quality settings. The lines in the test patterns should be well resolved down to at least the 5.0 lp/mm line set. If the target is used at the intended distance, you won't be using anything past the 2.8 or maybe 3.2 line sets anyway.The lens testing resolution patterns are based on those used for the NBS 1010A test chart. There's a high contrast set on the left of center and a low contrast set on the right of center. In the center is a checkerboard pattern which is used for focusing. Within the resolution patterns on each side is a Siemens star pattern.
Below the test patterns is a line which is exactly 100mm long that can be used for calibration as described later. There are also a small texture patches and text blocks on the left and right at the foot of the chart which can be used for visual assessment of image quality. A black border is used to look for chromatic aberration as will also be described later.Version 2 of the chartUPDATE:I've updated the chart to make it more useful. I found that in testing I rarely used the low contrast resolution pattern on the right side of the chart so I replaced it with a sine Siemens star pattern with small text in the center.
In the printed chart and in the full size file the star lines go all the way to the white center disk. The pattern you see here on the page is a consequence of downsampling the image to 600 pixels wide. The original is 6600 pixels wide.
The sine version has smoother black to white transitions than the square wave version on the left. In place of the texture patterns (which I also didn't find useful) I put a sine wave logarithmic horizontal resolution target on the right, two concentric circle patterns (at 1.8 lp/mm and 2.8 lp/mm) and two grey patches (useful for looking at image noise), one at RGB 128 and one at RGB 192.
There's also a grey scale test strip at the top right RGB values 0, 64, 128, 192, 230). The 2.8 concentric circle doesn't look like concentric circles in the image above, but it does on the printed version. The image above is again showing the effect of downsizing to 600 pixels wide, at which size the circles are not resolved.You can as a zipped jpeg file.Below is a typical lens test setup. In this case the 4x6 charts are attached to a sheet of white pegboard (about $5 from Home Depot!). The white background is good and the regularly spaced holes provide both distance calibration and can be used for visual assessment of lens distortion and camera alignment.Three lens testing charts are used here, but you can put one at each corner if you want, and the charts can be rotated if you wish.
In this case the corner charts are placed at the corners of a 15' x 22' rectangle. As noted above, this fills the frame of an APS-C DSLR, the magnification ratio is approximately 1:25.
For a full frame DSLR (or SLR) use a 24' x 36' rectangle.DistortionIf you use a background with a regular rectangular grid pattern, such as the white pegboard suggested above, you can use that pattern to assess the degree of distortion of the lens. Notice that in the image above the holes line up in a very straight line along the edges of this image showing the lens (EF 50/1.8) has very low distortion.Shooting the lens testing chartUse a tripod. I don't care how steady your hands are or whether or not you have image stabilization, if you want accurate and reproducibleresults, you need a tripod.
A cable release doesn't hurt either, especially if you are testing a long lens and/or your shutter speed is slow. You can even use MLU (Mirror Lock Up) if you have it and want to be certain that camera movement isn't affecting sharpness, though MLU probably isn't necessary with lenses shorter than 100mm.Set the height of the tripod so that the center of the lens is level with the center of the chart. The move the camera away from the chart until the 15' x 22' area just fits in the viewfinder. Now make sure your camera is square on to the chart. Set up your lighting (if you're doing this indoors), adjust your white balance and take an exposure reading from a gray card. If you don't have a gray card and you're using a white background as in the example above, just set your exposure compensation to about +1.5 stops. That should make the whites white rather than a mid gray.It's very important to get the alignment of the camera/lens and chart right.
There are three things to align:. The plane of the sensor must be parallel to the chart. The optical axis of the lens must intersect the center of the chart. The horizontal/vertical axes of the sensor must align with the horizontal/vertical axes of the chartThere are a number of ways to ensure alignment, here's one of them:Take a long, straight, thin rod and mount it so that it sticks out of the center of the chart at right angles. You can set this up geometric ally using a T-square.
Then when you view the chart, only the tip of the rod should be visible. If you can see any of the sides of the rod, the alignment is off. If you can see only the tip, then conditions #1 and #2 of the above list have been met.
To meet condition #3 you just need to rotate the lens until the sides of the viewfinder are parallel to the sides of the chart. If not quite parallel (due to distortion), then then the left and right and top and bottom gaps should be symmetrical.Any uniform illumination source is OK, as long as the lighting is truly uniform.
Shooting outdoors in daylight would be ideal, but you can also shoot indoors with diffused artificial light. You can use flash, fluorescent of incandescent lighting. Again, just make sure the illumination is uniform and you're not getting reflections from the test targets (which means that using on-camera flash isn't a good idea).For most lenses you'll want to shoot the first image at maximum aperture, then a series of shots stopping down by 1 stop for each one.So for an f2.8 lens, shoot at f2.8, f4, f5.6, f8 etc. Past f8 you'll probably start to see some drop in sharpness due to diffraction effects.If you know your AF is good, you can use AF. If you're not sure, compare a few manual focus shots with autofocus, or do an autofocus test as described. Select the 'one shot' AF mode and use the center AF zone only.
If you want to make absolutely certain that all the shots use the exact same focus setting, you can focus the lens using AF, the switch to manual focus. If your camera has a 'Live View' option, use it. For manual focus it will show you exactly when the image is in best focus with no ambiguity. A viewfinder screen has to be perfectly aligned to show best focus, but the same sensor is used for Live View manual focus as for the actual image, so if one is in focus, so will the other be in focus. Most cameras provide for a magnified live view image for even better focus accuracy.Analyzing Lens Testing Chart ImagesThe images you shoot can be analyzed for sharpness and chromatic aberration quite easily. The numbers on the lens testing chart represent theline pairs per mm (lp/mm) values of the test patterns they are next to. These patterns should be pretty accurate down to the set marked 5.0.
Past that set, most printers can't really generate accurate patterns, but we're not going to need them anyway.To find the resolution on the sensor we need to know the magnification. For example if the magnification is 1/26 (1:26) and the line set at 2.5 lp/mm is resolved in the image, that really corresponds to a resolution of 26x that on the sensor, i.e. 2.5 x 26 = 65 lp/mm. There are three ways to determine the magnification. Note that resolution can only be determined in steps of about 12%. If the 2.5 lp/mm group is resolved the resulting calculated resolution is 65 lp/mm. The next group (2.8 lp/mm) corresponds to 73 lp/mm.
So you get 65 or 73 lp/mm. You can't really get anything in between from this chart. If you're using a pegboard background with 1' spacing between the holes, you can just count the number of hole to hole spaces across the image. If you count 22.5, the image is of an area 22.5' wide, which is 571.5mm. Divide this by the actual width of your image sensor. For an EOS 40D this is 22.2mm. So you divide 571.5 by 22.2 to get the magnification factor, which in this case would be 25.75x.
You can also measure the length of the 100mm line on the chart in pixels. Let's say you find the line is 670 pixels long. With an EOS 40D we know the sensor is 22.2mm wide and contains 3888 pixels across the width. Therefore the image of the 100mm line on the sensor must be(670/3888). 22.2mm = 3.825mm. The magnification factor is therefore 100/3.825 = 26.14x. If you have the corners of the 15'x22' area at the corners of the frame, the magnification will be around 26x.
This is approximate, but probably good enough!So now look at the image at 100% in an image editor and see which is the finest set of line patterns which are resolved. Let's say you can see the 2.5 lp/mm set is resolved, but the 2.8 lp/mm set isn't. Let's say you determined your magnification factor to be 26x.
The resolution at the sensor would be 2.5 x 26 = 65 lp/mm.Now the problem here is that the sensor is limiting the resolution. For example, theEOS 40D sensor has a pixel spacing of 5.71 microns so there are 175 pixels per mm. There's a theorem in information theory called the Nyquist sampling theorem, which says that in order to reconstruct a sine wave you must sample at at least twice the frequency of the sine wave.
If you were to assume that the same principle applies to line pattern charts, the very best you could do would be to resolve about 87.5 lp/mm with a sensor which has 175 pixels/mm. However you can't strictly apply the Nyquist theorem for a number of reasons. First there's an anti-aliasing filter over the sensor which cuts off response close to the Nyquist limit to eliminate spurious effects such as moire patterns on images of closely spaced lines. Even without the anti-aliasing filter, you're not looking at reconstructing sine waves. You're looking at bar patterns with higher frequency components. So you're not ever going to get 87.5 lp/mm resolution, no matter how good the lens is.The basic upshot of all this is that even with an EOS 40D sensor (which incidentally has about the same pixel pitch and therefore resolution as the fill frame 22MP EOS 1Ds MkIII), you're not going to see more than about 75 lp/mm resolved.
Since most lenses are capable of resolving quite a lot more than 75 lp/mm, this in turn this means that most lenses will give you the same resolution based on these chart measurements, even used wide open. Now this doesn't mean that all lenses are equally good at all apertures. What it means is that strict resolution measurements aren't the best measurement of lens quality. So why measure it?
Well, if you don't see a resolution of over 60 lp/mm in the center of the image, you know you either have a really bad lens, a really bad sample or a lens or a focus problem. In the image corners you may see lower numbers, especially from low cost zoom lenses.On the left is a 100% crop from a test image. You can see the 2.5 lp/mm line set is quite well resolved. In fact even the 2.8 lp/mm set is just resolved. For this particular image a magnification factor of 26.3 was measured, so here we're seeing a resolution of 2.8 x 26.3 = 73 lp/mm. Incidentally, this shot was taken using a Canon EF 50/1.8 lens at an aperture of f5.6.
If you see something like this, there's nothing at all wrong with your lens. With an 8MP sensor APS-C DSLR you may see slightly lower resolution and with a 12MP APS-C DSLR you may see slightly higher resolution, but the difference will likely be quite small. On the left is a 100% crop from an image shot with a 12MP Sony Alpha 700 using a 100/2.8 macro lens at f5.6.What makes a good lens better than a poor lens then, if the digital images they produce all have similar center resolution? The answer is contrast, or more specifically MTF (modulation transfer function) at resolutions less then the theoretical limit. For an explanation of MTF see the article on this website. Higher contrast (MTF) will result in images which look sharper, and will in fact contain more information. That's why these charts have a high and low contrast set of patterns.
Better lenses will better show finer detail in the low contrast patterns than poor lenses will and so may give a better indication of a better lens.There are also samples of text and patterns (grass and waves) along the bottom of the chart. These can also be used to make a visual assessment of the lens. With the charts shot at the design magnification factor of about 25-26x, the smaller text should be on the limit of readability. On the left is a 100% crop taken from a test chart of the text block at the lower left.Chromatic aberration will show up most on the sides of the black border lines of the chart in the corner of the image. If you see one color on one side of the line and a different color on the other, you're seeing chromatic aberration. On the left is a sample from the corner of an image shot with a Canon EF-S 18-55/3.5-5.6 lens at 50mm and f5.6.
You can see a purple fringe on the right side of the line and a yellow fringe on the left, which indicates the presence of chromatic aberration. The more intense the colors and the wider the color bands are, the worse the aberration is. I'd say this amount chromatic aberration is noticeable, but quite acceptable for a low cost consumer zoom.The Siemens star pattern, particularly the high contrast version on the left of the chart can tell you allsorts of things and may be the most useful single element of the chart for visual assessment of the image quality.
This is a pattern consisting of alternating black and white thin 'pie shaped' segments. As you move towards the center of the star the lines get closer and closer together. The higher the resolution of the system generating the star pattern, the closer to the center of the star they will appear to merge. Below are actual siemens star images taken using an EOS 40D and an EF 50/1.8 lens at apertures of f16 (left) and f5.6 (right).The image on the left was taken at f16 and is softened due to the effects of diffraction. The image on the right was taken at f5.6, probably one of the sharpest aperture settings for this lens.
As you can see, the lines are visible closer to the center of the star in the higher resolution image on the right. You can also see some patterning close to the center disk. This is caused by interaction of high frequency components of the image (high lens resolution) with the sampling pattern of the pixels. In the image on the left there are no such patterns because there are no such high frequency components in the image. The lines are akin to Moire patterns.The star patterns above show what happens when focus shifts. On the left the image is focused and each image to the right shows what happens when focus is shifted by approximately 0.5, 1 and 1.5 times the total lens DOF.
You can see rings of apparent sharpness in the star image on the extreme right (blurred in the center, then a ring of sharpness, then a blurred ring). This is a classic indication of optical phase reversal which occurs under defocus conditions.