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Searching for a free eye chart to check your vision at home?

You can download version 2 of the resolution test chartas a zipped jpeg file. Below is a typical lens test setup. In this case the 4x6 charts are attached to a sheet. The top 100% crop (1 on the chart above) shows a pattern that is slightly to the right of center – for evaluation of the center performance of the lens (typically the sweet spot for a lens). The middle 100% crop (2 above) is taken from the position that, when shot on a full frame body, represents a corner on a APS-C ( 1.5x or 1.6x FOVCF. One way to test your own lenses is with the USAF 1951 lens testing chart. The best way to do this is to download a high resolution file of the chart from the link at the bottom of the page, and print the file out at about 3x3 inches in size on photo paper at your printers highest resolution (with many printers this is 1440 or 1880 ppl).

Home Learn Lenses USAF Lens Testing. Lens Testing with the USAF 1951 Chart. One way to test your own lenses is with the USAF 1951 lens testing chart. The best way to do this is to download a high resolution file of the chart from the link at the bottom of the page, and print the file out at about 3x3 inches in size on photo paper at your printers highest resolution (with many printers. The basic steps in testing the lens are: Purchase or print an SFRplus or eSFR ISO test chart and mount it on a flat surface. Photograph it in an environment with even, glare-free lighting. Use the View/rename Files module to rename the files to more meaningful names, based on EXIF data contained in the file. For example, the name.

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. But it could help you identify potential vision problems that demand professional attention.

Need the JPEG version? Click here.

How to Use the Eye Chart

1. Print the free eye chart on regular 8 1/2 x 11-inch paper
2. Tack or tape the chart to a windowless wall in a well-lit room at eye level
3. Measure ten feet from the wall
4. Cover one eye (if you wear glasses for distance vision, keep them on)
5. Have another person point to each line as you read the letters out loud and keep track of which letters you get right
6. Continue to the bottom row or until you can no longer read the letters
7. Write down the number of the smallest line where you identified the majority of letters correctly (Ex. If you were able to read 5 out of 8 letters on line 8, you would write 20/20.)
8. Cover the other eye and repeat steps 5-7

What Do the Results Mean?

That depends on the age of the person being tested. A 3- to 4-year-old should be able to read the 20/40 line, and a 5-year-old the 20/30 line. Older children and adults should be able to read the majority of letters on the 20/20 line.

If you notice any results that fall outside these standards, be sure to schedule an eye exam with your Vision Source doctor.

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This post describes an autofocus test chart I've developed, why I thinkit's better than others I've seen, and how to use it to test the autofocusof your camera and lens:

My Tech-Related Photography Posts

• Lightroom Goodies (lots of plugins)

more...

1. How to Print It (downloading offered here)

1. Introduction

If you suspect that your modern SLR or SLR lens has autofocus problems, youcan use a chart like the one offered on this page to perform some tests.

The basic premise of a chart like this is that when photographed at anangle, the paper slices through the plane of focus. Depending on thesituation, that plane can be very thick, encompassing the whole chart(leaving the whole chart in focus), or, more usefully, it can be very thin,leaving only the areas of the chart that intersect with that thin region infocus.

The illustration below shows proper autofocus and the result one mightexpect from it, where the region of what is actually in focus is more orless centered on the focus target. It also has views for the problems ofback focus and front focus, where for some reason, the lensactually focuses on a region either behind or in front of the focus point(resulting in the in-focus area on the chart to be above or below the focustarget). Mouseover the buttons under the chart to see those views.

It has to be said....

It must be said on a page like this that while there is certainly broken ormiscalibrated equipment out there, it's probably safe to say that themajority of “is my autofocus broken?” concerns are rooted notin miscalibrated equipment, but in the user's misunderstanding of theequipment, or of proper technique.

If you suspect you have equipment with autofocus problems and mention it ona photography forum, such as those at Phil Askey's most excellent Digital Photography Review, be prepared forthree types of responses: “me too”, “are yousure?”, and “it's all your fault.”

About that last group, sadly, some people seem to enjoy being combative andignore all evidence that the user actually does understand the issues athand, and persistently, incessantly post “it's all your fault –you don't understand anything” type responses. Just ignore them.

However, the “are you sure, did you consider...” responses tendto be from respectable forum members trying to help. Even if it does turnout that your equipment is at fault, you can always pick up good hints andtips from these people, so embrace and appreciate them. I certainly learneda lot when I went through this with my own autofocus problems. It turns outthat the major cause of my problems was broken equipment, but intrying to understand where the problem lay, I learneda lot and improved mytechnique.

Oh, and I came up with an excellent autofocus test chart....

2. What Makes a Good Test Chart?

Many factors make an autofocus test chart good or bad, but the two mostoverwhelmingly important are:

1. the ability to ensure that the autofocus locks on to the exact location you intend, and
2. that it allows you to clearly interpret relative crispness of focus across the scene.

Without the first, the test has absolutely no meaning, and without thesecond, you can't grasp the meaning it holds.

Two popular autofocus test charts I've seen and used (and learned muchfrom), by Tim Jackson (no longer online) and LeonGoodman, don't address either of these concerns as well as they could,which is why I developed the chart presented here.

Update (December 2008) — If you'd like to do serious focus-related lens calibration, also considertheLensAlign Focus Calibration System from the Michael Tapes Design,the makers of the WhiBal.

I consulted with them on the product during its design(“consulted” in the “had discussions and lent my expertise and opinions” sense, not the “got paid for my time” sense, though they did kindly give me one),and believe it's a fantastic tool for lens testing and calibration.

It's better than my chart in every metric... except price. 🙂

I'll go ahead and add a third important characteristic of a good test chart– perhaps the most important – proper instructions,because it's easy to draw the wrong conclusions from an improperly usedchart. The two chartsI link to in the previous paragraph excel in thisarea, and reading through them is highly recommended.

Ensuring Autofocus Lock

Toward the first goal, the middle my test chart has a high-contrastblack bar – the autofocus target – surrounded on all sides byliberal expanses of low-contrast gray that the camera autofocus can notlock on to. For an illustration of this, see the “What the AutofocusSees” button in the super-shrunk view of the chart, at right.

(Testing to ensure that your camera's autofocus system can indeednot lock onto the low-contrast gray is an important step in Preparing to Use the chart.)

Below is a small section of the chart at full resolution, from slightlyleft of center, showing part of the black bar that is the focus target(with the red line added here just to indicate the vertical centerline ofthe chart).

However, here's what the autofocus sees:

This provides a clear target for the autofocus. The low-contrast grayextends quite a bit from side to side, but more importantly, it extends thefull height of the page. This means that even when the chart is viewed at asteep angle like that shown in the photo at the top of this page, a largearea of low-contrast gray still presents itself around the target,buffering it from anything else that the autofocus might lock on. Thus,with the precautions discussed later, you're sure that the autofocus locksonto the target bar if it locks onto anything at all.

Ease of Interpretation

Toward the second important feature – allowing you to interpretrelative focus – I've filled the area around the target with linesand boxes that, when viewed at an angle, make it quite clear how focusprogresses as you inspect up and down the page:

It should be readily apparent how useful the lines and blocks are whenviewed this way, but it's perhaps useful to contrast this with a differentmethod I've seen.

I've seen autofocus test charts that use lines of random LoremIpsum text, because, as the author correctly notes, we arehypersensitive to the crispness of text, so it makes for good test fodder. The problem with this approach is that we don't really care aboutabsolute crispness, but want to scan up and down to gaugerelative crispness. For this, random lines of spaced text are not asgood because there's no continuity as you scan vertically.

On the other hand, this chart's vertical lines make for something thatwould be smooth and consistent throughout a vertical scan if focus wereperfect throughout, but since focus is not perfect throughout, the smoothand consistent nature of the lines highlight clearly what is and isn't infocus.

Additionally, when some of the lines are dashed, the individual blocksmaking up the line become convenient visual markers of distance from thefocus target, allowing you to quickly compare a blockabove thetarget with its counterpart the same distance below the target.

3. How to Print The Test Chart

The test charts are provided as 2,449 × 3,299 pixel grayscale GIFs,which, when printed at 300 ppi, fit nicely on US Letter or A4-sized paper.

The key to properly printing the test chart is to get a result in which thegray areas are faint enough that your camera's autofocus system can't lockonto them, but are distinct enough that you can use them to gauge theresults. With my printer on my paper in the light Iuse for tests with my camera, “25% gray” seems to be thesweet spot.

Your situation may be different from mine, so I've built seven differentversions of the test chart, each with differing levels of gray for thelow-contrast areas. The 5% version is the most faint, while the 35% is thedarkest:

I'll suggest that you start with the 25% version thatworked for me, printing and testing as instructed below, returning to try alower-contrast version if need be.

Be sure to print at “100%” without any “fit topaper” option so that the resulting print is as crisp as your printercan produce. Any resizing necessarily incurs fuzziness, which can makeinterpretation of the results slightly more difficult.

It makes sense to select the “center image on page” option, if yourprint dialog offers it. Depending on the printer, you may have to selectborderless printing in order to fit the full image onto the page, or justas well, let a tad be cropped off. Either are better than resizing to fitthe page.

If printing from within Photoshop, be sure to tell Photoshop that it'sto be printed at 300 ppi byselecting “Image > Image Size...” and unchecking the“Resample Image” box, then change the Resolutionto 300, as shown at right.

If possible, print on high-resolution matte photo paper, such as Canon'sMP-101. Besidesgiving a crisper print, the paper is more sturdy, which makes it resistantto curling and warping (both of which are detrimental to its successful usein testing focus).

After printing, test the appropriateness of the low-contrast gray bylighting the chart well, filling the viewfinder with the gray area, andchecking to see whether the autofocus can lock on to it. If it can, youneed to move to a lower-percent gray.

The version you've printed holds promise if the autofocus system can'tlock onto the gray, but the quick test you've just done is onlypreliminary. Be sure to check again under actual conditions after setting upfor the real shot, as described below.

4. Preparing to Use It

Using the test chart involves taking a picture of it, but it's importantthat it's done under the right conditions.

The Setup

• Bright — you want enough light for the autofocus system to do its job, and to allow for a fast enough shutter speed, to reduce overall shake-induced blur.

• Wide open aperture — the wider the aperture (that is, the lower the f-stop number), the more shallow the depth of field becomes, thereby accentuating any focus-related problems. Using aperture-priority exposure mode makes this easy to ensure.

• Autofocus On — lest you forget 🙂

• Fast Shutter — you want a fast enough shutter speed to eliminate the blurring effects of camera shake. As one suggestion, you should have enough light to get at least a ¹/1,000^th second exposure at ISO 400 or lower. (That should cover most bases, but if you're skilled enough to think this page might be useful, you're probably skilled enough to know what shutter speed you need to eliminate shake-induced blur.)

  You might also consider using a tripod, and a remote shutter release.

• IS? / VR? — I've never heard of an image-stabilization or vibration-reduction feature causing changes in focus accuracy, but just in case, if you have equipment with these features, it might make sense to test both with and without them.

• Low ISO — the ISO sensitivity is not particularly important in its own right, but it's important to realize that while increasing it allows you to get more sensitivity from the sensor, but it does nothing for the autofocus system. If you know you have plenty of light for the autofocus system and merely want to push the shutter speed really high, increasing ISO is fine. Just realize that pushing it too high adds some level of noise-induced blur to the overall picture.

  Personally, I like to keep the ISO under 400 on my Nikon D200, but I'll go to 800 if needed when doing a quick focus test that I'll not put too much stock in.

• Square to the Chart — it makes interpretation easier if you're exactly square to the bottom of the chart (that is, you're aiming straight to the chart on the horizontal plane, without any side-to-side angle). The top-center and bottom-center of the chart has small black “sightlines” that can be of help.

• Chart is Flat — if the chart is not perfectly flat, its slight rumples can have a large impact on the focus. It's okay if just the corners of the chart float a bit off the table. The focus is on the main part of the chart, so to speak, and that's the section that must be perfectly flat.) Using a heavy-stock paper helps a lot.

• Exposure Compensation — especially in really bright light (e.g. direct sun), automatic exposure tends to underexpose the fairly uniform brightness of the chart, so you may need to dial in some compensation. A few sample shots should make it clear whether this is necessarily.

• Enough Distance — nothing will work if you're closer than the minimum focus distance of the lens, so be sure that you're far enough away that you're not bumping up against that limit. Note that some lenses have a different minimum-focus-distance between manual focus and autofocus. Zoom lenses can have a different minimum depending upon the focal length in use.

Deciding how far away you should be from the chart is influenced by whatkind of test you want (some lenses display autofocus accuracy that varieswith subject distance), the focal length of the lens, and the nature ofyour autofocus sensors.

Understanding Your Autofocus Sensors

It's possible that the active area of the camera's autofocus sensorexactly matches the indicator you see in the viewfinder, but it's notlikely. More likely is that the active area of detection is a bit larger,perhaps asymmetrically so.

The photo sequences below illustrate the pitfalls of not understandingyour camera's autofocus sensors. The right-hand image is just a closeup ofthe central part of the full-frame left-hand image. Mouseover the fourdescriptions below the pictures to see the story unfold...

The final result above appears as if the camera/lens has back focus, but inreality, it's just picking an unexpected spot to focus on. It would be niceif the indicator in the viewfinder exactly matched the active area, butsince that's not the case, it behooves you to understand your sensors.

I should be clear that the illustration above is just to make a point, andit's unlikely that any camera actually has an autofocus sensor shaped likethat shown in steps 3 and 4.

I'll leave the detailed description of mapping your autofocus sensors tothe link in the previous paragraph, but in short, one way to measure theactive area is to get close enough to the chart so that the low-contrastgray area fills the width of the viewfinder, and place the sensor indicatorright in the middle of it. Having nothing high-contrast anywhere nearby, itshouldn't be able to lock onto a focus, but instead, should hunt arounduntil it gives up. (If it can lock onto the low-contrast gray areas, you'llneed to use one of the lower-percent gray versions offered in the How to Print It section.)

Then, as you move the aim toward the high-contrast black running up anddown the sides of the chart, keep trying the autofocus, and once it's ableto lock on, note where the black begins relative to that edge of the sensorindicator. You can do the same moving the other way, and up and down, andso “map” the true active area of your sensor.

If you have multiple sensors, they may well each have their owncharacteristics, so you may wish to map them all, but for the purposes oftesting your autofocus system, it's sufficient to use only the middlesensor.

Of course, the more you can fill the viewfinder with the chart, the better.Even if the scenario above results in a valid test, chart is too small toreally make out much detail from it, even with the resolution my10-megapixel SLR affords.

5. Taking the Shot

After taking into account everything in the previous section, it's a simplematter to take some shots. Take multiple shots at different angles ofattack. A lower angle shows the depth of the in-focus field the mostclearly (and with it, the accuracy of the autofocus system), but requiresthe most care to ensure that the autofocus sensor does not see the top ofthe chart instead of the intended target.

(There's an example of a valid test with very low angle of attack in the next section.)

While shooting, keep the following in mind:

• Keep double-checking that you're square to the chart

• Be very careful not to allow yourself to move the camera between achieving focus lock and actually taking the picture. Some people, for example, have an unconscious habit to move forward a half an inch in the process of taking the shot, and such movement would absolutely destroy any meaning to this test.

• If you're near the minimum-focus distance of the lens, keep that in mind as well.

• Pause occasionally to double-check that the autofocus can't lock on the low-contrast gray (by pointing at the wide expanse of it in the upper half of the chart and confirming that focus can't be found.)

Once I've set up for a particular shot, I take it, then point the cameraat something far and autofocus there, then return to autofocus on thechart, taking a second shot. I then do the same with something near (if I'mnot already near the minimum-focus distance of the lens) and return for athird shot. This way, I feel sure that the autofocus is starting fromscratch each time.

I got used to taking multiple shots set up the same way because my lenswas giving somewhat random results. I guess that's one symptom of beingbroken, because after itwas fixed, the results were consistently spot on.

6. Interpreting the Results

After taking the shots, I load them into Adobe Lightroomand make a couple of quick adjustments to make the results a bit easier tosee (I convert them to grayscale, and adjust the contrast a bit toaccentuate the low-contrast region). Lightroom is excellent for thisbecause it lets me quickly zoom and pan on an image (more quickly even thanPhotoshop), and to quickly flip back and forth among multiple images.

In interpreting the results, I look at both the vertical progression ofthe lines, and the numbers that run up and down the sides. Consider this example:

The depth of field here is only about 8 millimeters (a third of aninch), so it's fairly easy to see the effects on the chart as it slicesthrough the in-focus region. The more clearly focused parts of the verticallines seem to be properly centered on the target stripe, and comparablenumbers (e.g. the left-side “2” above the midline and the left-side“2” below the midline) seem to be about equally fuzzy.

Actually, in this case, there might be the slightest bit more sharpnessto the numbers above the midline, but it's so slight that it could wellbe because the autofocus picked the top edge of the target stripe ratherthan the bottom edge. That's how good the autofocus was with this shot.

(Frankly, it could be that the bottom sets of numbers have theslight edge in sharpness.... the more I stare, the fuzziereverything becomes!)

Let's look at another example...

Here, the depth of field is almost 10 times larger (7.6 centimeters; 3inches), which makes it more difficult to draw conclusions from thevertical lines. In this case, I focus more on the numbers (haha, I'm so witty). Clicking through to the larger version and comparingthe two “5” above with the two “5” below, they feel fairly close in theirfuzziness, although the upper pair are a bit sharper. In this case, that'sto be expected because we're close enough to the chart that the depth offield is not evenly distributed in front of and behind the focus point:about 3.5 centimeters are in front, and 4.1 behind.

Depth of Field Measurements

Lens Testing Chart

Pedantically speaking, “Depth of Field” doesn't really refer tothe region that's “in focus”, but rather, “of acceptablefocus.” The difference reflects the fact that as you move in front orbehind the focus point, the focus starts tapering off immediately, andcontinues to do so indefinitely. What's considered “acceptablefocus” changes depending on the intended use and the resolution ofthe medium capturing the image (that means, among other things, that thedepth-of-field calculations are camera dependent). The current Wikipediapage on Depth ofField has a good presentation of the concepts.

My OnlineExif Viewer reports on the depth of field if the image data containsall the requisite data required to compute it. Some of this data is in theMaker Notes section of metadata, which Photoshop strips, so for bestresults, check with an original straight-out-of-the-camera image.

A Few More Samples

Here are a few more samples to inspect. As with most images on this post(and on my blog, for that matter), clicking through on the image brings youto a larger version.

At first glance this one might appear to be okay because the target is clearly focused, but comparing, for example, the lower-left “4” with the upper-left “4”, shows that there's a bit of back focus here.

This was taken with a 200mm lens from a medium-close distance (about three yards), which results in a depth of field evenly split on either side of the focus point. Had it been taken with a short focal length at a close distance, it's possible that the depth of field would start to skew more toward the rear, and as such, a result like this might be expected. That's not the case here, so this shows back focus.

Some pretty severefront focus

Front focus

Severe backfocus

7. Conclusion

I would expect that the only people who actually read this far are thosesuffering from really bad autofocus problems, and are desperate tounderstand them. You have my sympathy, and I hope that my test chart andwhat I've so verbosely presented here are helpful.

Testing Lens Test Chart Download For Windows 7