|
Testing the Accuracy of SLR Camera Focusing system (by Francis Poon) First posted: 9th February 2003 Update: 27th April 2003 (a thicker center focus line is used in the AF test chart) Update: 5th May 2003 (a re-designed A4 size AF test chart with true metric scale and better arranged lines) Update: 14th May 2003 (a modified A4 size AF test chart with flip up large size focus target for wide angle lens test) One
of the basic element of a good photo is sharpness. The sharpness of
a photo is affected by many factors: How
accurate is your camera focusing system? The manual focus system accuracy depends on correct alignment of the focusing screen, the mirror glass, the photographer's eyesight and experience. AF Focusing System: The technology used is called contrast detection, the mirror glass is semi-transparent, a portion of the light passes through it and is reflected by a small second mirror behind it to an AF focusing module located at the base of the mirror box. The AF module is a horizontal and/or vertical array of CCD photo-sensitive cells for detecting image brightness, the signals from these cells are amplified and compared. When the image is in best focus the signal difference between adjacent cells is highest, a computer use this signal to control the lens driving motor in order to achieve best focus. The whole focusing system is so complicated that achieving fast and accurate focus is not an easy job. The focusing speed and accuracy is better under brighter light and higher contrast image. The accuracy also depends on the density of the CCD cells, this is evident when using wide angle lens with a lot of depth of field. You may test it by AF a wide angle lens from the infinity and from nearest distance setting on the lens, you will soon find out the the camera focus at different points in the two cases due to not enough resolution of the CCD array. Reports showed that current AF resolution is about 60 lp/mm which is less than the resolution of current high grade lens. That means manual focusing is better used when critical focusing accuracy is required. For a good zoom lens (not a varifocal zoom lens), it is better to focus at the longer end first and then zoom out for framing at the wide angle end. In general professional grade camera have faster and more accurate AF system than consumer grade camera. Larger aperture lens will enhance AF accuracy since it allows more light to pass through and thus results higher contrast (white to black ratio) image for the AF sensor to detect, provided the lens is sharp enough at wide open. A larger aperature lens which is soft at wide open may not have advantage over a smaller aperture lens which is sharper at wide open, because the softness will degrade the contrast. So at lower light and with softer lens, the AF focus accuracy would get worse. The AF accuracy depends on how accurate the mirrors and AF sensor module are aligned as well as how you manage it. We often heard users complain about front focus and back focus problem of their camera from internet and it seems not an isolated case. The popularity of digital camera makes this problem more pronounced since the image are loaded into the computer and viewed in pixel to pixel scale. So we need to test our own cameras to ensure we have the best tools on hand to get sharpest photos out of them. For a new camera, you have the chance of replacement by reputable shops within a week or so. You may also send it to the service department to correct it for free within the warranty period. Usually the manual focusing screen is quite accurate but the AF focusing system may have problem due to its complication. You may run a simple check by AF focusing and then by manual focusing to see whether the results match each other. If the image on the focusing screen looks a little bit sharper in manual focusing then there could be focusing problem in your camera. How
to test?
It's not easy to test the absolute focus accuracy of a camera due to depth-of-field, the image looks sharp enough within the DOF range. Here I'll introduce a simple focus accuracy test method which is easy enough for everybody using a test chart with the aid of depth-of-field calculator, the following websites have such DOF calculator on-line: http://www.dofmaster.com/dofjs.html http://users.aol.com/clemmi/Jv%5Fdof.htm & http://www.dpreview.com/learn/Glossary/Optical/Depth_of_field_01.htm . What you need is a DIY test chart, a sturdy tripod, your camera under test and film (no matter positive or negative film of ISO100 is OK, for DSLR no film is required). You can print the chart yourself using a good quality inkjet or laser jet printer. I printed mine using Epson photo inkjet printer 895 on A4 photo quality inkjet paper or photo paper at 1440 dpi and black colour. A crop of the test chart is shown below: |
|
 |
Place the chart vertically, you may simply mount it on a wall or the vertical side of a large box and place it on a table, place the chart horizontally on a table is also OK. The short side of the chart is perpendicular to the lens axis, and the long side of the chart is in 45 degree to the lens axis. To reduce perspective effect and get shallow DOF, it is better to use long lens of about 200mm with maximum f2.8, because critical focus accuracy is required in such shallow DOF situation. I use an EF 70-200mm f2.8 USM zoom lens for my Canon EOS 33 camera and an AFS 80-200mm f2.8 zoom lens for my F100. Mount the camera and lens on a sturdy tripod (I use Gitzo 1228 carbon fibre tripod) and load it with film. Use mirror lockup, remote shutter release or self timer whenever possible to reduce any camera shake. Set the camera focus point to the centre spot, make sure the lens are leveled. |
|
A
crop of the AF test chart
|
|
|
Caution: You should select only one single focus point in one shot mode, make sure the center focus line is in the middle of the focus bracket. For shorter focal length lens (wider angle), you should shorten the camera to chart distance accordingly (2m for 200mm lens, 1m for 100mm lens, 0.5m for 50mm lens etc. is recommended, make sure the distance is more than the shortest focus distance of the lens.) to ensure the chart is not too small and the center focus line is not too thin, also the focus bracket (the actual focus sensor could be a bit larger than the bracket, leave 50% more allowance) covers only the center focus line, otherwise the camera might not focus properly. You should test under enough light, better not less than 12 EV, about f5.6 1/125s, or f4 1/250s, or f2.8 1/500s etc. at ISO 100 film speed. Follow all the above to ensure you get a reliable test result. |
|
 |
Develop the film, print 4R or above if it is a negative film, and look for the result directly. If it is a slide film, use a 10x or above loupe to examine the result. You can also use a film scanner to scan the film at 2700 dpi or better resolution, open the file in Photoshop and zoom to appropriate scale for viewing on the monitor.
|
|
Test
setup photo, Nikon F100 with DG2 (x2) eyepiece magnifier attached.
|
|
| For digital camera, load the file directly to the computer and open it with appropriate photo browser, examine it on the monitor. If the focus is accurate, the lines are tack sharp at the 0 position and gradually blurred in front and behind it. Compare the lines in the front and back of the center line, check the DOF of the lens using on-line DOF calculator and see whether the result matches or not. You can easily distinguish the exact focus point by comparing the out-focus lines in front and back, thus you can verify the accuracy of the AF system, not a difficult job to do. | |
|
Practical test example 1 Camera:
Nikon F100 From the DOF calculator, for 35mm film format, 105mm lens at f4.5, distance 1m, the DOF is from 0.9975622608338539 to 1.0024496825402924m, that is 0.244cm in front and 0.245cm behind. The DOF is very shallow and nearly the same in the front and back. Here are two cropped test shot samples base on the old chart (the new chart has thicker center line, true metric scale and better lines arrangement), the first one is AF from infinity and the second one is AF from 0.5m: |
|
 |
|
 |
|
|
By comparing the in-focus lines and out-focus lines in the two shots to the DOF information, we can judge the focus point is quite accurate, the 0 mark lies well in the middle of the DOF range and matches the result of the DOF calculator. The manual focus is not too accurate in the same test due to my incapable eyesight, I need to test it later after I buy the DG-2 eyepiece magnifier. The result of this test shows the focusing accuracy of my F100 is very good. I'll test the AFS 80-200mm f2.8 lens later. Also I'll run the same test on my Canon EOS33 later. Right click and download the AF Test Chart. The
AF Test Chart is a 1 bit B/W BMP format file in A4 size, you could open
it in Photoshop and print it on an A4 photo quality Further Improved AF Test Chart (14th May 2003 update) |
|
 |
|
|
A
crop of the improved AF Test Chart
|
|
| The first AF test chart is already good enough and reliable if you follow the instruction carefully, the chart to camera distance should be close enough to ensure the the focus line is not too thin, otherwise this will challenge the limitation of the AF system. It is safer to follow the recommended distance for a given focal length. In testing wider lens below 50mm focal length, due to the limited shortest focus distance of the lens, the AF test chart would be too small and the focus line may not be thick enough for the camera AF sensor to work flawlessly. So I improve the AF test chart further with a much wider focus target, also the target is flipped up 45 degree so that it is perpendicular to the lens axis. The target consists much wider B/W lines pattern to ensure the AF sensor has the best contrast to work with. There are some fine lines also for achieving precise manual focusing using the focusing screen with the naked eye. I recommend you to use an eyepiece magnifier (x2 or more) in order you can focus precisely in the viewfinder. The crop of the improved AF chart is shown above. | |
 |
|
|
Close
up of 45 degree flip up focus target (photo taken by Nikon CP995)
|
|
| Print
out the chart on a photo paper (it is thicker, suitable
for flip up target), then cut the three sides of the target
with a cutter, don't cut the side that is in line with the 0 mark, flip it up to 45 degree (fold the two 45 degree wings also, see photo above) so that the target plane will be perpendicular to the lens axis when the chart is placed in 45 degree to the lens axis (see photo below). |
|
 |
|
|
Test
set up diagram
|
|
| Right click and download the AF Test Chart (flip up focus target). | |
 |
The AF test chart is mounted on a clip board, it is in a 45 degree slanting position against a box (X coordinate = Y coordinate) , fix the clip board simply by some ahesive tape. |
 |
Since the focus target is much wider, the camera to chart distance could be 2 to 4 times of the previous recommended distance. That's good for testing wide angle lens. The photo here is only for demonstration and reference, it is better to shoot under enough outdoor daylight. Remember to use timer or shutter release cord. |
|
Nikon CP995 Digital Camera Focus Test |
|
 |
The camera is set at macro mode and use the left side focus point, the shot showed that the focus is quite dead on. The CP995 performs excellent in macro with excellent AF focus accuracy. Normal AF Test (photo below) This shot showed a back focus of about 6mm, but well within the DOF range, it's not bad actually. Please note that the DOF range and the exact focus point is an approximation by visually comparing the sharpness of lines and letter "a" only, a quite acceptable method without the need of any sophiscated instrument and everyone can do it easily with some care. |
|
|
|
|
Conclusion The methodology introduced here lets every camera owner test the focus accuracy of their camera easily without the need of sophiscated testing instrument. I've tested my Nikon FM3A, F100, CP995 DC and the Canon EOS 33, EOS 1V with some lens combination. The manual focus of the FM3A and F100 is nearly dead on, the F100 AF is quite accurate. The manual focus of EOS 1V is OK but not as good as the F100, the EOS 33 is not good enough, both EOS 33 and EOS 1V showed more pronounced degree of AF focus shift than F100, EOS 1V AF is a little better than EOS 33 but it is not satisfactory for a camera aimed for the professional. An interesting result is found in testing the EOS 1V with the EF70-200mm f2.8 usm lens, the focus shift is quite a bit backward at the 70mm side, the focus is good in the 100-135mm range and OK at the 200mm end. This showed that both the camera AF accuracy as well as the lens sharpness play a part in final image sharpness. Overall Nikon F100 AF and MF is more accurate than Canon EOS 33 and 1V, also the AF accuracy of F100 is more consistant with different lenes. If you get the result that the focus point is outside the DOF range, then you would get soft or blurry photos. Replace one from the dealer or send your camera and/or lens to the service center together with your test result as a proof for a fix/calibration. Unfortunately my film scanner had some problem recently, I can't scan those AF test results at this moment, I'll do the scan later and post the result when the scanner is fixed. Here is some AF accuracy test results with my EOS 1V, EOS 10D (bought recently and returned after about a week due to inaccurate AF with my EF70-200mm f2.8 USM lens) and my Nikon F100 in combination with several lens that I have. http://www.hkdotnet.com/FrancisPhotographyChannel/AF_Test/AF_focus_test_result.htm |
|
|
Please sign my Guest Book and give your comments there, thank you for visiting! All rights reserved! Francis Poon - www.hkdotnet.com |
|
