The place for all photography and linux fans


by Jacek Góźdź

    What is front focus and back focus?

    When your camera sets the focus too close or too far it may mean that it suffers from so called front or back focus. This mean that autofocus system is not properly calibrated and it interprets the focus wrong. This kind of issue may be corrected only by a professional service (in fact there are some instructions over the internet how to do it but I personally think it is too dangerous, leave it to pros).

    The test

    Remember that 99% of errors are made by photographer not his equipment but if you think that this is not the case, you may perform a very simple test to check the camera. Some experts claim that it is needed to download and print a special chart, I think it is a waste of money and time. Here is what you can do to perform a quick but effective test:



    Take a ruler and put it on a table



    Set your camera on a tripod and make sure that the lens is pointing at the ruler. There should be 45 degrees angle between the ruler and lens optical axis. This is not a must, but it allows to do some additional calculations.



    Select one big line on the ruler (this will be your target line) and set the focus manually to take a perfect focus shot of it. Use the biggest aperture (the smallest F number) to get small DOF. If the photography shows the image as it should look like (selected line in perfect focus, equal blur in front and back of it) it mean that your camera focusing system works (it still does not mean that autofocus is good). If not, try to take another few shots to be sure if it is not your fault.



    If your camera passed the first test start testing autofocus. Make sure that your center focus point is set directly on the target line and there are no more details that could confuse it. Make sure your camera is set to use center focus point only. Take the shot using autofocus. If the target line is in focus your camera works properly, if not try to take few another shots and make sure you did everything you could not to confuse autofocus.
    ruler Image on the left shows result of my test. I did a shot at 45 degrees angle of a ruler from my school times. Target line was 10cm, aperture was wide open (f/2.8). Image shows perfect sharpness of target line and equal blur in front and back of it. The lens, camera and autofocus are therefore fine. To make the test complete one should do several shots of all of the lenses at different focal lenghts (for zooms).
    Additional information may be obtained by doing some calculations. If angle between ruler and lens optical axis is known (the easiest one to get is 45 degrees, that's why it is suggested) you can calculate depth of field of your lens at current aperture. Check where the blur is unacceptable, how far (on the ruler) it is (in my case it is not more than 2mm). Use some trigonometry to find out what is the real depth of field (calculate the projection of those 2mm on optical axis). If the angle was 45 degrees and I accept sharpness of line that is 2mm from my target line the depth of field would be 2⋅2mm⋅cos(45°)=2.8mm. This is more fun rather than scientific measurment, but done properly can give some usefull information.


Copyright 2007, Jacek Góźdź
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