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Discussion in 'General Technical Discussion' started by Greg, Apr 9, 2005.
Can someone give me a "brief" explantion of what back focus is and how to test for it?
Back focus is an autofocus problem where the plane focussed on is behind the plane you were trying to focus on. Front focus is just the opposite. You can test it by focussing on, say a line of text on a page held at an angle to the camera, and observing where the intended focal plane and the actual focal plane lie.
Beware, however, there are numerous errors you can make in such a test. Do you believe you are experiencing such a problem?
I'm not sure but have not been happy with my results. About 4 out of 200 today were what I feel to be ok. were are handheld with strobe and 50mm Sigma Macro and various F/stops. Not sure if the good ones are the errors and just luck or if my whole problem is between the ears.
Images seem soft with manual or auto focus. They look bang on in the viewfinder. I'll try the test you mentioned tomorrow as well as other controlled tests with a resolution target and tripod with release. I assume this test you mentioned should be performed at maximum aperture (wide open)?
Thanks for the advice. I'll try to post results.
By far the biggest problem that causes lack of sharp photos is camera stability. Especially if you are doing macro work. If you are working at a distance with a macro lens, be sure it's capable of sharp photos when used in that manner. The Nikkor 60, for example, is somewhat soft at large apertures when focused near infinity.
Greg, Chris is right. Do the test using a ruler. Focus using your center bracket and see the results. Use various appertures and make sure you keep your camera "bolted on cement". :!:
If you want to give a Spielberg touch in your test, create a layer in Photoshop resembling your cam's focus brackets. This will show you immediately if you have a BF or FF ...syndrome :wink:
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Comments welcomed. if there is any question about these I may do a reshoot and tweak the exposure and white balance better.
50 mm f3.5
70-300 at 200mm f6.7