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Old May 19, 2006, 10:25 PM   #1
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In continuation of our previous post about AF techiquie with 30D (or any other multifocus DSLR), I've done some test for myself as well as others who may be intersted in further experiments.

I've done just few comparison shots and that may not be enough to draw the conclusion what method is actally better: one with center point only and then recompose, or using all 9 AF points or just one which is the closest to the 'point of interest'.

My personal conclusion, and I would like to see other people try both, is that using only center AF point and recomposing will give sharper picture.

Test 1 (detail on the left is with center AF point):
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Old May 19, 2006, 10:28 PM   #2
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This was just a test picture, hand held, with built in flash, center AF point and recomposed. Please note that AF will still show in the center even though it was focused on the little sticker on the top of the botle:




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Old May 19, 2006, 10:30 PM   #3
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This one is taken buy 'suggested' method of using most appropriate (closest) AF point:
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Old May 19, 2006, 10:32 PM   #4
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And other test picture, this time some fruits, same two techiques tested; here the difference is not that obvious-maybe due the fact that I didn't use the tripod. The reason is that I do not use the tripod much for casual shooting which is in my case most of the time:


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Old May 19, 2006, 10:33 PM   #5
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Fruits - center AF and recomposed:
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Old May 19, 2006, 10:33 PM   #6
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Fruits 2 - Rightmost AF point used:
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Old May 19, 2006, 10:38 PM   #7
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I do not want this to be continuation ofarguing that this or the other way is better. Maybe there is someone interested to try similar thing and tell us about that.

Using center AF will not give bad results or not worse than using any other AF point. It tends to give the better results. My explanation for that is that lense is the best at its center. Camera takes measures with maximum apperture and the pictures are even more soft further from the center (usually with less quality lens).


Maybe tests with some other lens (maybe some L lens) will not be that different. I am using just cheap Sigma 17-70.

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Old May 20, 2006, 4:42 PM   #8
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For all that have replied to my previous post, thanks. The idea of prefocussing made sense - I had only made use of exposure locking in my SLR days but then my 14 year old SLR did not have 9 AF points to contend with.

GM has just highlighted something else with his first set of test pics in that the exposure is different - prefocussing could cause the exposre to be incorrect. I see from the manual that if the focus mode isNOT AI Servo and the metering mode IS evaluative then depressing the shutter button halfway will cause AE lock. The exposure will be based on the selected focus point if done manually or the active ones if in auto mode. (hope that made sense)

One could set C.Fn-04 to 1 in order to use the * button to lock focus and the shutter button would do the exposure part to compensate for the above.

This causes me to ask another question about digital - I know much has been said about noise but is it possible that an underexposed pic could influence the sharpness/focus of a pic?

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Old May 21, 2006, 11:45 AM   #9
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Quote:
prefocusing could cause the exposure to be incorrect.
Good eye and I would say yes and no, again depends what exposure method is used (manual, evaluative, partial, spot ...). Then other question is: if I focus on the side of the frame (where my subject is) do I want exposure to be metered there or in the center? Again, that is another issue. If exposure is critical (difference will be noticeable if there is a contrast difference in the scene), the best is to use manual setting then.

Other interesting thing in relation to AF points and taken from 30D manual, pg. 78:

Quote:
The center AF point is a HIGH-PRECISION, cross-type sensor sensitive to both vertical and horizontal lines of subject. ... remaining AF points are vertical (or horizontal) line-sensitive.
That also could be one of the reason why center AF point focusing may give the more accurate results.

And again, as flash pointed out, using center AF point is not possible with AI Focus or Servo. We will use this mode for moving objects where using center AF point may not make any sense, unless we can predict the object moving direction and quickly recompose.


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Old May 22, 2006, 12:44 AM   #10
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i've found that using the center point only works better for me. i get more consistently clear, sharp images than with the full 9-point AF. and i also find that the center AF works just fine with AI Servo or AI focus, though i don't tend to use the latter much.
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