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Fine Tune Calibration and Wide Angle focal length

elec164

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elec164 Silver Member Nikonian since 15th Jan 2009
Tue 12-Apr-11 04:46 PM

Somewhere in all the information I read in the various threads discussing focus issues I seem to remember reading a reference to a caution with calibrating wide angle lenses. I have not been able to find that source again to link to it, so I can only rely upon the fact that I seem to remember reading some sort of reference to wide angle lenses.

Having tested my 17-55 f/2.8 with the angled chart at 55mm and determining it focused pretty much dead on I stopped there. This image shows the area of the angled chart were the focus point was centered upon.

Click on image to view larger version


What I particularly liked about this image (and hope everyone else can also see it) is that in the paper pulp detail shows the DOF surrounding the focal plane. It appears the focal plane is the top side (would have been the back edge being that an angled chart provides depth to the target) of the line because at the focus distance the distribution of the DOF would have been 49% front and 51% back. I could show you the graduated rulers on each side indicating a possible back focus issue, but to me this clearly shows the camera focused pretty much where I expected it to within .5mm or less. And a flat chart mounted on the wall using 55mm focal length had pretty much the same result.

But then I recalled reading a mention of the wide angle caution, and decided to try the flat parallel chart test shooting at the extreme wide end. What I discovered was quite disconcerting. At about 2.5 feet and 17mm I found Phase Detection AF quite unreliable with live view getting better focus and more consistent results. At best it was a 50-50 shot. I could half-press and achieve a lock; release the shutter then half press again and get a totally different distance. In fact at close distance I would liken it to a jumping bean on a hot plate at certain times. Wondering if it was a defect in my 17-55 I mounted the kit 18-105 with pretty much the same results. Taking the camera outside with a real world wide-angle scene and the jumping around was not as drastic, but still occurred. Now I know in actual application the DOF would more than cover the slight focus error under normal enlargement. But it makes me wonder if all this hand wringing and mental gymnastics about fine tuning for the average enthusiast is really worth it in the long run (especially with wide angle focal lengths).

I tried to research this a bit but have come up empty so far. I know that wide angle lenses tend to show more aberrations especially at the outer edges of the lens. And being that it appears the Phase Detection system takes a sample from opposite sides this could be a major contributing factor I would think, coupled with the fact that shorter focal lengths will provide a greater DOF at a given distance because of the small aperture at a given f-number. But after that experience, if I am shooting wide-angle and the scene is static, I am more likely to use live view or manual focus than Phase Detection AF (especially if the DOF is narrow).

This brings up my next point, 100% view. It seems it was possible that this same issue occurred with 35mm film AF cameras. After all how many times did you make a 40x enlargement of one of your captures and view the results at an 18 inch viewing distance? I would guess the answer would be no one. At best most only analyzed 8x10 prints with a few analyzing larger ones. So at best a few might be judging maybe 20x enlargements maximum whereas most only viewed about 7x enlargements. But now D7000 users are routinely viewing captures at about a 60x enlargement and claiming a softness issue. I would think if you viewed your favorite sharpest 35mm film image at 40x enlargement (a similar enlargement factor for the given format size) you would see a similar issue, am I wrong in assuming this??

Hopefully this post makes sense to everyone; for I have viewed others resultant images, read and pondered so much about all that has been posted about this (plus all the reference links) that it sometime seems to be becoming a foggy mess in my head.

Any comments about my thoughts would be greatly appreciated.

Pete

Pete

Visit my Nikonians gallery.

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