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Old June 30th, 2011, 08:28 PM
Asher Kelman Asher Kelman is online now
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Quote:
Originally Posted by Nick Masson View Post
Hey all,
I have a question about technique when it comes to dealing with long depth-of-field and lenses that go soft in the edges when they get stopped down...

I am shooting with some older Nikkor lenses which I have chosen to work with because their small profile and weight allow me to bring them places I otherwise wouldn't or couldn't go (20mm Ai-S f/3.5, 24mm Ai-S f/2.8, 28mm Ai-S f/2.8, etc...).

Most of the time this setup is best suited for images with strong subjects and potent image value, but I do enjoy my share of tripod-mounted landscape work. These are not the best for lanscapes (given good wide lenses now, 17-35 etc...), but they are what I have.

To the point: When my subject is far off, everything is effectively at 'infinity' in as far as focusing. Sure nothing is truly at infinity, but there is no discerning between an object 1mile off and an object 2 miles off when focusing.

I have been told that it is still wise to use a small aperture in this case to maintain sharpness throughout the photograph, but is there really a difference between f/8 and f/22 when my subject is very far away?

I only ask because these small older lenses tend to get soft edges when stopped down, and are sharpest around f/8 (usually the case with most lenses), so I fear that whatever I may gain in shooting at f/22 instead of f/8 I would loose due to softening edges in the small-lens optics...

Any insight would be appreciated!
Thanks,
-Nick
Nick,

Your findings are not unique to "old lenses"!

Yes , your intuition that lenses "may not be sharp stopped down, especially at the periphery", is due to the fact the most lenses are not flat field, are better in the center and have higher resolution at worthwhile contrast when open at just a little off from the widest aperture. Some lenses are even designed to be totally sharp wide open. As one closes the aperture, the depth of field increases as you have sought with your f22 choice. However, there are costs! At smaller apertures, the resolution often gets less! Also, there's a second complication, namely diffraction. One eventually approaches practical degradation of the focus, due to the tiny aperture itself! In this respect, as a rule of thumb, with digital sensors and small format, one is O.K. up until 5.6, in almost all cases, and then by f11 there's some degradation. At f 16 or f 22, with tiny photosites, the degradation may actually become visible to you at the size you choose to print. Test it with your lens.

In order to get the best of DOF and sharpness with small sensors, a procedure to consider is taking pictures, where possible, using 5.6 to 11.0. To do better, you might look into "focus stacking" to build images with greater DOF, sharp through the zones of your interest. Especially if you are in one fixed place and with a tripod, focus stacking and exposure bracketing is possible. There are even programed stages for this. We've just discussed this with macro images! The great advantage of a digital camera is that it obeys rules! As long as you collect the right set of images, you can do whatever you wish!

Asher
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