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Old Aug 14, 2009, 10:35 AM   #1
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Default To stop down, or not stop down?

I shoot with either the Nikon 18-200VR, or the potent 17-55 F2.8 on a Nikon D300, or Finepix S2 Pro. My question is about lens diffraction when stopping down. I love to shoot detailed landscapes and I need deep DOF for either the foreground, or the background. Recently, while perusing a few magazines I noticed many photographers, shooting digital and stopping down to the likes of F22, F32 etc. If we do that, and there is some slight defraction and subsequent softness in the final image will sharpening bring that back? Or, should we stick to not stopping down to those extremes?
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Old Aug 14, 2009, 10:56 AM   #2
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Usually those extreme apertures are not wise and at most focal lengths for landscape they are not adding anything.

Take a look at http://www.bobatkins.com/photography...ffraction.html
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Old Aug 14, 2009, 11:22 AM   #3
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The diffraction limited f-stop of a 12MP APS-C dSLR is f/17.6.

See http://diglloyd.com/diglloyd/free/Di...ion/index.html
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Old Aug 14, 2009, 11:27 AM   #4
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Because aperture as expressed as f/stop is designed to express a ratio between the focal length of the lens and the size of the aperture iris, you have to take focal length into consideration when computing diffraction limitations. ;-)

The article that Mark1616 linked to has a calculator for that purpose.

But, in addition to diffraction related issues, you also have to take each lens on a case by case basis in order to determine the optimum aperture for a given focal length for sharpness/contrast purposes.
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Old Aug 14, 2009, 11:30 AM   #5
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Quote:
Originally Posted by JimC View Post
The article that Mark1616 linked to has a calculator for that purpose.
My bad. I thought he linked to a different article. Here's one with a calculator:

http://www.cambridgeincolour.com/tut...hotography.htm
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Old Aug 14, 2009, 11:33 AM   #6
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I'm wrong. Focal Length doesn't matter. You'll see this section at the bottom explaining why:

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Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out.

http://www.cambridgeincolour.com/tut...hotography.htm
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Old Aug 15, 2009, 11:04 AM   #7
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To pick a nit: diffraction effects not changing with focal length might be reasonable for a specified sensor/film size, but not otherwise. The old timers who used 8x10" (or larger) format cameras shot landscapes at f/64. If you tried that with an APS sized sensor the results would not be good - as shown at the site Mark linked to.
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Old Aug 15, 2009, 11:44 AM   #8
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How an individual lens behaves can make a big difference, too (outside of diffraction related issues).

In the case of the Nikkor 18-200mm VR the OP was asking about, I'd probably stick to f/14 or wider apertures with it.

Chances are, around f/8 to f/11 is going to be the sharpest for it at most focal lengths, and you've got lots of depth of field with that lens on it's wider end with a camera using an APS-C size sensor at those aperture settings. If you look at Steve's D300 Review Samples, you'll see this image at f/9 taken with that lens at 18mm. Note how much apparent depth of field you see:

http://www.steves-digicams.com/2008_...s/dsc_6879.jpg

If you stick to around f/9 and focus on something reasonably close to the hyperfocal distance (see this Depth of Field calculator for more info), it should be pretty nice at most print/viewing sizes for sharpness if you're shooting landscapes using it's wider zoom settings, without going to more extreme aperture settings.
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Old Aug 15, 2009, 12:43 PM   #9
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Quote:
Originally Posted by BillDrew View Post
To pick a nit: diffraction effects not changing with focal length might be reasonable for a specified sensor/film size, but not otherwise. The old timers who used 8x10" (or larger) format cameras shot landscapes at f/64. If you tried that with an APS sized sensor the results would not be good - as shown at the site Mark linked to.
Quite true ...

There are two independent issues here. One is the resolution/diffraction limit set by the sensor. This is controlled primarily by the photosite size (not the megapixel size as stated in the post that gives a link to a good description where the concluding chart clearly shows that the D3 and D300, both 12mp cameras, have radically different critical f/stops). This limit is independent of the focal length.

There is a second diffraction limitation controlled purely by the lens' characteristics. Unlike the sensor-based limitation, this diffraction limitation issue, know properly as Dawes' Limit, is a factor of the actual physical size of the aperture relative to the frequency of the light being focused. This translates to the limit occuring at different f/stops for lenses of differing focal lengths.

The bottom lines is that to get the sharpest results, you should not stop down below either limit. With short focal length lenses, the lens' limit may occur at a wider aperture than the sensor's and stopping down as far as the sensor's limit would not produce the sharpest results at the point of best focus.

BTW, SLR Gear's review of the 18-200 Nikkor VR (they haven't tested the new VR-II version yet) indicates optimum performance at between f/4-f/5.6 at the wide end and around f/8 at middle and longer focal lengths. Stopping down further begins to significantly reduce sharpness as diffraction takes its toll.
http://www.slrgear.com/reviews/showp...uct/250/cat/13
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Old Aug 15, 2009, 12:55 PM   #10
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Quote:
BTW, SLR Gear's review of the 18-200 Nikkor VR (they haven't tested the new VR-II version yet) indicates optimum performance at between f/4-f/5.6 at the wide end and around f/8 at middle and longer focal lengths. Stopping down further begins to significantly reduce sharpness as diffraction takes its toll.
http://www.slrgear.com/reviews/showp...uct/250/cat/13
The center may be a little sharper at f/5.6 on it's wide end. But, it looks like it may be a bit better in some areas of the frame at f/8 on the wider end to my eyes. Note the bur index at the top edge of the frame in the middle (furthest away from you in their graphics), as well as the corners (see the one closest to you using their blur index chart for a better example of improvement at f/8 versus f/5.6).

Most lenses are sharpest about 2 to 3 stops down from wide open (although you really have to take each lens on a case by case basis).
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