Why do lenses have smaller F stops than the optimal front to back clarity F stop ...

samfurlong

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10 Feb 2012 - 9:44 AM

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Quote: But what situation would you need a smaller F stop than the one that gives the sharpest image? I am confused

Because you may need the creative effect that a smaller / larger f number gives - such as varying the depth of field or achieving a certain shutter speed.
A big part of being a good photographer is understanding your kit, it's limitations and how / where to compromise on one aspect to achieve a desired result with another.

For example:
You might be shooting a landscape where you need a small DOF and low ISO for best quality so you decide to compromise on shutter speed so you get the tripod out and work carefully, using a slow shutter, possibly making your exposures in between gusts of wind to keep the trees still.

or you might want the very fastest shutter possible to freeze action so you open up the aperture and accept the slight drop in lens sharpness and up the ISO and accect the increase in noise to achieve your shutter speed.

the list goes on...

You may of course decide that you want to shoot everything at the optimal f number in which case just select aperture priority and f8 (most lenses sweet spot is around f8 - ish) and away you go. You will just have to keep an eye on your shutter speeds and ISO's and compromise on one or other of them to balance the other one out. It just depends what you want from your kit.

Graywolf

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10 Feb 2012 - 3:03 PM

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Photography is like life. Sometimes everything is exactly right, but most of the time it's a compromise.

LenShepherd

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11 Feb 2012 - 12:33 PM

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Quote: As a matter of interest, how do you measure '50% resolution'?



Most lab tests indicate a result using a "never encountered in real photography" 1000:1 black and white line edge target.
This obviously interests "nerds" - but has little direct relevance to the variable contrast levels encountered in every day photography.
I do my own tests using first a 1000:1 target to ensure no optical faults and then test the way I use a lens in the lighting I expect to use it.
Going back in time to the B&W film era most photographers knew contrast influenced resolution, and knew how to go about getting the result they wanted.
This is something "now out of fashion" but if you critically examine your work when working to high standards it is not difficult to do your own tests.

LenShepherd

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12 Feb 2012 - 10:14 PM

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Quote: That still doesn't explain how you define and measure '50% resolution'

The same way as "everybody else" who wants to do it
Fuji did it (and still do) for their films, comparing resolving power in lp lines/mm with a 1000:1 target and a 1.6:1 target.
You can achieve something similar by
1/ scanning a 1000:1 target and printing out a copy having significantly reduced the target contrast so you can compare 100 0:1 contrasts with lower contrast targets
2/ photographing a 1000:1 contrast target at the lowest and highest ISO's on your camera and comparing the resolution
3/ comparing central resolution with an f1.4 lens at f1.4 with corner resolution.
As guidance Nikon's 50mm f1.4G in Nikon's official f1.4 MTF scores 0.9 and 0.65 at 10 and 30 lpm at the centre and 0.35 and 0.15 in the extreme corners of FX.
Whilst home testing might produce slight differences; if it is to a very high standard you should get broadly similar f1.4 centre and edge differences.