One lens vs. multiple lenses

Started Jul 30, 2013 | Discussions thread
Gerry Winterbourne Forum Pro • Posts: 14,223
Re: One lens vs. multiple lenses

trut_maluglist wrote:

I use 6.3 instead of 5.6 because I learned that F8.0 is the optimum F stop on most lenses.

Then you "learned" wrong.  Considering IQ only, in perfect shooting conditions, you need to think about what you mean by "optimum" - do you mean (1) best centre resolution, (2) best edge resolution or (3) most uniform resolution across the frame (even if some of it is lower than the maximum possible)?

If you mean (1), the best centre resolution typically comes about 2 stops down from the widest available.  So for all the fast primes and zooms out there it's more likely f/4 or f/5.6 than f/8.

If you mean (2), it typically comes a stop or two down from (1) so it does occur at f/8 sometimes - but not on "most" lenses.

If you mean (3) it typically comes when diffraction smoothes out difference across the frame, somewhere around f/11, which is beyond f/8.

Or, if you try to blend all these factors into one overall assessment, it typically comes between (1) and (2), so it's at f/5.6 as often as it's at f/8.  Looking at your 18-200 on Photozone, its "optimum" performance varies at different focal lengths: I wouldn't pick f/8 as optimal for most of the FLs on this lens, never mind "most lenses".

For your bull shot, where (I presume) the bull was more important in the image than the buildings behind it, the lens you used has better centre and edge resolution at f/5.6 than at f/8; and the centre resolution - where it really matters - is better at f/3.8 than at f/8.  So f/3.8 would give you better IQ where it matters - which is how I interpret "optimum" - than the f/8 you've "learned".

Next we consider noise: this always erodes resolution to some extent, so what the camera-lens delivers in the test lab at base ISO beats shooting at higher ISO.  Choosing f/3.8 instead of f/6.3 would bring your ISO down from 1600 to about 560, which loses less resolution.

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Gerry
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First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
gerry.winterbourne@ntlworld.com

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