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06-16-2016, 09:34 PM   #1
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Prime Lens IQ at Different Focus Distances

With a zoom one might see in a review how a lens' IQ varies at different focal lengths.

I've wondered if IQ varies in a prime lens depending on how far or close one is focusing. I have been testing that idea, and it seems to me that focus at infinity typically exhibits the poorest IQ. Other than at infinity, I haven't yet been able to decide much. Is there any wisdom, or studies, that address image quality of a single-FL lens focused at different distances?

06-16-2016, 10:46 PM - 2 Likes   #2
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It depends on lens design. Those with floating elements should be best corrected for various focal distances. Those without are likely weakest at minimum focus distance. Perhaps portrait lenses are optimized for that working distance. Process lenses are optimized for 1:1 reproduction.
06-16-2016, 11:52 PM   #3
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QuoteOriginally posted by les3547 Quote
With a zoom one might see in a review how a lens' IQ varies at different focal lengths.

I've wondered if IQ varies in a prime lens depending on how far or close one is focusing. I have been testing that idea, and it seems to me that focus at infinity typically exhibits the poorest IQ. Other than at infinity, I haven't yet been able to decide much. Is there any wisdom, or studies, that address image quality of a single-FL lens focused at different distances?
One problem with testing IQ at infinity is that meteorological visibility (or other weather condition) can vary very much from one day to next.
It may be hard to find a day that has perfect weather conditions for testing IQ.
06-17-2016, 01:55 AM   #4
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Infinity focus may be tested good at distances further then the hyperfocal distance. For most normal and wide angle lenses weather wouldn't be a real problem. For tele lenses it may be a problem.

06-17-2016, 02:15 AM   #5
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QuoteOriginally posted by civiletti Quote
It depends on lens design. Those with floating elements should be best corrected for various focal distances. Those without are likely weakest at minimum focus distance. Perhaps portrait lenses are optimized for that working distance. Process lenses are optimized for 1:1 reproduction.
Good point - If you read about the Pentax A* and FA* 85mm lenses you'll see that some people suggest the A* is superior to the FA* at infinity - different lens designs - is one more of an "all rounder" and the other optimized for portraiture ?
06-17-2016, 07:52 AM   #6
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QuoteOriginally posted by Fogel70 Quote
One problem with testing IQ at infinity is that meteorological visibility (or other weather condition) can vary very much from one day to next. It may be hard to find a day that has perfect weather conditions for testing IQ.
Very true. Actually I haven't been able to confirm that infinity focus is worse than any other distance.

I know with my 180mm, I take the worst shots at infinity when I'm shooting hand held (i.e., it's harder to steady the camera), but I do better when focusing at closer distances. That makes me wonder too since doesn't 180mm FL remain 180mm whether one is focusing near or far?

Last edited by les3547; 06-17-2016 at 08:56 AM.
06-17-2016, 08:22 AM   #7
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Interesting thread; I've wondered about this issue myself. Just like every lens has a 'sweet-spot' aperture for best IQ (it isn't necessarily f8), does every lens have its own 'sweet-spot' focus distance?

Two cases in point:
1. When using my Pentax-M 50mm f1.7 with either close-up lenses or extension tubes for macro, will I get the best IQ if I focus to minimum, maximum, or in-between?
2. I've read that my Vivitar 400mm f5.6 gives best IQ when focused close to the minimum distance, 13 feet. The problem is that I use that lens for stuff that's far away.

06-17-2016, 07:31 PM   #8
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QuoteOriginally posted by LennyBloke Quote
Good point - If you read about the Pentax A* and FA* 85mm lenses you'll see that some people suggest the A* is superior to the FA* at infinity - different lens designs - is one more of an "all rounder" and the other optimized for portraiture ?
I would say that LennyBloke is correct, in my experience (from quite a few years ago).

I will also say (which may or may not be related to the above) that, while the FL of the FA* 85/1.4 is likely close to 85mm (or at least similar to other older Pentax 85's) when focused at close to infinity, it would seem to be a bit less than 85mm (or at least a bit shorter than other older Pentax 85's) at closer focus differences. Take a look -



06-17-2016, 08:03 PM   #9
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Even the "best" shows this.

One example I have experienced is with my Leitz 24mm f/2.8 Elmarit-M (the original version made in the '90s). The lens is optimized for near range focusing and just sings wide open focused at 4-7 ft or so. Beyond 15ft or so it begins to decline in sharpness etc. It was made for close distance reportage in lower light situations and Is beautifully rectilinear with as little distortion at the edges as is probably possible with any lens of that focal length. Not to say it is not a wonderful lens at any distance or f-stop, just how it was optimized. I really don't think there is a lens made that will be "superb" at all f/stops and all focus distances. It is the nature of optics. There has to be a "sweet spot" of some kind.

Last edited by jeverettfine; 06-18-2016 at 08:26 AM.
06-17-2016, 08:22 PM   #10
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QuoteOriginally posted by jeverettfine Quote
One example I have experienced is with my Leitz 24mm f/2.8 Elmarit-M (the original version made in the '90s). The lens is optimized for near range focusing and just sings wide open focused at 4-7 ft or so. Beyond 15ft or so it begins to decline in sharpness etc. It was made for close distance reportage in lower light situations and Is beautifully rectilinear with as little distortion at the edges as is probably possible with any lens of that focal length. I really don't think there is a lens made that will be "superb" at all f/stops and all focus distances. It is the nature of optics.
Interesting. It would be useful, to me anyway, to have charts for the optimal focus distances for lenses, but that's not commonly part of reviews.
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