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11-02-2015, 11:57 AM   #1
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Is there a list of Pentax lenses along with optimal apertures?

Has anyone compiled a list of Pentax lenses and their optimal aperture ranges for sharpness / lack of CA / lack of diffraction? Seems to me this would be useful resource. I'd guess most of the information can be gathered from the in-depth and user lens reviews here on the site. If it hasn't already been done, I may pull one together... Otherwise, if one exists, I'd be grateful if someone could point me towards it Thanks...

11-02-2015, 12:57 PM   #2
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...or you could put your Pentax camera in MFT Program Line (assuming it supports it) and see what the Pentax engineers think is optimal for any given exposure.

Michael
11-02-2015, 01:30 PM   #3
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QuoteOriginally posted by MJSfoto1956 Quote
...or you could put your Pentax camera in MFT Program Line (assuming it supports it) and see what the Pentax engineers think is optimal for any given exposure.

Michael
Only for FA and newer lenses...
11-02-2015, 01:38 PM   #4
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QuoteOriginally posted by MJSfoto1956 Quote
...or you could put your Pentax camera in MFT Program Line (assuming it supports it) and see what the Pentax engineers think is optimal for any given exposure.

Michael
Hi Michael... good idea, but I'm not sure that would give the whole story. The kind of information I'm talking about would, for example, tell me that my 50mm f/1.7 lens is soft wide open with significant lateral CA, performs best *between* f/4.0 and f/11, and after that diffraction kicks in.

For the lenses I use often, I know this stuff anyway... But, if I think about my DA15 and DA70 - neither of which are getting much use right now - I honestly couldn't tell you the optimal range of apertures. If I was out shooting and decided to use either of those, I'd have to make a best guess. It would be nice to have a crib sheet with me.

Also, for those who don't yet have a particular lens, it might be useful to know where it shines.

As I say, if such a compilation doesn't exist, I'm happy to pull the information together for myself (and others, if interested)...

11-02-2015, 01:40 PM   #5
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Fairly consistently maximum sharpness is at f5.6 to 8 on a huge variety of SFL and zoom lenses. CA and aberrations are also commonly best around f5.6~8, but not always.
11-02-2015, 01:44 PM   #6
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QuoteOriginally posted by MJSfoto1956 Quote
...or you could put your Pentax camera in MFT Program Line (assuming it supports it) and see what the Pentax engineers think is optimal for any given exposure.
Something I wonder about: I know that (say) the K-3 has multiple program lines to choose from, and one of them is for optimal MTF; but (say) my K-30 only has one, and the manual doesn't say anything about its philosophy. Has anyone ever tried comparing to see which program line the one on the lower-end cameras is comparable to (if not equals exactly)? Or has Ricoh ever said anything about it somewhere else?
11-02-2015, 02:13 PM   #7
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QuoteOriginally posted by WPRESTO Quote
Fairly consistently maximum sharpness is at f5.6 to 8 on a huge variety of SFL and zoom lenses. CA and aberrations are also commonly best around f5.6~8, but not always.
Agreed, and for the lenses where I can't remember the optimal range of apertures, this is where I tend to end up... but some are very adequately sharp, with little to no CA, at f/2.8 or f/3.5, and that gives a decent amount of extra light to play with if you're aware of the fact...
11-02-2015, 02:29 PM   #8
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QuoteOriginally posted by MJSfoto1956 Quote
...or you could put your Pentax camera in MFT Program Line (assuming it supports it) and see what the Pentax engineers think is optimal for any given exposure.

Michael

Could you please explain what the MFT Program Line is, where it is accessed, and how it is used? Or any additional information?
I don't believe I've heard of it before. (sorry if that makes me a goof!)

11-02-2015, 02:32 PM   #9
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QuoteOriginally posted by BigMackCam Quote
Agreed, and for the lenses where I can't remember the optimal range of apertures, this is where I tend to end up... but some are very adequately sharp, with little to no CA, at f/2.8 or f/3.5, and that gives a decent amount of extra light to play with if you're aware of the fact...
Lenses sharp and with little CA @ f2.8~4 are generally those that have a maximum aperture one or two stops wider (i.e. F1.4~2). Few lenses perform best at their maximum aperture*, so any f2.8 will not be best @ f2.8, nor an f4 @ f4.

* FYI: A perfect lens, in theory, preforms best at its maximum aperture and will show IQ degradation from diffraction even one stop down. Some APO enlarging lenses are near perfect, but** only at specific, rather short lens-to-subject distances so they are not really suitable for general photography. Some macro lenses will reach best IQ only one stop down. In the era of view cameras, some lenses for 4X5 and larger formats were specifically designed to operate best @ f64 (usually called "process lenses"), but such a lens design is impractical and would not give equivalent IQ for small-format cameras.

**The "but" here I should clarify. Enlarging lenses only have their best IQ at particular, short lens-to-subject distances. All lenses have an optimum L-2-S distance for best IQ, otherwise any random lens would be suitable for macro.

Last edited by WPRESTO; 11-03-2015 at 04:48 AM.
11-02-2015, 02:49 PM   #10
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QuoteOriginally posted by yucatanPentax Quote
Could you please explain what the MFT Program Line is, where it is accessed, and how it is used? Or any additional information?
here's a link from a previous thread: https://www.pentaxforums.com/forums/38-photographic-technique/8208-program-line-mtf.html

on my K-3 II you set the Program Line value by going to: Menu/CameraTab/2/Program Line and choosing the program that is appropriate for your shooting style.

Michael
11-02-2015, 02:53 PM   #11
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QuoteOriginally posted by WPRESTO Quote
Lenses sharp and with little CA @ f2.8~4 are generally those that have a maximum aperture one or two stops wider (i.e. F1.4~2). Few lenses perform best at their maximum aperture*, so any f2.8 will not be best @ f2.8, nor an f4 @ f4.
Right, so... as a rule of thumb, then... an f/1.4 lens should start performing really well from, say, f/2.8 onwards, while an f/4 lens would start kicking in nicely around f/8 - yes? What about the point where diffraction starts to become an issue?

If I can get to the point where, instead of a lens by lens list, I can reliably work off a formula, I'd be genuinely happy to do so.

QuoteOriginally posted by WPRESTO Quote
* FYI: A perfect lens, in theory, preforms best at its maximum aperture and will show IQ degradation from diffraction even one stop down. Some APO enlarging lenses are near perfect, but only at specific, rather short lens-to-subject distances so they are not really suitable for general photography. Some macro lenses will reach best IQ only one stop down. In the era of view cameras, some lenses for 4X5 and larger formats were specifically designed to operate best @ f64 (usually called "process lenses"), but such a lens design is impractical and would not give equivalent IQ for small-format cameras.
Very interesting!
11-02-2015, 03:09 PM   #12
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QuoteOriginally posted by MJSfoto1956 Quote
here's a link from a previous thread: https://www.pentaxforums.com/forums/38-photographic-technique/8208-program-line-mtf.html

on my K-3 II you set the Program Line value by going to: Menu/CameraTab/2/Program Line and choosing the program that is appropriate for your shooting style.

Michael
Thank you. I'm doing some reading in that thread now. Much appreciated.
11-02-2015, 03:36 PM   #13
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Take a look here>Diffraction Limited Photography: Pixel Size, Aperture and Airy Disks and here>https://en.wikipedia.org/wiki/Diffraction-limited_system
11-02-2015, 03:38 PM   #14
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QuoteOriginally posted by BigMackCam Quote
Right, so... as a rule of thumb, then... an f/1.4 lens should start performing really well from, say, f/2.8 onwards, while an f/4 lens would start kicking in nicely around f/8 - yes? What about the point where diffraction starts to become an issue?
diffraction is usually visible at f/8 on crop and f/11 on ff, because that's where they both have the same dof... but since there isn't any universal standard for the actual aperture sizes, it's all based on whatever the lens manufacturer makes it; you have to test your lenses for diffraction softening... view at 100% size.

so in general f/8 and smaller apertures on crop would probably be a last resort, only go there if you need the dof.
11-02-2015, 03:49 PM   #15
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And let's not forgot diffraction is also defined by where you start to see it - so unless you print very large and view from very close, the practical limits are higher than the simple math and the pixel peepers may suggest.
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