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09-05-2014, 02:48 PM   #1
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Differences K24mm f2.8 and f3.5

I am not an optical designer, but generally faster lenses tend to be bigger. Why is the K 24mm f3.5 larger than the f2.8? Is one sharper than the other?

09-05-2014, 03:01 PM   #2
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This doesn't directly answer your question,but the K24/3.5 is quite simply a Takumar 24/3.5 with a bayonet mount to replace the m42 mount. The K24/2.8 is a different optical design. Perhaps the 3.5 is simply an older, less efficient design made before the advent of SMC to allow smaller lenses.

K24/3.5 from Boz


K24/2.8 from Boz


My personal opinion is both are wonderful lenses, but each has a unique character. I think the 3.5 maybe a bit sharper but less contrasty and the colors aren't as punchy. The 2.8 produces a more robust image with higher contrast - and that maybe hides a tine bit of sharpness. Some of that may be attributed to sample variation and/or changes to coating. They're both excellent lenses.

[EDIT:] In the Forum reviews Phil (gofour3) speculates that the 24/2.8 is built and feels closer to an M than a K and was released at a time concurrent with the M lenses, replacing the K24/3.5. He supposes that had Pentax been able to design this lens with the 49mm filter ring they would have labeled an M-series lens.

Last edited by monochrome; 09-05-2014 at 05:36 PM.
09-05-2014, 04:09 PM   #3
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QuoteOriginally posted by monochrome Quote


My personal opinion is both are wonderful lenses, but each has a unique character. I think the 3.5 maybe a bit sharper but less contrasty and the colors aren't as punchy. The 2.8 produces a more robust image with higher contrast - and that maybe hides a tine bit of sharpness. Some of that may be attributed to sample variation and/or changes to coating. They're both excellent lenses.
How do the two lenses compare for distortion?
09-05-2014, 05:39 PM - 1 Like   #4
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QuoteOriginally posted by Swift1 Quote
How do the two lenses compare for distortion?
It has been long enough now that I can't really recall the difference and I lost a hard drive full of images from the period when I had the K24/3.5. Reviews here mention barrel distortion several time for the 3.5 but that is apparently not evident in the 2.8 - not in my copy. Sorry I can't help personally.

09-05-2014, 08:35 PM   #5
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I have the 24mm f/3.5 (super takumar) and I don't find distortion excessive. I need a setting of +2.5% in photoshop (lens distortion) to fix the barrel distortion and this is not an excessive amount (IMO). I also find it to be (physically) small, but that is a question of what you are used to. Actually it is an excellent lens--I think of/like the 24mm, 28mm, and 35mm f/3.5 Takumar lenses, which except for the 28mm lens, are then carried over into the K lenses. [Of course one cannot expect resolution on the same (high plane) as that 35mm lens, which is superb/really unusual.]

Not that the 24mm f/2.8 isn't very good--based on comments I have seen from people I trust, it likely is excellent. I suspect either lens will do well unless the 2/3 stop difference ls important. For me--I prefer the takumars--in wide angles, as I zone focus and use auto. aperture priority (instead of manual).

---------- Post added 09-05-14 at 08:47 PM ----------

Oh--as regards size--there was a trend at the time (and since then) to reduce lens size, and (thus) also possibly some loss of robustness. I have seen large differences in even AFAIK identical optical formulas, I am thinking of my Schneider 135mm f/3.5 lens for Alpa. Some of the lighter lenses may be/are more prone to damage from hits on their side (I recall this was an assertion about the fast Nikon 50mm AI f/1.2 lens).

Last edited by dms; 09-05-2014 at 08:58 PM.
09-05-2014, 09:25 PM   #6
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Thanks for the feedback, the responses were just what I was looking for
09-05-2014, 09:55 PM   #7
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PM gofour3 and bring his attention to this thread. In the mean time, you may have seen his review.

https://www.pentaxforums.com/lensreviews/SMC-Pentax-K-24mm-F2.8-Lens.html?page=2#review1905
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