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05-29-2011, 10:05 AM   #1
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Optimally sharp aperture, FA77, DA15

Are there good published tables to find out where diffraction starts to break down sharpness in Pentax (or other) lenses?

I have become mostly an available light shooter over the last few years, and am usually in the f/2.8 to f/4.5 range. Yesterday I had occasion to shoot with my strobes, and I wasn't too worried about sharpness with the lenses I was using, so rather than turn my light down, I just stopped down to f/9. I was shooting with 3 lenses that are normally very sharp close to wide open: the DA15, the FA77, and my Nikkor 35-70. In evaluating results, there was a noticable decline in sharpness at f/9. Nothing too horrible, the pics were still quite usable, though I was a bit more aggressive with P/P. But it just got me thinking about this, and the need to be a little more thoughtful under these circumstances.

05-29-2011, 10:19 AM   #2
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I was just reading a fine set of published tables on the da15 but need to get back home to find it. I can tell your that depending on how you want the max sharpness on the edge versus center, apertures of f6.3 and f7.1 were indicated as best on the charts I just read. When I find the site I will post again.

EDIT - I remembered, its the dpreview.com review and the MTF50 chart. Although my requirements differ from much of the reasons the review was so-so on the lens, I have no reason to believe the numerical analysis is not accurate.

Last edited by imtheguy; 05-29-2011 at 10:24 AM.
05-29-2011, 10:40 AM   #3
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QuoteOriginally posted by Todd Adamson Quote
Are there good published tables to find out where diffraction starts to break down sharpness in Pentax (or other) lenses?

I have become mostly an available light shooter over the last few years, and am usually in the f/2.8 to f/4.5 range. Yesterday I had occasion to shoot with my strobes, and I wasn't too worried about sharpness with the lenses I was using, so rather than turn my light down, I just stopped down to f/9. I was shooting with 3 lenses that are normally very sharp close to wide open: the DA15, the FA77, and my Nikkor 35-70. In evaluating results, there was a noticable decline in sharpness at f/9. Nothing too horrible, the pics were still quite usable, though I was a bit more aggressive with P/P. But it just got me thinking about this, and the need to be a little more thoughtful under these circumstances.
Never shot FF77 below 2.8 but good to know your findings.
05-29-2011, 11:29 AM   #4
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w/ the 15, it's almost positively f8 in my experience.

w/ the 77, I'm not so sure, but I don't think it "matters" much past f5.6, where it is just crazy sharp.

05-29-2011, 11:43 AM   #5
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You can also see the test results on photozone.de. While I think their testing methodology may be exaggerating the difference between center and corner sharpness, I do tend to agree that f/4 and f/5.6 are both actually sharper in the center - which is to say, probably 90% or more of the image - than f/8 is, but that if you are shooting a scene in which the corners are actually in focus (rare) and you need them sharper, going to f/8 helps.

We're splitting hairs with regard to center sharpness, though - it's still clearly pretty amazingly sharp through f/11. but if you want *optimal*, I'd go with f/5.6, both based on experience and photozone.de's numbers.

Last edited by Marc Sabatella; 05-29-2011 at 11:50 AM.
05-29-2011, 11:43 AM   #6
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That's a good question Todd. I'd like to know the answer too with those lenses. I would have assumed it was safe to go near f/16 before seeing a noticeable loss in image quality. But then I'm using the K10D, not something like the K-5.
05-29-2011, 11:51 AM   #7
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QuoteOriginally posted by builttospill Quote
I would have assumed it was safe to go near f/16 before seeing a noticeable loss in image quality.
Noticeable loss in IQ, you are likely right. I think theoretically, though, f8 is going to be optimal all over. I think that the optimal aperture is related to "wide open", but this I am not so sure of.
05-29-2011, 02:29 PM   #8
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From first hand experience I can say that the "optimum" aperture on the DA 15 is f/8. Below that, corner softness is quite noticeable. If you don't really care about the corners, then it is stunningly sharp in the center at any aperture up to and including f/9 (one of the sharpest photos I've ever taken was at f/9). Above that, it's slight, but if you look closely you will see some diffraction softening.

For the FA 77, no question about it: f/5.6. I've pixel peeped extensively at that aperture and it is absolutely flawless corner to corner at that aperture. There is no reason to go any smaller than that unless you want more DOF.


Last edited by Cannikin; 05-29-2011 at 03:34 PM.
05-29-2011, 02:42 PM   #9
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Thanks for all the info everyone.

QuoteOriginally posted by Marc Sabatella Quote
but if you want *optimal*, I'd go with f/5.6, both based on experience and photozone.de's numbers.
I just did a quick test, and for the apertures I used, I would agree with this.

I'm not sure why I am in the mood to pixel-peep, but I guess I am. I'm really good at doing half-ass lens tests that come with many caveats, and today's test is no exception. I shot a book cover at four different apertures. In this case, the main caveat is that at f/10, my shutter speed was slow.....a half second. I shot from a tripod with SR turned off, and I did the f/10 shot three times and chose the sharpest one. The other caveat is that I cropped and also down-sampled a little bit for the results, just for convenience.

The result is HERE.

The first panel is a B&W version of the crop showing the approximate focal plane. If you inspect a couple of critical spots along that line in the other image, and particularly if you have them in a layered document and switch the layers off and on for comparison, as I did, you'll see the following:

f/1.8 - Not very sharp. Some fringing, and a somewhat reduced contrast. All expected wide open, of course.

f/3.2 - Pretty darn sharp. Fringing still present, but much reduced. Maybe a slight bit of haziness still, if you stretch your imagination.

f/5.6 - Best quality in this test for sure. Fringing is gone, contrast looks great (keeping in mind this is an unedited raw file). It's sharper than f/3.2, but not by a significant amount. It actually took me several times going back and forth, looking at different spots, to convince myself that it was sharper at 5.6 than at 3.2. I am convinced, though.

f/10 - Not as sharp as f/5.6, but again the discrepancy is not really significant for most of us. I'd say f/3.2 is a hair sharper than f/10, but the sharpness is still quite respectable, and the results usable at f/10. There IS some noticeable loss of contrast at f/10, but it's of a level easily remedied in post without loss in final IQ.

Not to take anything away from the FA77, because I think it's a phenomenal lens, but overall sharpness at any aperture doesn't measure up to my Zeiss lenses and some of my Nikkors. Pure sharpness isn't the only parameter, or even the most important one, by which to measure a lens. This lens has character and micro-contrast approaching my Zeiss lenses, and better than many, if not all, of my Nikkors. And of course, it has that Pixie Dust thing going for it.
05-29-2011, 03:15 PM   #10
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QuoteOriginally posted by Cannikin Quote
From first hand experience I can say that the "optimum" aperture on the DA 15 is f/8. Below that, corner softness is quite noticeable. If you don't really care about the corners, then it is stunningly sharp in the center at any aperture up to and including f/9
I guess I haven't shot with the DA15 enough to get the feel, but yeah, I'm really impressed with the center sharpness even wide open. At first I was disappointed with the sharpness at f/9 yesterday, but in revisiting the files, I see that my focal point (the eyes) was pretty far off center. LOL:

05-29-2011, 04:32 PM   #11
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Yeah, f/4 or f/5.6 is where a lot of lenses I like to use seem to hit their center sweet spot. Here's what photozone found (notice the DA 15ltd's performance in the center wide-open, better than at f/8!)


.




05-30-2011, 03:27 PM   #12
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Thanks for posting those, Jay.

QuoteOriginally posted by jsherman999 Quote
notice the DA 15ltd's performance in the center wide-open, better than at f/8!
Very interesting!
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