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07-02-2011, 07:05 PM   #1
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Question about FA 31mm sharpness at f/1.8

Hello All,

I am a new member even though I have been a regular visitor of this forum for a while now. I am a K5 user and love it, and I bought a FA 31mm limited (yeah !!!... or so I thought) about a week ago.
I have been taking a lot of experimental pics at home to test out the lens, like putting at fixed position on a tripod, focussing on the fine print of a scotch bottle (black text on a white background) kept about 5 feet away, with single point focus, spot metering, M mode, manual focus etc. and I found that the lens is not as sharp at f/1.8 or f/2 , compared to the sharp-as-tack images that I got between f/4-f/5.6 range.
Question is, is this normal? Having read numerous reviews claiming that the fa31 has legendary sharpness even at 1.8, I must admit I am a tad disappointed with the results.
Or am I expecting too much? Or did I get a bad copy? Mine is an AIV one. I was thinking of returning it and getting another copy just to make sure, in case there are QC issue with the one I have.

Comments please.

Thanks.

07-03-2011, 03:34 PM   #2
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No response from anybody. Have I committed blasphemy by daring to question the venerable 31 limited in a pentax forum? :-)
07-03-2011, 03:36 PM   #3
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Pentax SMC-FA 31mm f/1.8 AL Limited - Review / Test Report - Analysis

f4 and f5.6 should be a lot sharper, so I'd say this is normal
07-03-2011, 03:41 PM   #4
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QuoteOriginally posted by mr.sinha Quote
No response from anybody. Have I committed blasphemy by daring to question the venerable 31 limited in a pentax forum? :-)
No, you haven't committed blasphemy by "questioning the venerable 31". However, without any samples to look at it's impossible to say if the difference in sharpness you're seeing when stopped down is normal or not. Certainly the 31 is sharper stopped down, as pretty much all lenses are.

07-03-2011, 03:46 PM   #5
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Perfectly normal. I find f/1.8 is "sharp enough" for real world use, I have no hesitation using mine at 1.8 anyway, although there is a noticable increase in sharpness by f/2.5+
07-03-2011, 03:46 PM   #6
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Well, at f/4-f/5, this lens is the sharpest. So if you are comparing f/4 sharpness with that at f/1.8, it will always be 'less' sharper.


Abhi
07-03-2011, 03:47 PM   #7
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danko, thanks for the article. Its reassuring that nothing is at least "wrong" with my lens.
07-03-2011, 03:57 PM   #8
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Did some real world testing in NY Central Park today even though it was very cloudy, raining and not bright and I had to be extra careful shielding the lens (with my K5 body up, lens down while walking around). Shot a lot of pics with f/1.8. Haven't downloaded yet, but looks quite good on the small lcd screen. Will post some samples later. So I guess you folks are right, I should not be comparing f/4 sharpness with f/1.8.

07-03-2011, 06:56 PM   #9
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These are the photos from my FA31mm review (posted in the lenses section).

I would say you should be able to get at least as good.

Index of /Photos/HelpandStuff/31mmReview

f1.8 isn't razor sharp but it falls in the good enough range for me.

07-03-2011, 07:39 PM   #10
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QuoteOriginally posted by mr.sinha Quote
Hello All,

I am a new member even though I have been a regular visitor of this forum for a while now. I am a K5 user and love it, and I bought a FA 31mm limited (yeah !!!... or so I thought) about a week ago.
I have been taking a lot of experimental pics at home to test out the lens, like putting at fixed position on a tripod, focussing on the fine print of a scotch bottle (black text on a white background) kept about 5 feet away, with single point focus, spot metering, M mode, manual focus etc. and I found that the lens is not as sharp at f/1.8 or f/2 , compared to the sharp-as-tack images that I got between f/4-f/5.6 range.
Question is, is this normal? Having read numerous reviews claiming that the fa31 has legendary sharpness even at 1.8, I must admit I am a tad disappointed with the results.
Or am I expecting too much? Or did I get a bad copy? Mine is an AIV one. I was thinking of returning it and getting another copy just to make sure, in case there are QC issue with the one I have.

Comments please.

Thanks.
I have the same combo, when I got my FA 31mm, I also found it was not very sharp at 1.8, similar problems with other "legendary" lenses, I tried adjusting the AF fine adjustment, sharpness improved a lot.
07-03-2011, 07:41 PM   #11
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You also may find that without precise fine AF tuning there may be some slight front or back focusing that may account for obvious softness wide open.
07-03-2011, 11:10 PM   #12
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I honestly don't find my 31 to be very sharp at f/1.8, period. It's also got quite low contrast and quite a bit of fringing wide open, which doesn't help matters.

However, I do find that I get a much better "keeper" rate if I focus it manually between f/1.8-2.8. I don't think it's front- or backfocus. I think that the Pentax AF system just isn't quite reliable at wider aperture settings, and I don't think I'm the first to suggest this.
07-03-2011, 11:20 PM   #13
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Folks,
Is there any place in NYC where someone can professionally check and if necessary, do the AF tuning? I bought mine at B&H. In my whisky bottle tests indoors, at f/1.8, I went thru the whole range of AF adjustments (-10 to +10) but didn't notice much diff, so AF adjustment *may* not a problem here, but just want to rule it out.

Anyway, uploaded some photos on flickr from my shooting today. Light conditions were not optimal, sky was heavily overcast with light rain, but damn if I would sit at home shooting more fine prints on whisky bottles, so off I went, trying to protect the lens from rain as much as possible.

A walk in Central Park on a rainy day - a set on Flickr
07-03-2011, 11:22 PM   #14
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Oh, forgot to add, out of the 20 shots on flickr, 5 were shot at f/4-f5.6. Rest at f/1.8, which is what I wanted to test anyway.
07-03-2011, 11:27 PM   #15
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Hey deadwolf, I see you have the Panasonic GF1 too. I have it and love it, but ever since the K5 arrived, I haven't used it much :-(
Question for you, do you happen to know of a good 4/3 adapter to mount Pentax FA and DA lenses on the GF1?

Thanks.
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