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08-18-2009, 10:47 AM   #1
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FA 31mm Limited troubles or is it me?

Just bought a mint FA 31. But when I use the the max aperture, it is fuzzy. I am not sure it is because me, the lens or my k20d. Thanks.

The flower seems fuzzy, but the ammeter seem ok.
The plate seems fuzzy, especially when I got a 100% crop.

All in K20D F/1.8 1/60 ISO 200

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08-18-2009, 10:55 AM   #2
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1/60th is pretty slow to be hand-held. Try it when you've got full daytime brightness and shutter speeds up above 1/250th.

Wide-open is almost never the sharpest for any lens, and I have no personal experience with the 31 so I can't help beyond that for that point.

Also, the license plate almost looks out of focus, are you sure you didn't move at all, between the step of focusing on the plate and actually taking the picture? Depth of field (what is acceptably sharp) is pretty thin at f/1.8 on any lens and if you moved your head forward 2", that might be enough to throw it off.
08-18-2009, 11:53 AM   #3
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Out of focus is the most common problem wide open for a fast lens like the FA 31. For a more rigorous test:
  • On tripod, SR off
  • Flat subject (newspaper tacked to a wall)
  • Well-lit
  • About 0.6 meters from the subject
  • Manual focus, best of five

Steve
08-18-2009, 11:55 AM   #4
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its just you

that licence plate photo is nearly a 400% crop

F1.8 is not known for being sharp, these are good photos for wide open!

08-18-2009, 12:06 PM   #5
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Original Poster
QuoteOriginally posted by stevebrot Quote
Out of focus is the most common problem wide open for a fast lens like the FA 31. For a more rigorous test:
  • On tripod, SR off
  • Flat subject (newspaper tacked to a wall)
  • Well-lit
  • About 0.6 meters from the subject
  • Manual focus, best of five

Steve
Thanks. I will post some photos by your advice later.
08-20-2009, 07:25 AM   #6
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That's sharp!

Those pics are very sharp considering it's wide open and F1.8.
08-20-2009, 07:42 AM   #7
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I have the 31 Ltd. and love it. Your pics are just fine. The license plate may be very slightly OOF, but to my eye this is sharp "enough", considering it is 1.8, not to mention flat light. Mind you, sharpness and contrast are also related, we percieve low contrast as lack of sharpness and high contrast as sharp.

The flowers are hard to tell, because Pentax AF is really not predictable which such a subject. I mean the hughe AF sensor could find focus anywhere in this scene ;-)

Don't worry, enjoy your lens...
08-20-2009, 07:44 AM   #8
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Nothing wrong with thos shots at all.

What sharpening (if any) have you applied ?

08-20-2009, 07:47 AM   #9
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Here are the Photozone MTF evaluations for the lens. You're going to get decent to very good results wide open, but as the other posts have indicated, make sure you technique is perfect to use that wide of an aperture. I would suggest using the lens a lot wide open and trying a 1000 different types of photos to get an idea of its capabilities at that aperture. I think what you might find is that it is going to be incredibly sharp when properly applied but it's never going to be as sharp as when you stop it down a couple of stops.
08-20-2009, 08:06 AM   #10
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I have found my FA31's AF a little inconsistant with the DS (haven't had enough experience with the K-m yet). Your 3rd shot looks slightly OOF. The rest are too small to tell anything because any soft shots look sharp when downsized.
08-20-2009, 09:13 AM   #11
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On the flower picture especially, DOF is probably shallower than your subject - there's probably no way to get all the flowers in focus at that aperture, focal length, and shooting distance.
08-20-2009, 11:35 AM   #12
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They all look fine to me. And the crop does not matter at all...don't expect a crisp image from such a crop of an object that far from fills your frame. A better test is to fill your frame with the subject and look at that to see if the image works for you.

To help with the DOF and what to expect will be iin focus this DOF calculator is a pretty good one:

Online Depth of Field Calculator

After you play with it for a while you will have a feel for what each of your lenses will give at different apertures. It helped me a lot the more I used it. I even downloaded the PalmOS version to my old PDA to carry around just in case.
08-20-2009, 01:03 PM   #13
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As others have stated, those are actually pretty sharp photos for f/1.8, I'd be pleased with those. That plate looks about what I'd expect for a 1.8 shot from that distance from a 31mm lens (not exactly telephoto) .

Actually, the shot of the flowers and the meter just made me want this lens again
08-20-2009, 01:13 PM   #14
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quick look at the photos and the feeling is it's you. Too picky @1.8 in my opinion.
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