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07-09-2011, 11:18 AM   #1
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31mm at 1.8 vs 2.8. something wrong?

I recently bought a Pentax K5 with some lenses, including tha famous 31mm ltd. I bought this lens for its wide-open capablities to go withe the smooth bokeh. I understand that it cant be as sharp at 1.8 as on 2.8. But, to me it looks like something is wrong with my copy of this lens. Look at the transition from white to darker areas, it is kind of leaking white into the darker areas. And the sharpness is not great either. The picture is taken on 15-20 meter distance. If taking pictures at a closer range, the result is the same, ie, the number "3d" engraved in my Pentax 15mm leaks onto the black lensbarrel in the same way.Can someone who knows this lens help me with this? You can fin a picture here: http://chals.zenfolio.com/p296791352/h133dcd4


Last edited by chals; 07-09-2011 at 11:46 AM.
07-09-2011, 11:38 AM   #2
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If you attach your image rather than use some url we have to go to, you might get more help.
07-09-2011, 11:49 AM   #3
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I hope it work better now. I dont think I can do better from this smartphone. Please put in an effort.
07-09-2011, 11:51 AM   #4
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I only see the hair on your lens or sensor...don't see bleed...

07-09-2011, 11:55 AM   #5
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I've noticed such 'haze' with other fast lenses when things weren't quite in focus. The DOF is so narrow with these lenses that just an inch can make this sort of fuzziness. Try shooting the same shot in live view, zooming in for critical focus on the while thing (looks like the edge of a roof, drip rail, and fascia).
07-09-2011, 12:03 PM   #6
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I am certain focus i OK. Done focus with live-view and not live-view, the results are always the same. The linked picture is representative.
07-09-2011, 12:27 PM   #7
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QuoteOriginally posted by chals Quote
I am certain focus i OK. Done focus with live-view and not live-view, the results are always the same. The linked picture is representative.
did you do manual or auto-focus?

if you didn't try to manually focus, try doing that one first. and if the results are still the same, maybe you'll want to get another copy.
07-09-2011, 12:29 PM   #8
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QuoteOriginally posted by chals Quote
I recently bought a Pentax K5 with some lenses, including tha famous 31mm ltd. I bought this lens for its wide-open capablities to go withe the smooth bokeh. I understand that it cant be as sharp at 1.8 as on 2.8. But, to me it looks like something is wrong with my copy of this lens. Look at the transition from white to darker areas, it is kind of leaking white into the darker areas. And the sharpness is not great either. The picture is taken on 15-20 meter distance. If taking pictures at a closer range, the result is the same, ie, the number "3d" engraved in my Pentax 15mm leaks onto the black lensbarrel in the same way.Can someone who knows this lens help me with this? You can fin a picture here: Zenfolio | Carlo Fals | div 2
The f1.8 image appears a bit out of focus, but that is just an aside. The "bleeding" that you are seeing on the f2.8 image is CA and can be easily fixed in post processing. A lot of lenses have CA, and can be taken out when the lens is designed, however at the expense of something else within the design. So the lens designers tend to leave some of the CA in since it is so easily corrected for, and thus do not have to compromise in some other area of the lens' optics.

CA is Chromatic Aberrations (CAs). Here are some links, explaining what it is, why it occurs and how to correct (or fix) it on your images.You can also, look up in your post processing tool - Chromatic Aberrations (CAs), and it will provide specific directions on how to repair it. Or you can google you tool with Chromatic Aberrations (CAs) and probably find additional instructions.

Also, if you scroll down in the first link you will see the MTF (resolution) section. f4 is the aperture for the highest resolution for the lens in the center and f5.6 for across the full lens (center and boarders). Scrolling down to the Chromatic Aberrations (CAs) section you will see that f5.6 provides the best aperture in terms of minimizing Chromatic Aberrations.

hope that helps ....


Last edited by interested_observer; 07-09-2011 at 12:39 PM.
07-09-2011, 12:29 PM   #9
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the picture shows you use your 31mm @f1.8, 1/2000sec.

1) too much light for such aperture :
- that's probably why the white seems to leak into the dark area.
- that's also why you have some CA

2) the f2.8 picture is out of focus

3) this lens gives its best around f5.6-f8 so use more those aperture instead of 1.8 @1/2000 sec -_-'
07-09-2011, 01:09 PM   #10
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If you take a look at photozone's review on the 31mm, there's noticeable difference between f1.8 and f2.8.
07-09-2011, 01:16 PM   #11
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thank you very much for the respons.(kenyee not counted) When I am back home I will post pictures based on your tips. It might be in a couple of weeks though. I still hope on some more contributions, I can read, but not post new pictures from where I am now.
07-10-2011, 06:56 AM   #12
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I never understood what you meant by "white bleeds into black"...from the different responses, it seems others are guessing at what you meant too
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