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09-03-2008, 05:52 PM   #1
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DA 16-45 decentered?

I recently picked up a used 16-45. I did some brick wall tests initially and it seemed fine, though my tests weren't exhaustive. I've been shooting it for a bit now and some of my pictures seem to be a bit out of focus in places.

What do you think of this photo (16mm, f/6.7, 1/180s, iso100, center AF, full size):



Center 100%:



Upper Left 100%:



The upper crop seems not so great to me. Do you think this is unusual? My understanding is that this lens is supposed to be pretty good... Any comments?

09-03-2008, 05:55 PM   #2
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it looks more like a DOF issue than decentering from this example
09-03-2008, 05:58 PM   #3
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QuoteOriginally posted by k100d Quote
it looks more like a DOF issue than decentering from this example
The DOF calculator suggests that my depth of field should be from around 2 m to infinity in this case if the focus point is a few hundred metres away... Am I misunderstanding things?

Edit: focus point might have been further, say a few km, but that doesn't really change anything (makes it better for distant objects if anything).
09-03-2008, 06:07 PM   #4
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Another example... in case the focus was off in the first one. Look at the mountain at upper right...

45 mm this time, f/6.7 1/180s, iso100, center AF, full size



09-03-2008, 06:11 PM   #5
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QuoteOriginally posted by sewebster Quote
Look at the mountain at upper right...
The snow and sky are completely blown out (overexposed), there's no detail left. This isn't anything to do with decentering.
09-03-2008, 06:13 PM   #6
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What I am seeing is atmospheric distortion. Basically you are looking at high humidity close to saturation. You are looking through a lot of space toward clouds.
09-03-2008, 06:14 PM   #7
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QuoteOriginally posted by audiobomber Quote
The snow and sky are completely blown out (overexposed), there's no detail left. This isn't anything to do with decentering.
Hehe, well, it wasn't supposed to be a stellar picture, just an example. I wasn't looking at the blown parts... compare trees near centre to the rocks at upper left... isn't that a reasonable comparison? Maybe not, how about rock at upper left vs rock at upper right? (I'm talking about the second example here)
09-03-2008, 06:17 PM   #8
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QuoteOriginally posted by sewebster Quote
Hehe, well, it wasn't supposed to be a stellar picture, just an example. I wasn't looking at the blown parts... compare trees near centre to the rocks at upper left... isn't that a reasonable comparison? Maybe not, how about rock at upper left vs rock at upper right? (I'm talking about the second example here)
These photos are not what you want to use for checking decentering. They're too complex, too many variables. Do the test everyone else does, photograph a brick wall or resolution chart, and use a tripod.

09-03-2008, 06:25 PM   #9
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QuoteOriginally posted by audiobomber Quote
These photos are not what you want to use for checking decentering. They're too complex, too many variables. Do the test everyone else does, photograph a brick wall or resolution chart, and use a tripod.
Ok, that sounds reasonable. One thing I wonder about though is distant or infinity focus. Maybe decentering is not the issue, but I wonder if I could get good results on a brick wall that is a few feet away, but poor results on a mountain that is a few km away. I photography mountains more often... and it seems that sometimes my shots seem a bit poor in the upper portion of the frame. It also seems potentially orientation dependent, which is a bit odd, but things could shift inside the lens I guess...?
09-03-2008, 06:41 PM   #10
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in terms of depth of field, even if the calculator says you get infinity, i believe you get more sharpness by stopping down further

maybe you can try changing the orientation the other way and taking the same photos. that or try to take photos without a foreground subject to test the infinity focus clarity.
09-03-2008, 08:51 PM   #11
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Agree

QuoteOriginally posted by k100d Quote
in terms of depth of field, even if the calculator says you get infinity, i believe you get more sharpness by stopping down further

maybe you can try changing the orientation the other way and taking the same photos. that or try to take photos without a foreground subject to test the infinity focus clarity.

In landscape type pics you want lots of DOF and not what the calculator says, you should be at f8 minimum IMO, and f11-f13 would be better. If you look in the center you are getting the DOF but the calculator does not calculate the complex things that start happening when going out into the corners, like sharpness dropping. Thats one of the reasons I got the Sigma 17-70mm over the Tamron 17-70mm, I like landscapes and for that you will never need f2.8 or f4 or f5.6.

Also the pic is too complex to analyze, I downloaded the full size pic and saw no decentering problem so far but its hard to tell, IMO its not, its your not using a small enough aperture. Post another
09-03-2008, 11:52 PM   #12
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Interesting. I've been shooting a lot in P mode, because I haven't had much time to adjust the camera. With the 16-45 it often selects f/6.7, presumably because it decides this is the aperture of maximum sharpness. With the kit lens it often chooses f/8. Perhaps I need to up my apertures to higher f/numbers for these landscapes.

I shot a newspaper though, and it is somewhat disturbing. The top of the frame isn't so good, especially upper right, whereas the bottom left corner seems totally sharp. I reversed the orientation of the camera, and the problem is much less severe, but the top is still worse. The only thing I can think of is that something shifts inside the lens. But perhaps it isn't that bad. What do you think? It is noticeable, even in this scaled down version. Maybe I should be happy that at least one of the corners is sharp? Hehe, I just don't know what to expect.

45mm, f/8, 1/60s, iso100, tripod w/2s timer, fairly careful alignment (full size)



PS Thanks for the help!
09-04-2008, 12:09 AM   #13
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I really don't like using newspapers for decentered testing because you have to be so close that your results can strongly be affected by the curved plane of focus as well as a camera that is not perfectly perpendicular to the paper. The fact that when you reversed your camera the top was still soft may prove this point that the camera was not perpendicular to the paper. If it was perpendicular and you had decentering, the bottom would have then been soft. That is why I prefer to use brick walls outside or something similar.

I see no obvious decentering in your first image. In fact I think it is a very nice image.

Last edited by PentaxPoke; 09-04-2008 at 12:15 AM.
09-04-2008, 02:57 PM   #14
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QuoteOriginally posted by PentaxPoke Quote
I really don't like using newspapers for decentered testing because you have to be so close that your results can strongly be affected by the curved plane of focus as well as a camera that is not perfectly perpendicular to the paper. The fact that when you reversed your camera the top was still soft may prove this point that the camera was not perpendicular to the paper. If it was perpendicular and you had decentering, the bottom would have then been soft. That is why I prefer to use brick walls outside or something similar.

I see no obvious decentering in your first image. In fact I think it is a very nice image.
Thanks for the compliment on the first pic

As for the newspaper, I did tilt the camera up and down a little bit to compare, and saw little difference between the shots, making me think that it was not a curved focal plane issue.

However, I did shoot a brick wall. Here are two shots from about the same place shot with a tripod, one with the shutter button "up" and the other in the opposite orientation, button down. Both shots are f/8, 1/250s, iso100, from about 3 metres from the wall. Default lightroom raw conversion.

Click for full size.





In both cases the image is less sharp at the top than the bottom. Worst is the upper right in the first photo. This was also the worst orientation in the newspaper test. It seems fairly repeatable. I am a little disappointed in the corner performance here at f/8. I think it is worse than the kit. But most disturbing to me is that the bottom corners seem fine...

Has anyone seen anything like this before? Orientation-dependent lens performance? I found this dpreview review of the 16-45 and they mention something similar ("portrait format softness"). I wonder if this is "normal" and I should just shoot all portrait shots with the shutter button down, but that's a little annoying.

Thanks for any comments or advice.

Last edited by sewebster; 09-04-2008 at 03:06 PM.
09-04-2008, 03:55 PM   #15
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Yes you have found a problem I clearly see it and IMO 90% for sure you have softness in the corner; however its a common problem with that lens because it extends to zoom out. Here is a quote from dpreview review of that lens go take a read of the whole review if you like -

"One clear issue with our sample was extreme softness at the edge of the frame when shooting at wide angle and in portrait format, especially with the lens angled sharply upwards (perhaps not a common shooting situation, but distinctly noticeable in some architectural shots). We can only speculate that this could be a decentring phenomenon due to the play in the lens barrel when fully extended at 16mm."

So it seems you are not alone and the barrel is prone to movement. I am not sure what to advise but the softness at f8 is not that bad and is fairly deep into the corner.
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