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05-24-2011, 05:05 AM   #1
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DA 15mm corner blur

Hi everyone,

after heading to Tibet with my DA15 I was wondering about strong corner blur especially in bottom left and right. Therefore I shot a wall around the corner to see what you think about my copy. Here is one of the shots where I find the corner blur in the bottom a little ugly.

f/8


Center:


Bottom left:


Bottom right:



Here are the shots of the wall.

AT F/8:
f/8


f/8 Center:


f/8 Bottom left:


f/8 Bottom right:


f/8 Top left:


f/8 Top right:



AT F/11:

f/11


f/11 Center:


f/11 Bottom left:


f/11 Bottom right:


f/11 Top left:


f/11 Top right:



I find the corners in the top to be better than the corners in the bottom. Any advices?

Ben

P.S: For interested one's here are also the F/4 and F/5.6 shots:

AT F/4:
f/4 Overview
f/4 Center
f/4 Bottom left
f/4 Bottom right
f/4 Top left
f/4 Top right

AT F/5.6:
f/5.6 Overview
f/5.6 Center
f/5.6 Bottom left
f/5.6 Bottom right
f/5.6 Top left
f/5.6 Top right


Last edited by Marc Sabatella; 05-24-2011 at 11:31 AM.
05-24-2011, 06:32 AM   #2
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there may be slight optical decentering in your copy. My DA15 Limited is quite even across the frame though the extreme corners of the imaging field are blurred it's just the nature of the beast -
stopping down to f/8~f/11 remedies the problem. This lens has a rather high amount of field curvature as well so take that into consideration when you are working with near-far compositions.

I used to use a 16-45mm f/4 and at 16mm f/5.6 that lens was far worse.
05-24-2011, 11:28 AM   #3
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It's important to realize that DOF is not infinite even at f/8 or f/11, no matter what some DOF calculator says. Those calculators aren't meant to for pixel peeping; DOF is *much* thinner if you need an image sharp at 100%. Also, as mention, the focus "plane" is not actually flat, it's curved, so a brick wall is guaranteed to be soft in corers in it happens to curve in exactly the same way as your lens. Both of these effects will tend be more pronounced with a wider angle lens than a normal or telephoto, because the wider field of view makes it that much more likely the corners won't be in focus when the center is. In fact, it would be almost unheard of.

Bottom line: the corners aren't in focus in any of those shots, and that explains why they are not sharp at 100%.

BTW, I'm editing your post to use IMGWIDE tags for the wider images. Please get in the habit of using these for images that don't fit on screen.
05-24-2011, 12:35 PM   #4
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Distance can really affect a test like this. In the first series, I would strongly suspect distance is the whole problem, since the bottom corners are what, five feet away? The distances change so rapidly from the bottom of this image to the top, that it's very likely that the bottom corner simply wasn't in focus.

With the wall, it looks to me like you were pretty close to the wall (or those bricks were big). With my 16-45, I can get about the same number of bricks in a frame at 16mm, just 3 feet away from the center. But it's more like 6 feet from the corner. Holding the camera at only a slight tilt can easily make a 30% difference between the distance from each corner to the sensor.

I wouldn't condemn the lens based on these images. I might test again with a wall at fifteen feet.

05-25-2011, 06:34 PM   #5
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QuoteOriginally posted by Digitalis Quote
there may be slight optical decentering in your copy.
That's also my guess.

QuoteOriginally posted by Marc Sabatella Quote
Bottom line: the corners aren't in focus in any of those shots, and that explains why they are not sharp at 100%.
I totally agree with you for the first shot! But I don't see it for the brick wall. How can the bottom part of the wall be out of focus? Shot with a tripod, leveled with soft- and hardware, wall seems to be straight...

QuoteOriginally posted by Marc Sabatella Quote
BTW, I'm editing your post to use IMGWIDE tags for the wider images.
Thanks! Didn't know this tag!

QuoteOriginally posted by Just1MoreDave Quote
I might test again with a wall at fifteen feet.
Will check this out within the next days. Maybe at my camping trip to the great wall on this weekend. Very special brick wall

I also did two short test shots out of the window to check for decentering. In the second shot I turned the cam 180. Both shots with same settings. The bottom right corner appears to be much sharper in the 180 shot. I don't know much about decentering. Is it easy fixable by the service?

f/8


f/8 crop bottom right


f/8 180


f/8 180 crop bottom right
05-25-2011, 08:51 PM   #6
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yep looks like there is something not-quite-right with your DA15 Limited. Decentering can be tricky to fix especially in wide angle lenses.
05-26-2011, 12:17 PM   #7
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QuoteOriginally posted by 123ben Quote
I totally agree with you for the first shot! But I don't see it for the brick wall. How can the bottom part of the wall be out of focus? Shot with a tripod, leveled with soft- and hardware, wall seems to be straight...
Exactly - the wall is straight, but the focus "plane" is curved, as I said before. That means the focus along the edges will be in front of the wall unless the wall actually curves toward you as well. Now, one might expect that to affect the top as well as the bottom, so to the extent it doesn't, that might indicate a decentering. I personally didn't see enough difference between the corners in your brick wall tests to be concerned about, though.

QuoteQuote:
I also did two short test shots out of the window to check for decentering. In the second shot I turned the cam 180. Both shots with same settings.
Very clever test! I'm still not convinced there's enough difference to be worth worrying about, though - again, there is no possible way those corners would be in focus anyhow.
05-26-2011, 01:49 PM   #8
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I like that test, it takes care of a lot of the distance concerns.

I was going to say the Great Wall might not be plumb.

QuoteOriginally posted by 123ben Quote
... I don't know much about decentering. Is it easy fixable by the service?
There's a test instrument called an optical colliminator to align each element's optical center with the other elements. But there is not a lot of adjustment for each element, so an element ground substantially off-center will be easier to replace. They may not bother and simply exchange lenses with you.

05-26-2011, 05:06 PM   #9
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QuoteOriginally posted by Marc Sabatella Quote
Exactly - the wall is straight, but the focus "plane" is curved, as I said before. That means the focus along the edges will be in front of the wall unless the wall actually curves toward you as well. Now, one might expect that to affect the top as well as the bottom, so to the extent it doesn't, that might indicate a decentering. I personally didn't see enough difference between the corners in your brick wall tests to be concerned about, though.
Thanks for the explanation! It's much clearer now!

QuoteOriginally posted by Just1MoreDave Quote
I was going to say the Great Wall might not be plumb.
I also thought about it plumbness Therefore I brought it directly to the service here in Bejing. They told me (exactly, they told my girlfriend as my Chinese is very bad and nobody speaks English there) it would take about 1-2 month to fix it and that the service in Shanghai is responsible for this lens. They also said that it would be definitely fixable... But I don't trust that people at the counter very much as they took a picture with flash against a white wall (dist: about 4ft) to test what I've said and showed them at pictures before Fortunately they also have to send it in and after some arguing they told us they will inform us about the results in 1 week and that I can pick it up in at least 2 weeks. So we will see where this goes as they told us they are not allowed to change lenses?!?!
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