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11-28-2012, 07:33 PM   #1
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New DA 15mm Sharpness/Fringing Issue

Hey guys,

So i had the DA 15mm a month a half ago, but it came with a focusing ring being mis-aligned so I sent it back to Adorama. It finally came back in stock after black friday and they sent me a new copy. I was testing it today and I ran into some problems. For instance, I was testing if the corners were all consistent using a tripod/shutter release etc and this setup:

All the images are unedited straight from lightroom:
Here is the center at F4:

I personally think it looks fine, for wide open.
Here is the corner at F4:

Soft, but I think that as expected given the reviews of the lens.
Center at F10:

Very good!

Here is my issue! Corner at F10:



I feel like there is barely any improvement over F4!?!?!? And whats up with purple fringing? Am I doing something wrong here?

Like I said, I used a tripod and shutter release for no camera shake. The camera was not moved or anything between the two shots.


Last edited by oxidized; 11-29-2012 at 05:36 AM.
11-28-2012, 09:30 PM   #2
Ole
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Not all your images show.

Note that a super wide angle lens like this inevitably exhibits field curvature. This means that it will not project a flat surface like your wall perfectly flat onto the sensor. If the center is sharp the corners are not at their best, and vice versa. The lens is designed for field use, i.e. photographing 3D objects.

Macro lenses are designed to map a flat surface onto the (flat) sensor. But you won't find a 15mm macro lens anywhere!

The fringing is be due to the subject being out of focus.

If the corners are equally out of focus your lens is OK. If some corners are visibly better than other corners then some lens elements could be decentered, i.e. the lens is faulty.
11-29-2012, 05:38 AM   #3
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Original Poster
Okay, fixed the images now. I dont know, I knew the lens had soft corners wide open going in, I guess I just didn't expect they were going to look like that even at F10.
11-29-2012, 06:22 AM   #4
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The "Corner at F10" shot looks like lateral CA, owing to the green-on-one-side-purple-on-the-other fringes along
tangential edges only, not along radial edges (or is it the other way round?)

Lateral CA doesn't improve on stopping down. It arises from different magnifications of the different wavelengths.
The upside is that lateral CA is easily fixable in post or in-camera.

Regards,
--Anders.

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