Chromatic aberration / fringing?

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potato
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Chromatic aberration / fringing?

Post by potato » February 23rd, 2013, 12:45 pm

I went to a nearby camera shop to test out a lens I saw in their inventory. It's a Pentax autofocus 50mm macro lens (link).
I was definitely impressed by its ability to focus up close!

Comparing some photos at home though, I'm seeing a lot of cases where there are funny colors on the edges of things.
I realize this is somewhat expected, especially with the aperture open (f/2.8 in these 4 photos), on high contrast edges, etc. but is this amount really normal?
Just wondering if it could be a bad lens. My current lens only goes down to f/3.5 so maybe I'm just not used to having that bigger aperture and therefore more aberration.
IMGP9708-1-s.jpg
IMGP9711-s.jpg
IMGP9719-1-s.jpg
IMGP9721-1-s.jpg
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Steve20050
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Re: Chromatic aberration / fringing?

Post by Steve20050 » February 23rd, 2013, 5:22 pm

That is why most lenses preform best at a medium f stop setting. If you pick up a lens that opens another stop then you should have moved the best fstop too, though there certainly are other factors. I suppose it does often come down to, you get what you pay for as well. An L or aspheric lens will often run more money, but that wide open shot should have noticable less aberration. I sometimes see these for a reasonable price, dependant on elements of the normal lens and what changes were made to incorporate the aspherical element. There are lens tests done for comparisons on many links from consumer reports out there. I would use these for info and comparison as they usually have specs worth looking at before you purchase a lens.

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Waffle Stomper
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Re: Chromatic aberration / fringing?

Post by Waffle Stomper » February 23rd, 2013, 5:43 pm

That seems like an excessive amount of aberration. To put this into perspective for me, are these cropped?
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Re: Chromatic aberration / fringing?

Post by potato » February 23rd, 2013, 5:46 pm

Thanks Steve. I'll look at some consumer reports. I've seen some good reviews for this lens but maybe they were biased somehow.

These photos are all cropped, some more than others... can post the originals later if that helps.
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Waffle Stomper
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Re: Chromatic aberration / fringing?

Post by Waffle Stomper » February 24th, 2013, 7:55 am

Just one uncropped would be helpful to see. Even my top end Canon lenses have aberration at either end of the apertures.

What are you using to process your photos?
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Re: Chromatic aberration / fringing?

Post by potato » February 25th, 2013, 7:08 pm

Here's the full view of the bottom picture for comparison.

I use RawTherapee currently. It does have a defringe option, but it didn't clean it up very satisfactorily.
these 4 pictures haven't really been enhanced. just took the raw and saved as jpg in the most neutral profile.
IMGP9721-s.jpg
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Re: Chromatic aberration / fringing?

Post by jdemott » February 27th, 2013, 9:46 am

CA is caused by the fact that different wavelengths are refracted to differing degrees, so it will always be most noticeable at the edges of an image and will usually show a red fringe along one side of an object and a blue fringe along the other side. Fringing in the center of an image is unlikely to be CA because there is much less refraction. Purple fringing along areas of extreme contrast is usually not a lens issue--I think it is caused by the difficulty of correctly interpreting the color and position of edges when the sensor is recording adjacent pixels as all red or all blue or nearly black (the sensor uses an array of pixels that are sensitive to different wavelengths).

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