Nelson Minar 10:39pm, 7 April 2006
I recently got a Tamron 28-75 f/2.8 and love it. Except one thing. In regions of high contrast I sometimes get some color bleed. For instance if you look at this photo full size, the two dead trees framed against the blue sky show the effect. The edges of the branches are weirdly blue. The big branches are a nice woody dark grey, eight or so pixels wide, with 2 pixel wide blue fringes. The small branches are pure blue fringe.

Where does this color bleeding come from? Is it the lens, the sensor, or the RAW conversion? I've only started noticing it with the Tamron lens, but it may also be the RAW conversion that's doing this. I get the effect even if I use full auto settings in Photoshop / Adobe RAW, so I don't think it's me doing something stupid.

It's not the usual chromatic aberration you see at the edges of a lens; it's something else. Any ideas?
phoneyman [deleted] 11 years ago
It could be the sensor. I know that bleeding is a problem in some sensors, in general (I don't know any specifically.)

Pierre
fotobydave 11 years ago
Why not try a different raw conversion program like rawshooter?
Nelson Minar 11 years ago
Thanks for the suggestion. Same problem in Rawshooter. I guess I can understand it as a sensor limitation. If the resulting JPG has a black bit that's only 2-3 pixels wide, then the sensor itself only has roughly 2-3 pixels of non-blue to work with. That suggests you'll get some color aliasing effects. Now, how to correct for it?
MerlinsMan PRO 11 years ago
Something to try would be DXO (dxo.com/en/), which claims to remove such fringing for specific combinations of lens and camera body. The Tamron 28-75 on EOS 350 is one of the combinations they support. There's a free trial download. It would probably be worth giving it a go.
Marco Wessel Posted 11 years ago. Edited by Marco Wessel (member) 11 years ago
Looks a bit like Chromatic Abberation. Very common phenomenon in lenses.
photosam PRO 11 years ago
If it's Chromatic Abberation, RawShooter automatically corrects it.

Photoshop CS2 has a manual adjustment feature in the RAW converter, but the results don't look as good to my eyes.
Nelson Minar 11 years ago
I spent some time trying out different raw convertors; see this image comparison or my blog post. Long story short, I think the colour fringing is because of how small the detail is; a limitation of colour fidelity in digital cameras. Different raw convertors have different results. DxO seems best for this particular issue.

(It's not what's usually called chromatic aberration. That refers to colour distortion that's strongest at the edges of the image caused by different colours diffracting at different angles. Fancier lenses have special glass to control this; for Canon it's either Fluorite or UD, Tamron calls it "LD (low dispersion)". This kind of chromatic aberration is also correctable in software.)
MerlinsMan PRO 11 years ago
According to this review:
www.nikonians.org/html/resources/software/DxO/index.html
DXO does all this and more.
1600 Squirrels Posted 11 years ago. Edited by 1600 Squirrels (member) 11 years ago
Nelson:

It sounds like your original post describes "purple fringing", which is due to transverse chromatic aberration. You can tell this phenomenon is not entirely due to the sensor, because it occurs on film, and on digital the width of the bleed in pixels varies depending on the lens and aperture used.

Avoiding overexposed skies should help reduce the problem.

What you describe as "chromatic aberration" is lateral chromatic aberration. Here's one reference that explains the two kinds of chromatic aberration: www.vanwalree.com/optics/chromatic.html
Nelson Minar 11 years ago
One more update; when tinkering with Photoshop I just realized that part of the problem is Adobe Raw's "color noise reduction". If I turn that setting down from 25 to 0, much of the blue tint disappears.

I'm still sure this problem is different from purple fringing. The problem occurs in the center of my image and the error is symmetric. But that's a great article on chromatic aberration, 1600 Squirrels. I guess purple fringing is due to lateral (aka transverse) chromatic aberration. I wonder if longitudinal chromatic aberration is playing a role here?

OK, back to being a photographer rather than an optics nerd.
1600 Squirrels 11 years ago
Yeah, sorry, I think I got transverse mixed up with longitudinal - I'm used to the terms lateral and axial. Purple fringing is due to axial chromatic aberration, not lateral chromatic aberration, precisely because it does not vary with distance from image center, but rather with aperture.

As to whether this problem is purple fringing or not, I don't know, as I don't really see much purple fringing in the original photo you posted. I do see sharpening halos that might be exaggerating any demosaicing problems.
Groups Beta