Problems with CA

[SwissJon]

I've noticed on my lenses, that when I take sillouettes, I get a fair amount of CA. It's not evenly distributed accross the picture, being wider at the edges than at the centre.

I mainly use Lightroom to fix this, using the Lens Correction tool, and using one of the profiles, and then tweak afterwards in manual.

What I find is that when CA is fixed in the centre, I'm left with quite a lot to the edges. Is there any way to fix this? Or do you think this might be caused by diffraction?

Below is an example of what I mean, if you look at Big Ben, no CA, if you look at the vertial edges on the tower to the left, a blue band on the left hand side, and a red one on the right. If I correct for that, the CA moves to the centre:




Camera Nikon D7000
Exposure 2"
Aperture f/22.0
Focal Length 24 mm
ISO Speed 100
Exposure Bias +5/3 EV
Exposure Program Aperture-priority AE
Lens 24.0-70.0 mm f/2.8

[OsmosisStudios]

That's beyond excessive: i'd almost suspect there's an issue with the lens, especially considering there isnt as bad CA on the right hand side.

[ravncat]

Yeah, I agree, given that i couldn't find many reports of CA issues with the 24-70 and you're stopped down alot for this scene (which makes LoCA completely disappear). I can only imagine that you've got a decentering issue in a lens element somewhere. That could lead to uneven CA (and sharpness) across the frame - check other landscapes during the day with good light at a lower F-stop and see if you can sense that the left edge is less sharp than the right edge when focused on something (like a brick wall) and the lens is parallel to it - that may be able to verify a decentering issue.

For now, I guess, I'd attempt to fix this by going with two conversions - one with the left side CA adjustment and one with the right side CA adjustment - and layer them and blend together to get the best mix you can.

[SwissJon]

Hmm.. The lens is covered by the warranty so I think I might get in touch with Nikon if this continues. First off I'll try what you suggested with the brick wall and take a few more.. Is it possible that the sensor is skew? I know that would do the same on all photos, but I don't photograph buildings in sillouette often enough to notice this phenomenon.

The sun went down just beside Big Ben, between the two smaller towers to the left, so the scene wasn't equally lit, and the tall tower is further away than Big Ben. maybe that helps explain this phenomenon, and it's a 2 second exposure, so perhaps there was some vibration.

I've had more of a play with this picture and it looks a little better now, (curious the preview here doesn't show the updated picture.) Not perfect, but better.

[RichardTaylor]

When you update the pic on Flickr the numbers in the URL do not stay the same (even within pic posts to flickr threads)

[SwissJon]

Interesting..

Anyway, I've updated the original photo.. I'm beginning to think it may have been me being heavy handed in PP.. This one's much better.