CA problem - expert advice please

PaulC · Joined: 23 Dec 2008 Posts: 2318

I have a friend who is asking about fringing on a defective efs 17-55mm f.28 IS USM. She sent it back to the company for replacement and they now say they have repaired it and want to send it back. Here is the problem:

"I wish it had just been my high expectations. Unfortunately, it was also showing up at the f10+ range - even at 14 and 16. If there's anything white: shirt collars, whiteboard isolations (with no lighting on the white board and in the shade!), and absolutely anything and everything with a sky in it. The worst was when one green leaf overlapped another green leaf, and there was fringing along the edge of the front leaf.

Quite honestly the CA was far far worse than on my crappy 18-55mm kit lens. And so much that it made the lens impractical because post processing to fix the problems was too lengthy.

I'd checked it out with the other photogs because I wasn't sure myself, and there was agreement (based on the images shown) that it was a duff copy."

She wants to know:
A) could a lens like this have something wrong with it that needs a straightforward repair, or does it have to be replaced?
B) If a repairable fault could cause this, what would need to be done to put it right, so she can ask the shop what they did to it and find out if they are just messing about or might really have fixed it.

It costs her a lot in postage so she doesn't want to get it back and find it needs to be sent back yet again.

Thanks

Orio · Expire: 2012-12-04

some image samples?

PaulC · Joined: 23 Dec 2008 Posts: 2318

I'll ask her to provide some.

PaulC · Joined: 23 Dec 2008 Posts: 2318

Here we go:

Sevo · Expire: 2012-12-03

Probably a misaligned/poorly centered group. Has the lens ever been dropped, or banged against another in the camera bag? Modern high end AF lenses are very complex, and must use rather delicate mechanics to keep the AF fast and low power - it is easier to throw them out of alignment than an old brass lens.

poilu · Expire: 2019-08-29

if those are 100% crop, it is quite good for a Canon

PaulC · Joined: 23 Dec 2008 Posts: 2318

They are 100% crop

Nikos · Expire: 2015-01-02

Sevo · Expire: 2012-12-03

It is easy to check whether it is due to a symmetric or asymmetric phenomenon (and force will rarely ever create a symmetric defect) - the CA will generally be about as symmetric as its cause. Only design limitations and assembly flaws, sensor artefacts or a (rare) purely axially offset element will create a strictly radially symmetric CA, i.e. matching juxtaposed seams at each corner.

Orio · Expire: 2012-12-04

visualopsins · Expire: 2025-04-11

I'm no expert, I agree with poilu and Orio, after seeing at a comparison between 18-55 IS and 17-55 IS here (scroll down to middle of page). The 18-55 CA is much worse, but I still see it in 17-55 example. The review also says 'CA in 17-55 is well-controlled'; imho, that means much better controlled than 18-55 (obvious in example), not necessarily in comparison with other lenses. The review also states 'this should be an L-series lens'. Laughing

The lens review also says prone to flare. I wonder if a hood would have helped your friend?

If CA in examples was much worse I would say as Sevo did much more eloquent than me there might be misaligned, incorrect(!), or even a reversed element or group.

Given all that, I think the lens is fine, that the "CA" present in photos is due to lens tendency to flare rather than CA.

When your friend asks repair what was done to the lens to 'fix it', expect to hear 'realigned element/group', 'found improperly seated element', 'tightened auto focus mechanism', or something similar.

PaulC · Joined: 23 Dec 2008 Posts: 2318