What could be causing this??

[imrtun]

Hi,

I've been having trouble with my sigma 70-200mm f/2.8 for some time now and am begging to get to the bottom of it all. My lens has spent the best part of 2 months in Japan and they send it back saying it's "within factory spec". I'm not satisfied, infact, I'm furious.

All images appear to have a kind of halo effect round areas of high contrast, I've taken images of the product box to illustrate this. The interesting thing is however, when you back focus the lens by turning the focus ring to the left by about 1mm all the aberations dissapear at the cost of some resolution.

You can either have blurrey auto focus pictures or OOF manual focus pictures.

Basically what I'm trying to find out is if this is normal for a sigma 70-200, if it's a repairable problem, or if it's a problem with my camera.

Any comments will be well recieved.

[Lester Wareham]

Well I notice the blue channel is clipped in the white areas of the image and wonder if this might have something to do with it. Try repeating the experiment with -1 stop of exposure compensation.

[troutbreath]

I'm considering buying a copy of this lens, so I am eager to see how this resolves itself.

[gasrocks]

My first guess is bad exposure. Can you show us another/better example?

[evandavies]

Thats pretty strange...

Could be a lens out of alignment or an element with some kind of residue on it.
But if Sigma has checked it out and sent it back the same what can you do next? Did you send it with example pics?

Does it do this if you get good focus using MF?

What happens when its stopped down?

If you don't get this with your other lens then its probably not the camera.

[imrtun]

It happens on pretty much anything with any degree of contrast, think harsh light, back lit subjects. I think the porblem is always there it's just not noticable all the time. If you MF the same thing happens if you back focus however the ghosted halo and intended image will reach a kind of equilibrium but by that time the picture is out of focus and you lose a fair bit of resolution.

This bird photo was taken under harsh mid day sun, and the bird was white, unsurprisingly the halo effect resulted. I am 100% sure this isn't motion blur as it was at 1/3200 and when you shoot at slower shutter speeds and smaller apertures, like 5.6, the problem goes away.

[pfogle]

looks to me like an element out of alignment - causing bad chromatic aberration. May be wrong though... it'll need repair :(

[imrtun]

Here's a similar image at f/5.6, notice the haloing has almost all gone. This lens spent 2 months in Japan and apparently they tested it and sent it back unchanged saying it's within spec. Make of that what you will, I know I'll be saving for a 70-200L IS now though or maybe I'll test a 120-300 in a shop and make 100% that it doesn't have any problems.

[Curtis N]

To be honest, I've noticed some of the same problem with mine, though maybe not quite as bad. It seems to be worst at f/2.8

Do you have the newer "DG" version of the lens? Are you using a UV filter?

[imrtun]

I'm not using a UV filter and it's DG, I have to say I've kind of lost interest in sigma and thier awful QC, it's just not worth the time and the hassle.

[hudsonch]

One question for IMRTUN: for the sample image of "70-200", is it in the center of the original picture, or a chop of any edges/corners? It appears that the halo effect is directional - more blurring at the top side than at the bottom side. If you have tested with another lens and you are sure that it is not your camera's problem, I would agree with Pfogle - your Sigma has a misaligned lens element.

Sigma said that the lens is "within factory spec" might be plausible. Their spec just allows such tolerances at full aperture.

[imrtun]

Good point hudsonch, it is pretty far in the top right corner, though the problem happens everywhere in the image it's slightly better in the center.

On over cast days it seems to do much better, this one was at 100mm at f/3.2 100% crop