Sigma 17-70mm soft auto focus

[Espike]

Hey everybody,

So I've had my T2i for a few months now and I am pretty comfortable with it. Recently, I acquired a Sigma 17-70mm f/2.8-4.5 through trade. I figured it would be a good replacement for my kit lens since I was noticing a lot of CA plus the Sigma has a longer zoom. I've been using it exclusively for about a month to really get to know it, and so far, I am not impressed. I was noticing that most of my photos had a soft feel to them. In some areas, white objects would almost seem to glow. Today, I figured I'd do a little test. I put the camera on a tripod and focused on a small fake shrub. As you can see, the auto focus seems to be a little off. When I manual focused it, it was better but still not as sharp as I'd like it to be. I have also included shots of the same subject taken with the same body but with a 50mm f/1.8 and the kit lens. All pics were shot at f/5.6, ISO 100, and at max focal length, AWB, Standard picture style. The AF point was set to the center point and was aimed at the same spot for each photo (which I have marked out for you on the first pic). I also took shots (not attached) with a Rebel XSi using the Sigma lens and got the same results. What, if anything can I do to fix this lens?

Manual Focus Sigma

Auto focus Sigma

Auto focus fifty

Auto focus kit

 

[sovietdoc]

1. Go outside when the sun is out
2. Tripod + remote shutter release, if you dont have remote, use 10 second timer and don't breathe
3. Use the point focusing mode and use center af only.
try focusing on a flat object, by flat I mean perpendicular to you lens, so what you're shooting is on one pane.
Try shooting at various apertures, usually if you stop the lens down a few stops it will be sharper

If you want to do this a more proper way, you can try downloading a focusing sheet from google, it will help you do a more precise test. I don't know if your camera is capable of remote capture through Canon's EOS utility but if it is, it will help to use it.

Look up AF microadjustment on google, although you're not trying to do it with your lenses, it might help you setup a better test to determine if your lens is missing focus or not. Some micro adjustment guides also talk about how to test the lenses with Canon OES Utility, so that might be useful.

 

[Espike]

I'll give it a shot today after work. I would do it at lunch but I don't bring my tripod with me to work. Also, about how far away should I be from the subject for a test like this, or does it not matter as long as it's consistent?

 

[Espike]

So, I wasn't able to do any testing today since after work, I went to my son's karate promotion ceremony, but I'll be bringing my tripod with me to work so that I can test some stuff out during lunch (in full sun). I'll bring my kit lens and my 50 just for comparison. I'm thinking a book would be a good subject to test on?

 

[Espike]

I did manage to take some pics of a book outside in full day light but the pics came out pretty clear. The only thing that I don't like is that there is a soft glow around bright spots. Is this just a characteristic of the lens? I didn't include those shots, but I can if anyone is interested in seeing what I'm talking about. I did however, print off a focus chart and do a test in my office with a light pointed right at the chart. both pics are shot at ISO 100, with the aperture wide open. The Sigma was at 70mm, and the 50 is a prime... It looks like the sigma is focusing a bit past the center and the fifty seems to be just a bit in front of the center line. Also, whats up with the purple and green tones in the fifty pic?