Need to get my lens calibrated

So I've had my Sigma 24-70 2.8 for a few weeks now, and have been trying to test it for focusing problems I think I somewhat figured out why some of my pics at 24mm, wide open have been showing up soft. Seems the lens is front focusing at wide end of the zoom. Now I'm not anywhere near technical when it comes to testing a lens' AF performance, but it seems like the lens is fine when I autofocus at 70mm, then just open up the lens to 24mm and take the pic. If I take the autofocus at 24mm, and view 100% crops it show's up noticeably soft. Again this is only what I've been noticing when taking the pics at wide open aperture(f/2.8 ).

Here's some examples....
(first one I autofocused at 70mm then just zoomed out to 24mm and took the shot, second the autofocus was taken at 24mm)

both exif are identical:
f/2.8
1/100
ISO 100
24.0mm
Auto WB

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send it in, I'm sure they'll easily fix that.

This hapens from time to time just send it to sigma,but have a good go with it first.

I tried two of these lenses and they both front focused on the Canon 350D. I'm sure Sigma will sort the issue out for you, but I wasn't impressed with having two lenses front focusing.

John

Does anybody know if Sigma matches return shippiing? Say if I 2 day air it to them, will they 2 day air it back to me?

You focused at 70mm, the zoomed back to 24 and took the shot without refocusing? Absolutely nothing is in focus on the 2nd shot. I thought my 24-70 had issues too and sent it in. It seemed to perform the same however after I upgraded to my 30D the lens has performed perfectly. I don't think they match shipping, but the return is pretty quick.

Mr. Clean wrote in post #2061730
You focused at 70mm, the zoomed back to 24 and took the shot without refocusing? Absolutely nothing is in focus on the 2nd shot. I thought my 24-70 had issues too and sent it in. It seemed to perform the same however after I upgraded to my 30D the lens has performed perfectly. I don't think they match shipping, but the return is pretty quick.

Yes, in the first shot I did not refocus at 24mm.

conflictingALIBI wrote in post #2061835
Yes, in the first shot I did not refocus at 24mm.

I don't think it's your lens, yet. If I focus in at 70mm and zoom out to 24mm, I always refocus.

Mr. Clean wrote in post #2062542
I don't think it's your lens, yet. If I focus in at 70mm and zoom out to 24mm, I always refocus.

Yeah, the 24mm shots with focus taken at 70mm come out MUCH sharper. But after reading a lot of other threads on here, I'm starting to come to a new conclusion that its the Rebel XT. Read some other threads where the 30D and XTi are more compatible with lenses that have a 2.8 aperture. Thus why your 30D works perfectly with the 24-70.

Ok, a little bit of an update and a question for somebody thats more knowledgeable on this than me. The shots taken in the original post was taken with center AF. So I was playing around a little bit and it seems like if I use any of the other AF points, the object I focused on comes out as "acceptable", slightly soft wide open but definitely with some sharpening in post, would come out to good pictures. Could this be the center AF of my XT? Has anybody else ever had this problem? Center AF is kind of important to me since I like to shoot sports.

conflictingALIBI wrote in post #2083751
Ok, a little bit of an update and a question for somebody thats more knowledgeable on this than me. The shots taken in the original post was taken with center AF. So I was playing around a little bit and it seems like if I use any of the other AF points, the object I focused on comes out as "acceptable", slightly soft wide open but definitely with some sharpening in post, would come out to good pictures. Could this be the center AF of my XT? Has anybody else ever had this problem? Center AF is kind of important to me since I like to shoot sports.

when you say soft are talking about the actual resolution or the focus? They are two entirely different things, if they are comming out "soft" when using the other points then i would say its most likel the fact that they are using part of the lens further away from the centre, most lenses do not perform as good around the edges than at the centre. If its a focus problem then it may just be that the non centre focus points had trouble locking onto a target, if the XT works as the 20D does, some are vertical sensitive some horizontal sensitive.You can see this in the viewfinder too, notice how the only true square focus point is the centre, this is the cross type, if i recall this is both sensitive to vertical+horizontal. With the others you will notice they are rectangles, just try focusing on a straight line with one of those points, change the orientation of the camera and be suprised lol

conflictingALIBI wrote in post #2083751
Ok, a little bit of an update and a question for somebody thats more knowledgeable on this than me. The shots taken in the original post was taken with center AF. So I was playing around a little bit and it seems like if I use any of the other AF points, the object I focused on comes out as "acceptable", slightly soft wide open but definitely with some sharpening in post, would come out to good pictures. Could this be the center AF of my XT? Has anybody else ever had this problem? Center AF is kind of important to me since I like to shoot sports.

Your "target" for focussing in the example shots would have been impossible for accurate focussing. The black area in the clock has no contrast for the autofocus to work with. I would suggest two things:

Try your tests again with a better subject - one with contrasty lines in the area of the active AF point(s).

Always focus the camera - either manually or with autofocus - separately at each focal length of a zoom lens. Even the best Canon L zooms don't hold focus perfectly for the whole zoom range. I know that because I own and use three of them (16-35, 24-70, and 70-200 IS).

Just use a book for testing auto focusing would be better. Open the book and place it at 45 degrees relative to you. Focus it at the center of the book and take the picture.

Then, rotate your camera 90 degrees and take another picture.

In this way, you will see if your camera/lens combination is front/back/side focusing.

Baybud, sorry I did mean the resolution was a little soft(understanding edge performance isnt as good as center) but the focus was pretty much spot on when using the other focus points besides the center. And if the center AF is supposed to be a "crosstype" isn't it supposed to have better accuracy? Why are my pictures worse when using center AF?!

Ok, I tried opening a book and placing it at a 45 degree angle, and its obviously front focusing when the camera is in a horizontal orientation. Again, this only seemed to occur when using center AF. When in vertical orientation, the shots seem to be a lot better. Using other AF points, no matter what orientation the camera was in, the shots seemed to come out good!

Using anything BUT the center AF, the shots seem to come out a lot better. Now is this a problem with my lens or my camera?!