Uh oh...focusing problem?`

I think I may have a focus problem with my Sigma 24-70. I've read the thread about the focus target, but I don't have a tripod available to be able to do a steady test - would it still behoove me to try?

Second problem - the Sigma is a secondhand lens. If it DOES have a focus problem, how hard is it going to be for me to send it in for repair?

100% crops from .RAW files done with Irfanview and saved as JPG, highest quality. Sigma 24-70 on the left, Canon 70-200 on the right. Both at f/4.0, 1/200 sec, 70mm and focused for each shot.

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I can't say if you have a focus problem or not. There are several focus tests you can download and perform. I would definitely do that first.

As far as returning it, I just sent mine back on Monday and I did have to send the receipt.

When you think you have symptoms like this, several tests need to be made.

First, you want to decide that it is front-focusing or back-focusing. In other words, if you have a perfect focus lock on a perfect target at 100 meters, do you get perfect sharpness at that 100 meters and maybe 95 to 110 meters? Or, does it seem to have perfect sharpness at 110 meters, and maybe from 105 to 120 meters? So, that is one issue to be sorted out. This might be the fault of the lens, or it might be the fault of the camera.

Second, when you do get the focus at 100 meters, is it sharp enough for you. Note that marginal lighting conditions will give you squirrelly results. That's probably a lens-only thing.

Now, if you think you have a lens problem, then you should be able to move it over to another normally operating EOS camera and get very similar results. Then when you move the lens back to your original camera, the symptoms will move back. Or if you think you have a camera body problem, then you should be able to move it to another normally operating EF lens and get very similar results.

Unfortunately, you might be in a position where it is difficult to get much help with swapping these things around.

When I got my first film SLR camera, I was in the military on the 38th parallel, and I couldn't find any support in a tough place like that. After shooting it for about one year, I figured out that the symptoms were probably due to lack of experience on my part.

---Bob Gross---

I don't think it's fair to compare those two images. The Sigma is at max zoom and the Canon is at its shortest focal length. It's normal for zoom lenses to get a bit softer toward the long end of their range. Lack of a tripod may also be a factor.

This doesn't rule out that you have a problem, but it doesn't prove that you do.

Thanks for the replies, everyone.

I didn't know if it would do me any good to try the focus chart test, as I don't have a tripod to put the camera on. I'll download the form and try to set it up as best I can, tonight.

The problem with sending in the lens is that I bought it secondhand off Ebay, so no receipt...

If you have a Sigma lens then Sigma can repair it or recalibrate it if necessary. Of course, it may cost you as much as the lens is worth if there is no warranty.

---Bob Gross---

respectfully suggest you buy a tripod and cable release first. Those two will improve your photos more than expensive glass.

I'd suggest seeing if you can try manually focussing the Sigma on something and seeing how that looks, or try an outdoor shot with good subject depth and see if things look sharp anywhere close to where you were focussing. A fence at a 45-60 deg. angle with a marker at your intended focus point (say the fencepost) would be good. Not the same as the defined tests, but it should give you an indication if you have a focus problem or something else altogether.