Is it hard to achieve accurate focus at fast apertures?

I am having a really hard time getting sharp photos from my 50mm sigma f1.4. It seems to be fine from f3.5 and above, but anything less and it front focuses. It is especially bad when the subject is far away from the camera.

But I am not sure if I am asking too much of it. I know that the DOF is really shallow at f2. Plus the focus point is much bigger than it appears in the viewfinder, so maybe it is just focusing on the wrong part of the scene?

Is this acceptable? How do your lenses perfom at f2? Is focusing really difficult and erratic because of the shallow DOF?

I already sent the lens in for calibration and it has come back like this. Which has me second guessing myself. I have a 40d so I cant use microadjustment.

Could be your technique, do you focus then re-compose?
Are you swaying back and forth after a focus lock?
Was your lens calibrated with the camera?
Shooting with with fast lenses is a challenge, technique has a lot to do with success.

At f/2 and 10 ft, you have less than a foot in focus. (sorry, I don't know if you still use these silly measures in Australia. At 3m, DOF is about .25m.) That does not leave much room for error, or for your camera selecting the wrong thing to focus on. I shoot candids around f/4, and even at that, I use center point only and recompose.

If you google, you will find lots of front-focusing tests. I would do one, ideally using a tripod.

I have the exact same problem. You need a new focusing screen ( third party or Canon ), EF-s or something like this.
EDIT: Oops, you mean with the AF ? That wouldn't be normal.

shniks wrote in post #13188677
I am having a really hard time getting sharp photos from my 50mm sigma f1.4. It seems to be fine from f3.5 and above, but anything less and it front focuses. It is especially bad when the subject is far away from the camera.
But I am not sure if I am asking too much of it.

All lenses will not be as sharp at f1.4-2 as it will be at f3.5-4. How do you know the problem is AF?
1. Use Live View 10X manual focus on a tripod focusing on the center of the image using a subject matter with decent contrast in good light. Go to AF, center point only and see if the focus position changes in 10X live view.
2. try different distances, nothing closer than 3m and apertures 1.4-5.6. AF take a picture, then MF using Live View 10X and take a picture.

If you are getting consistent front focus/back focus, especially at the more distant targets you may need to get the lens calibrated. If the focus is soft at wider apertures and there is no significant improvement with MF, then your expectations of sharpness are not too realistic. Stop down and be happy. There is a phenomenon called focus shift where focus point changes as the lens is stopped down to shoot, but it is almost always with f1.2 and faster lenses at closer distances.
Mike K

I think it's not that it's harder to focus with a wide aperture, it's just more critical because, of course, of the shallow DoF. I agree with the live view approach for improved focusing precision.

Thanks for the help everyone.
When I have been testing it I havent been recomposing, so it isnt that. It might be movement because I cant remember where I put my tripod....

I have never used live view before, will definitely give MF with live view a go.
I found a focus chart online and did a quick test without a tripod, yep front focusing by about 10-15 mm. But I wont be 100% sure until I get the camera on a tripod.

If I find it still front focuses depsite the tripod, should I send it back? I feel weird taking it back to get calibrated again, I would have thought they were professionals that knew what they were doing?

I would say that the shallow depth of field has a lot to do with it. Especially at close distances. Here is a really good depth of field calculator.

http://www.dofmaster.c​om/dofjs.html

shniks wrote in post #13188677
I am having a really hard time getting sharp photos from my 50mm sigma f1.4. It seems to be fine from f3.5 and above, but anything less and it front focuses. It is especially bad when the subject is far away from the camera.

Discussing DoF in this situation seems wrong-headed because the farther away the subject is from you the more DoF there is...

I think the problem is as you say, that the area being focused by the focus 'box' on the VF is actually larger than than the box itself....and at greater distances the area being focused will more easily be OUTSIDE the box you see in the VF....

Now, I can't say whether that accounts for it focusing on something CLOSER without seeing a photo, but it's a possiblity to consider nonetheless...

Are you using the center-box only mode....? It may be helpful to use it rather than the other boxes...