Sigma 70-200 f2.8 -- Autofocus is front focusing?

[rkoshy]

Hello guys... this is my first post so excuse any ignorant assumptions...

I got a 300D in the first week of May after almost a year of fighting the digital bug. Before that I was a Minolta SLR owner -- purely amatuer.

I've read this forum for about a good 10 hours and then decided to buy the Sigma 70-200 f2.8 lens... works great, the pictures are amazing...

BUT.. today I did a test but focusing on an envelope sitting on a table, propped up by a glass bowl in the back and a lens cap in front...

Looking at the pics, looks like the autofocus is approximately 1 - 1.5 inches short, i.e. in FRONT of the subject. I can manually focus on the envelope and it comes out great.

Any ideas? Is this even a decent test? (Dining room table with pretty decent lighting.. nothing too bright)

Thanks

[drisley]

Is it a white envelope?
That may be the problem.

Recently I did some tests of my own with my 70-200F4L using one of the printed charts and methods provided here.
Often I would get front focus, sometimes back focus.

BUT, when I tried real world pictures, like my alarm system on my wall (using the numbers on the keypad as reference), it worked perfectly.

Another great test was using an old radiator in my place. It worked fine too. But when I went back to the ruler printout on paper, results were again all over the place.

I think auto focus can have a hard time on low contrast objects like a white envelope, paper, wall. Additionally, the auto focus points extend slightly beyond the focus box shown in the viewfinder. So that can be a problem too.

[KennyG]

You need contrast for the AF to work. A plain light or dark color just won't work. Draw a big black cross on the envelope with a marker pen to create some contrast with the background and re-run the same test.

[robertwgross]

You need contrast for the AF to work. A plain light or dark color just won't work. Draw a big black cross on the envelope with a marker pen to create some contrast with the background and re-run the same test.

The other day somebody did this. They had the standard ruler with little contrast, and then they used a bright red sticky arrow at the zero focus point. I think immediately the camera and lens would focus on it correctly.

---Bob Gross---

[Tom W]

I did something similar - testing the 50 at 1.4 for some stupid reason (perhaps to reassure myself that my sometimes lousy photos were not the fault of the equipment), but I used a golf tee as the point of focus.

This is at f/1.4 - click on the image for a larger version:

http://images.fotopic.net/?id=4420551

[Chris1le]

My Sigma 70-200EX did the exact same thing. It is on it's way back to Ronkonkoma, NY right now. I'll let you all know how it turns out. Maybe Sigma calibrated in a little front focus to compensate for the 10D's previous backfocus problems. :lol: