Focusing Question

[trahanw]

Ok...I'm trying to figure this Sigma 28-70mm D 2.8 lens out.

Why when shooting at the same focal lentgh, ISO..do I get totally different sharpness. I know the F stop makes a huge difference but I'm really trying to figure out the 2.8 lens. Below are 2 Pics with one at f2.8 out of focus and the other at f4 in focus. Using center point, matrix metering.

When shooting portraits, I'm really having a difficult time with nailing the shot in focus where I want it....being mainly the eyes.

[Daniel Bates]

It could be that the lens is not calibrated, and the deeper DOF at f/4 just moves the subject into the acceptable range of focus. Try shooting a ruler test if you have the time.

[gbux61]

It's interesting that nothing is in focus on the first picture taken at 2.8. You should try shooting the ruler or at the very least try the 2.8 setting again and make sure that you have locked focus. Could it be that your camera is set to allow shutter release even if focus is not obtained?

[trahanw]

Thanks for the help on this. The camera is set to lock focus. Same settings on the pics below. 1st one 2.8; 2nd 4. Focus locked on 3". Thanks again for the help. Wouldn't suprise me if the lens needs calibration.

[Daniel Bates]

Yep, it does appear to be backfocused. It could be the camera, the lens, or some combination of the two.

[trahanw]

Don't believe it's the camera, fairly new and works great with the 18-200vr, 50mm 1.4. Do you know a price estimate or vendor in the DFW area that could repair. Or is this something that would have to be sent back to sigma

[SpeedRacerSlots]

Did you buy the lens new or used?
If new, and within 12mo., Sigma will service it for you for free. Info is on their site at www.sigmaphoto.com
If used, then Sigma will still service it, they will just charge you (after giving you an estimate).
They currently have one of mine in for service after finding out it was front focusing (after getting some L glass for Christmas).

I used a pretty slick focus chart that I will see if I can get posted today from my other computer (dont have the URL, just the PDF).

What is your AFpoint set to? All or just the center?

Interesting though, my boss rented some Sigma lenses (new 28-70 2.8, 50-150 2.8 and 70-200 2.8) After testing them on his 20D and 40D as well as my 300D, XT and XSi... all of the Sigma's in our hands at the time were front focusing. All the Canon and Tamron glass in our hands was all right on the money.

[bondarnes]

Did you do the test on a tripod?

[trahanw]

Good Point Mr Don, tought about this same thing, but I was to much in a hurry.