Strange behavior from a sigma 24-70 2.8

[Agnate80700]

Im pretty good with cameras and I like to think I understand my rebel XT back-to-front. However, I come here with the humble understanding I dont know everything.

I had the chance to co-shot a wedding a few days ago, and it was the debut scene for a new sigma 24-70 F2.8 DG Macro.

Upon review of my shots, I'd say about 80% of them were somewhere between "slighty soft" and "downright out of focus" and I cant figure out why. The strange thing is some of the shots came out tack sharp

I've been playing with the lens for some time before this wedding and had noticed that shots seemed to be a bit off, and after this wedding where I shot 200 some shots through it, I definetly dont trust this lens anymore.

Lets say I just want to take a shot of the groom, mostly filling up the frame.This is what I do, and experience: I have my XT set on manual, 2.8, with 580EX speedflash on a flashbracket. The camera is set to use only the center dot for focus aim. The lens side switch is AF, and the ring is set on AF (pushed forward.) I have the XT set on oneshot, and evaluative metering (if either of those even matter.) I push the shutter button down 1/2 way to autofocus, I hear the beep and the red flash, and take the shot. The shot will probably be moderatly soft to very soft.

Another thing to mention/add is that there always seems to be some part of the shot in focus, just not what I aim at (for example, bride in a grove of trees, I aim at the bride and focus, take the shot, and what ends up in focus is a section of trees a few feet behind the bride.

Do I have a falty lens? Is it not metering correctly? Or is that peticular lens just that soft at 2.8?

Whatever the reason, its rather frustrating.

[gus]

I would put a sample of a photo you describe up in the correct forum & the EXIF data for the tech heads to see.

[Eric&Susan]

Well with out an example it is hard to tell. Fell free to post one.

Is the whole thing soft or is part in focus? I ask because with the 2.8 your doing to have a very small DOF. Where did you focus?

Maybe try shooting in a controlled environment at different apperatures to determine if you have a soft lens or not.

Eric

[Agnate80700]

incoming

[Eric&Susan]

I missed the part about the birds. It sounds like your lens is back focusing.
Do a search for a back focus test, print out the chart and follow the directions.

Eric

[Tee Why]

something tells me that it's motion blur.
But without looking at the shot and the exif, it's very hard to tell.

[Agnate80700]

ok this isnt anything i'd put in my portfolio (ha...) but its a good example of whats going on. Not all of them are THIS out of focus, but this is the harsher end.

http://people.emich.edu/zhaslick/stats.JPG
http://people.emich.edu/zhaslick/IMG_2862small.jpg

[gus]

Wrong forum :wxwax ...anyway i'd say shutter way too slow & m'ment happening on your part. Take a shot outside in day light of a tree or something with the shutter at 1/500+ & show us along with the EXIF.

[Agnate80700]

Just browsing through the shots looking at the metadata, I could see how it might be a factor of motion blur, but that shot in peticular is 1/80th at 70mm which should be ok, no? I have a pretty steady hand too.

heres another one. this time, the metadata shows those numbers arnt even close.
http://people.emich.edu/zhaslick/stats2.JPGhttp://people.emich.edu/zhaslick/IMG_2887small.jpg

[Agnate80700]

sorry, wasnt sure what forum to post under. I was just curious as to wether or not it might be an open and shut case of lens crappitude.

[Eric&Susan]

The first one looks like blur/shake.

The second one appears to be in focus on the brick wall and some of the grass just pass the guys. Since it was 1/1250 I would guess you focused on the backgound on accident or your lens backfocus'.

I would set up a little test area, tripod mount and go from there. Try different apps and determine if it is the lens or simply user error.

Also why are your using 2.8? I would have gone at least f4 maybe 5.6 for the group. You still would have had a very useable shutter speed.

Eric

[gus]

The first one looks like blur/shake.

The second one appears to be in focus on the brick wall and some of the grass just pass the guys. Since it was 1/1250 I would guess you focused on the backgound on accident or your lens backfocus'.

I would set up a little test area, tripod mount and go from there. Try different apps and determine if it is the lens or simply user error.

Also why are your using 2.8? I would have gone at least f4 maybe 5.6 for the group. You still would have had a very useable shutter speed.

Eric

'zacly what they said.

[Tee Why]

Can't say about the first one. I would still consider camera shake even though the shutter speed is 1/80 at 70mm.

The second one has the brick wall in focus.

The area that the camera focuses is actually bigger than the AF point visible on the viewfinder, so it the brick wall is near the AF point it will pick that since it's more contrasty than the dudes.

[Agnate80700]

Ok, heres a shot in a controlled setting. The camera is not hand held. Take a look at this one. (I let it be a little bigger so you can get a good look in the difference of detail.)

1st, look at the screen. This is what I focused on. Its Obviously 'soft.'

Now look at the little jumble of cords below the screen. They are in focus, but they are roughly 2-3 inches *behind* the screen.

http://people.emich.edu/zhaslick/stats3.JPG
http://people.emich.edu/zhaslick/IMG_3097.jpg

[Blurmore]

I have never SEEN a case of backfocus this severe, but that is not to say it is not possible. To me it would seem as though the camera is catching focus and confirming something in the background as in focus. Make sure your AF mode (not just drive mode) is in One shot. Short of that I have seen similar though not as severe problems with my 30D and 17-55 f2.8 EF-S, and Ziggy has seen them with his XT and 18-50 SIgma f2.8, and the only thing we came up with is that we were using the center focus point. Which is super touchy and possibly covers a larger area in reality than is illustrated in the viewfinder. The problem is made worse at wide angle. I have solved the problem by using the 'all points active' option on my 17-55 and making focus confirm on more than one point of the subject(s) I want in focus. If it confirms and I don't like the points that are returning confirmation I hit it again to try to get different (more accurate) points to pick up on the subject. Try using all focus points rather than just the center and tell us how it goes.

[Agnate80700]

ok i tried turning it on 'all points' and it seems to work.

Now, its been a long time since i've had it turned onto that, but I seem to remember turning it off because I've missed shots due to it choosing the wrong thing, and I'd have to wait for it to refocus (possibly more than once.)

So it does seem that the center point has trouble... is that the lens? Or the camera? A combination perhaps?

[marklarry28]

I got my sigma 24-70mm 2.8 EX DG a week ago.
I took a few shots yesterday.

I upload 4 pix on my website, I didn't correct white balance, sharp, color and etc.

I open on my RawShooter, and converted then to JPEG. I took these pix aroung 6:30pm and 10:30pm without flash.

http://marcio.smugmug.com/gallery/1809295
:thumb