Backfocus?

JazzyJak

So I've been chugging along learning as much as I can about taking good images. And I was recently lucky enough to get a Sigma 70-200 f/2.8 DG, and it's great, I love it... I shoot a lot of sports with it, basketball lately and notice I have a hard time getting sharp images sometimes. In a poorly lit gym, camera shake is definitely an issue, but I've proven to have pretty steady hands, subject movement probably doesn't help either. But neither of these should be a problem here and so it's my most convincing piece of evidence to date. Can you help me come up with a good way to test this out and decide where the problem is?



I apologize for the large image but it's hard to see the problem in anything smaller. So you can see the focus point was in the green jacket of the girl and the man in the back row is obviously sharply in focus and everyone else seems pretty well blurred. What's up here?

gtc

depth of field
you would have been shooting wide open at f2.8 and that gives you a very thin depth of field and the subjects are either within the DOF or outside it-those outside it will not be in focus.

you were also shooting at a bit of an angle which would put some people in the front within the dof and others outside it-does this make sense?

solution is to either increase your ISO so that you can still get a high enough shutter speed at a smaller aperture,say f4.0 or f5.6 or use a tripod so that you can use a smaller aperture and not have to worry about shutter speed-also try shooting straight on to minimise the dof field problem

JazzyJak

Thanks for the respones gtc.

All that you've said makes perfect sense, but in this case I don't see how any of that could possibly put the "window in the DOF" that far behind the focus point.

Thanks again for responding.

4labs

I just had to send in two lenses and a cmaera for back foucs issues. Set up a shot with three objects farther enough apart so that you know the object you are focusing on shoudl be the only one in focus...You will know right away if you have a problem

JazzyJak

Thanks 4labs I'll give that a try as soon as we get some daylight again.

Harrison

You may also have different results from different lighting. Here at work, we sent both our Nikons in for backfocusing, but they still do it at times under low light. AF gets weird in low light. I switch to manual if there's any doubt! :

Mongrel

Not sure if it affected this particular shot or not, but your exif shows that your AF was set to AI SERVO.

My experience has shown that my 20D AF is not as accurate when shooting static shots in AI SERVO as it is when set to 'one shot' AF.

You may want to check your lenses on a static subject in one-shot AF before making any concrete conclusions.

Bob Bell

Mongrel wrote:

Not sure if it affected this particular shot or not, but your exif shows that your AF was set to AI SERVO.

My experience has shown that my 20D AF is not as accurate when shooting static shots in AI SERVO as it is when set to 'one shot' AF.

You may want to check your lenses on a static subject in one-shot AF before making any concrete conclusions.

Thats a good catch Mongrel. If you hold the shutter button down with AI Servo it constantly adjusts and you can take an image completely out of focus.

Looking at the AF point in focus, it looks fairly 1 color and somewhat consistant lighing, which can fool the AF sensor. Backing the lens down a few mm and moving the center point to her face should be a lot sharper image.

gtc

flat is best for shallow DOF
it looks like front of the people in top far left are in focus,the red head and the person behind-that is your field.I bet your focus point for the the bearded man did not light prior to shooting.

treat shallow DOF like you would a macro shot where the DOF can be wafer thin-like a photocopier the subject must have the sensor orientated so that the subect is flat in relation to it-an an angle will have some of the subject within focus and the rest OOF


e=JazzyJak]Thanks for the respones gtc.

All that you've said makes perfect sense, but in this case I don't see how any of that could possibly put the "window in the DOF" that far behind the focus point.

Thanks again for responding.[/quote]