trouble with Auto Focus

appreciate anyone who can help this noob with an issue Im having with my Sigma 70-200 2.8 on my 40D as it will occasionally fail to auto focus. shooting manually or AV usually so I can keep it wide open at 2.8 to reduce my depth of field, also shoot centre spot focus point. but sometimes it fails to lock on to whatever Im targeting leading to complete blurr. I can hear it trying to lock on but just wont happen. Maybe Im doing something wrong or could this be a lense issue ? appreciate any thoughts or input you may have.
Cheers.

Under what circumstances does it do it?

If you are too close to your subject, it can't focus if they are inside the minimum focusing distance. If you are trying to focus on a plain surface it will have difficulty focusing too, as it needs a decent contrast (preferably across a line) to work with, so it often works best if you find an edge to focus on.

If you can give an idea of when it does this, we might have a better idea where the problem lies.

Sorry should have included that. no it appears to happen across a range, usually shooting rugby from the sideline,/ halfway with play anywhere across the pitch. - or at least 1st half to mid pitch.
dont usually use the stabiliser but do use a monopod and its lense hood.

I am assuming you in AI servo mode? I would if not, track the target with back * button, and shoot multiple frames.

Yes, if you are shooting sports, you should be using AI servo which won't lock on, it isn't supposed to as it keeps tracking the subject. So, it is constantly refocusing all the time. I don't know what is causing it to miss the focus so that you get a blurred shot though. Are you trying to use servo and focus recompose at the same time? or is your focus point not staying on the target perfectly?

It is blur caused by being out of focus, and not motion blur you are seeing, due to too slow a shutter speed?

A couple of images, with the exif intact would really help us work out what is going wrong here, we are just blindly guessing at the moment.

sandpiper wrote in post #14878126
Yes, if you are shooting sports, you should be using AI servo which won't lock on, it isn't supposed to as it keeps tracking the subject. So, it is constantly refocusing all the time. I don't know what is causing it to miss the focus so that you get a blurred shot though. Are you trying to use servo and focus recompose at the same time? or is your focus point not staying on the target perfectly?

It is blur caused by being out of focus, and not motion blur you are seeing, due to too slow a shutter speed?

A couple of images, with the exif intact would really help us work out what is going wrong here, we are just blindly guessing at the moment.

yes, its definately a focus issue and I do use AI servo, you can hear the lense try to lock on but it just wont. - however I took it to my boys game on Sunday and removed the protector/ filter that was on it and did not have an issue all day. could this pissibly have been the issue ? I dont have any images that I can post right now but if it happens again Ill keep and post for better diagnostic, Thanks for your help Sandpiper.

shanky31 wrote in post #14879003
yes, its definately a focus issue and I do use AI servo, you can hear the lense try to lock on but it just wont. - however I took it to my boys game on Sunday and removed the protector/ filter that was on it and did not have an issue all day. could this pissibly have been the issue ? I dont have any images that I can post right now but if it happens again Ill keep and post for better diagnostic, Thanks for your help Sandpiper.

I'm not sure what you mean by the lens won't "lock on" in AI servo, it isn't supposed to, it should be continuously focusing in that mode. Do you mean that it won't track the subject?

Anyway, it seems that you may have solved the issue. Yes, a cheap filter can screw up focusing sometimes. They can also cause softness issues (even when focus is correct), loss of contrast and lens flare. If you feel you really must use one, get a really good one to avoid such problems.

It would not adjust to focus at all, at some points - others yes? hence the focus was intermittent in any mode. I was a lot happier with the images from Sunday with out the protector, they appeared a lot sharper, so will continue down that path at least for now. Thanks again for your input. - Im learning heaps !

Why put a $20 piece of glass in front of a $1500 piece of glass?

guess I learned the hard way, but I did learn !!

Use your lens hood as a protector and throw the filter away. I have never used a filter and shot through the backstop fence many times with several lenses such as your Sigma, 70-200 Canon, 300 f/2.8 and the 100-400. Never had a problem with focusing either.