Is my lens backfocusing?

I bought a 70-200mm F4 off a member here a few months back and had my first chance to really use it. For the most part it kept wanting to focus behind the subject. I was using an XT and had it in AI Servo and focus points were set to all and the depth switch was set to 1.3m. Maybe its just me, but I have never had this much problem with focusing on a moving subject with other lens.

Examples:
1.

sethmo wrote in post #8108646
I bought a 70-200mm F4 off a member here a few months back and had my first chance to really use it. For the most part it kept wanting to focus behind the subject. I was using an XT and had it in AI Servo and focus points were set to all and the depth switch was set to 1.3m. Maybe its just me, but I have never had this much problem with focusing on a moving subject with other lens.

Examples:
1.

2.

3.

4. This seemed to be a common theme, the focus usually tended to be on the orange barrel on the left.

5.


Any thoughts? How much does it cost for a lens calibration?

it's really hard to say because you seem to be focussing on random objects. i mean the pictures do look like crap but i'm not sure why .

ed rader

My guess is that your camera is choosing alternate focus points. It doesn't seem very consistent when it misfocuses. When my Tamron 28-75 was front focusing it was happening every time and it was very consistent. If I were you I would try printing a chart and testing it.

I found that real world tests are usually user error. When I sent my Tamron back and got a 2nd one I thought it was misfocusing again but turned out I was messing up.

Post a shot of a focus chart then you will really know.

I forgot about the chart, I will try that tonight and post results. Thanks.

From looking at the photos you attached, it looks like you missed focus (i.e., focused somewhere other than where you intended) in almost every one of them. There are points in each frame that are in focus - they're just not the ones you wanted. Try manually selecting a focus point instead of using all points - the outcome with all points is too unpredictable.

Veemac wrote in post #8108979
From looking at the photos you attached, it looks like you missed focus (i.e., focused somewhere other than where you intended) in almost every one of them. There are points in each frame that are in focus - they're just not the ones you wanted. Try manually selecting a focus point instead of using all points - the outcome with all points is too unpredictable.

I have to agree. looks like the focus was missed on all shots.

Yup......I think in every shot the focus is bang on whichever focus point the camera chose to lock on with. Just that it's not where you wanted it to be. Try centre point focus only perhaps

Matt.

The longer the lens is the more diffiult it is to focus. Often I have trouble focusing with this lens too. As for your shots I see the active focus point (the red one ) is on some unimportant object but as for the focus itslef it does seem to focus around that point. Not sure why - when all points are active the camera is supposed to pick the closest one, which is not the case in the examples. But that definitely is not the lens fault.

Thanks guys. The explanation makes sense. Not all shots turned out like this, I just found it odd that this is the first time that I had this much of an issue focusing on a moving object since owning my XT(Bought in Aug '07).

Thanks again guys!

Take a look back at your photos and see where the active (red) focus point is in each one. In #1 it's on the tire of about the third bike back; in #2 it's on the grass and the truck's left rear tire; in #3 it's on the curb behind the trailer; in #4, on the rider's hand and the asphalt behind him; #5 it's on the asphalt right at the white line, behind the rider.

That's the disadvantage to letting the camera choose the focus point. It really has no idea what you're actually trying to focus on and is just going to make it's best guess - which often isn't what you wanted at all. The rest of your settings look fine as far as I can see (except that you really didn't need AI Servo for the static shots of the parked bikes). I've never shot an XT, but if it allows you to manually select your focus points I'd try that - if you choose the point and put it on what you're trying to focus on, it's pretty much guaranteed you'll get the results you want.

sorry to hijack....what is the chart referred to in this thread?

They all seem to be in focus around the selected focus point. Your subjects are just not at those focus points. Manually select the center focus point and keep your subject pinned, you should have better luck.