autofocus issue - 7d + 400mm 5.6

[crockny]

Well I got my 7D -- went out for the first time using my 400mm 5.6 lens to shoot birds and had a terrible time with the autofocus. I use autofocus, AI servo, continuous for birds with center spot metering.

Sometimes I could move to another object at the same distance and get it to focus but I missed a lot of shots ...

Any idea what the problem is? It wasn't even that cold - above freezing anyway ...

[crockny]

Sorry about the double thread - I'm having trouble with this site -- anyone else? My posts weren't showing up ...

[inkista]

You need to update your signature.

I wouldn't worry. From what I've read, the 7D's autofocus system is one of the most sophisticated ones out there with a bunch of options that make it one of the tougher ones to master fully. Even experienced 1-series shooters take a little time to get used to all the extra zone options and controls.

For me, AI Servo is one of the worst modes to shoot in, because you're ceding all control of AF to the camera. Use either Single Shot or Servo. It's more work, but then at least you know you're in the right mode at the right time. Don't forget to use the focus limit switch on the lens if appropriate, to speed up AF lock.

And I'd give a good read through the manual on the back-button autofocus options--you've got a lot more options to customize how to operate the AF than on nearly any other Canon dSLR . If you're still half-pressing the shutter button to achieve focus lock, you may want to have a rethink.

[crockny]

Arrgh there are so many autofocus options I can't think -- and the manual doesn't say "for back button autofocus do this" -- I'm reading stuff online to see if I can sort it out ...

As for AI Servo - my only choices are One Shot, AI Focus or AI Servo ... not sure what you mean by using Servo ... I guess I thought since birds are constantly moving their heads that AI Servo would have the best chance of getting a sharp eye ...

I do use the switch on the lens - somewhat annoying actually when things are happening fast.

Your suggestions have been really helpful to me -- thanks!

[crockny]

If I understand the instructions correctly for back button focusing this is how it seems to work on my camera ... I can autofocus with the shutter halfway and then hit AF-On and it won't refocus as long as I have my thumb on the button -- is that the way it's supposed to work?

[ElCapitan]

Back button focusing is something you can apply by going into your camera's custom function. I don't have my camera on me right now, so I do not remember exactly what it is under, but you change a custom function to where pressing the shutter button does absolutely no focusing, but instead pressing the AF-ON button does the focusing.

The nice thing about this is you can focus using the AF-ON button and then shoot you subject without having to worry about focusing and recomposing over and over. Another nice option is when shooting sports or moving subjects using AI Servo, you can hold down the AF-ON button and keep focusing and shooting, but then take your finger off the AF-ON button to stop focusing but keep shooting in case something gets in the way that may mess up your focus

[inkista]

As for AI Servo - my only choices are One Shot, AI Focus or AI Servo ... not sure what you mean by using Servo ... I guess I thought since birds are constantly moving their heads that AI Servo would have the best chance of getting a sharp eye ...

By Servo, I mean AI Servo. My bad. I got the AI Focus and AI Servo mixed up in my head.

The 7D's back-button autofocus functions are much more extensive than on the other Canon dSLRs. Custom Fn. IV is where you want to look. You can individually assign functions to just about every button on the camera. What constitutes back-button autofocus on most of the Canons is different on the 7D. You can not only use the shutter half-press, AF-ON, and * (AE-lock) button for this stuff, but also the DoF preview button, and AF-stop button on the lens.

For example, when I'm birding, the way I have back-button autofocus set up on my 50D is that the half-press on the shutter button is for AE lock, the * (AE-lock) button is for metering and autofocus start. Therefore, by default, my camera has its autofocus locked. When I want the camera to autofocus my lens, I push the * button with my thumb; when I want to lock the focus, I take my thumb off the * button.

That way, I don't have to hold the shutter button half-press to hold focus, and I can tell the camera to keep looking if I don't think it's gotten it yet. This also makes it very easy for me to "touch up" on the AF with full-time manual focus by telling the AF not to touch anything, after I turn the focus ring on the lens. If the shutter half-press is assigned to the AF/start, as soon as I go to take a picture, the AF resets the focus, erasing my "touch up".