"back" auto focus button

[patriciaann]

I read an article on getting sharp images one suggestion was to use the back auto button, which I have tried to do in conjunction with the live view on my Canon 5d mk2. I understand there is some custom functions I need to set can anyone help.. I confess I dont understand this button totally as my lens have auto focus as well, can someone give me a better explanation of how they work together and what the custom functions would be

[arthurking83]

I'm assuming that you're referring to the AF-On button on the upper RH corner where you're thumb naturally falls?
Using this button doesn't really help in getting 'sharp images', but what you end up doing in setting up the camera in using this button, as opposed to the half press of the release trigger, is that you will end up having control of the cameras AF system.
The correct way to set the camera up by using the AF-On button is that you remove the focus ability of the release trigger, so that every time you want to make an exposure, the camera doesn't auto focus.

A brief description of what this set up helps to achieve is that if you like to focus and recompose, you frame the scene so that you focus using the AF-On button briefly(with your thumb), stop focusing by removing pressure off the AF-On button, recompose and then shoot. If you;re not used to this system, it may take a few attempts to teach yourself this new method, and I think it's the most natural way to do it(I have my camera set up this way too).
But it's very important to decouple auto focus from the shutter release as well. That is, you can have the AF system work with both buttons at the same time where both AF-On and the release will both AF if you want. This is not ideal and doesn't really provide any benefit. The real benefit of this method is to separate AF and exposure triggers.

Just had a quick look at the menu system, and it seems to be that you want C.Fn IV-2 and set AF-On to enabled. Hopefully this will remove the AF ability from the exposure trigger.
I don't understand the menu block just before the AF-On menu, but I would leave it at 0. Metering + AF Start

One other setting you may want to keep in mind too, is that this method of AF works best when you use continuous AF drive.. that is for as long as you hold the AF-On button the camera is constantly focusing. it's then your choice to stop focusing as you see fit.

Hopefully someone with experience with a 5D MkII will be along promptly to help you set it up correctly.

[bushbikie]

I probably read the same article! Have a look at this webpage: http://www.usa.canon.com/dlc/control...rticleTypeID=5
Cheers!

[mikspics]

seems to work better in A1 focus than just the shutter button, if you use them both together you can lock focus and recompose while using A1 focus ?
i use it all the time on a 1d2n and 1d3. regards mike.

[etherial]

I actually use mine for the reverse. I find I like the focus control under my shutter finger but there are times when I want it to stop focusing. So I have programmed the AF-On button to be AF-Off. This way I can just hit it when I want the focus to stop and hold where it is. It also makes a good cancel button from menus etc as it jumps the camera back to shooting mode and starts metering, also good if I just want to meter a scene without starting all the focus and IS etc.

[MarkChap]

I love the back button focusing, does take a small amount of getting used to, but once it is familiar you will never look back.

Page 201 of the Manual
Custom Function IV: Operation and Others
Custom Function IV-1 Shutter Button / AF On Button
The first part iss what the Shutter Button does and the second part is what the AF On Button does
0 - Metering and AF Start - Both Buttons do the same thing
1 - Metering and AF Start / AF Stop - The Shutter Button activates metering and AF Start / The AF On button activates AF Stop
2 - Metering Start / Metering and AF Start - This is the setting you want, The shutter button starts you meter / the AF On button starts auto focus and metering - AF is disabled on the shutter button

[rafikicat]

I have my camera set up this way - I find it much more natural and useful.

[dood]

I've used my 30D like that too (Custom function IV-2 if I remember correctly). I got quite used to it but it did confuse others if I handed the camera to someone else. The quick fix was to put it in green square mode.

The 30D doesn't have an AF-on button so it just swops the AF and AE lock functions around. This means that you focus/focus lock with the exposure lock button and expose lock with the shutter button.

Am I right in assuming that because the 5D MkII has both an AF-ON and AE lock button, you can use them for both independently? If so, then option 1 above seems the easiest to get used to if you haven't swopped buttons around ie use camera in normal mode with trigger maintaining focus. Only use AF-ON when you want to focus lock. Exposure lock is unchanged.

[mickello]

I use the back focus button, and yeah it is a hassle trying to explain it to someone when they want to take a photo. What is the green square mode?

[Wobbles]

I use the back focus button, and yeah it is a hassle trying to explain it to someone when they want to take a photo. What is the green square mode?

Mickello,
he means "Full Auto" mode all Canon DSLR's (except 1 Series) have the Green Square on the mode dial representing Full Auto which also (temporarily) overides any settings activated by the user in other modes.

[dulvariprestige]

Another good thing about back button focus is that if you want to manual focus, and your lens has full time manual focusing, you don't have to switch it to manual focus, just focus and hit the shutter button.

[mickello]

Woops, thanks for that... I need to look at my mode dial more often!

[DAdeGroot]

Mark has it one the nail. That's exactly how I have all my cameras set up.

As others have said, it takes a little getting used to, but if you're a control freak you'll never look back.

While in AI-Servo mode, just hold down AF-On to continually focus and press the shutter to fire. In One-Shot, you only need to press AF-On to focus where you want, then press the shutter to fire (i.e. no need to hold AF-On).

Invaluable when using the focus and recompose method, or when tracking birds that keep flitting behind something.

[Namus]

WOW! Huge thanks to Mark and Dave for their posts - as others have said, this feels much easier to use; can't wait to try AI servo mode with this during a sports event

Quick follow-up: is there any way to program the '*' key to be the AF-On button...? Would be easier for me to operate (as I have small hands) but not a biggie... (I have a 40D if that helps)

Thanks again

[agb]

Probably, I have it set that way on my 7D.