What's the "AF-ON" button for?

[JClishe]

I recently picked up a 50d. I pretty much have it figured out, but can someone explain what the "AF-ON" button is for and when I would use it?

Thanks!

 

[PhotoXopher]

Takes the place of the shutter release button to focus.

 

[JClishe]

Thanks. So when/why would I use that instead of the shutter release button?

 

[PhotoXopher]

One thing I can think of is that some people use the shutter release for exposure lock only. I'm not a big AF-ON fan so I never really tried to find a use for it lol. That said, some people can't imagine life without it, hopefully they chime in for you.

 

[Big Mike]

My camera doesn't have a dedicated AF-ON button, but I've configured it so that the * button is my AF-ON switch...I use it all the time.

I basically use it as a way to lock the AF and keep it separate from the shutter button. For example, if I'm shooting a subject who is not moving much, I will push the * button to activate AF and use the centre AF spot to focus on them. I can then release the button and recompose the shot (because I may not want the subject in the centre of the image). I can keep pressing or holding the shutter release button and not have to worry that the AF will pick up on something and refocus the image.
Another example is when my subject is moving. I use the AI servo focus mode, so when the subject is moving I hold the * button down so that the focus tracks the subject and constantly focuses on them. My finger is then free to fire shots when I want. If I want to stop focusing at any time, I just release my thumb from the * button.

I now find it extremely annoying when I pick up someone else's camera that focuses with a half press of the shutter button.

 

[Garbz]

So I'm out at night. I want to take a picture of something on the horizon, no AF, it's dark, can't see squat. But far off in the distance there's a light I may be able to get focus from. So I can either:
- Switch lens to autofocus, half depress shutter, switch to manual focus, recompose and shoot.
- Hit AF-On, recompose and shoot.

There are many situations in photography where you don't want to AF every time you depress the shutter button. In which case the AF during shutter press can be disabled in the camera. The end result is much less complicated then playing with the auto focus / manual focus switch.

 

[bigtwinky]

big_mike, I have that * configured to lock the exposure for when I am shooting in AV or TV, not the AF. I figure that if you are shooting with the * as AF lock, then you are almost always shooting in manual?

 

[JClishe]

So I'm out at night. I want to take a picture of something on the horizon, no AF, it's dark, can't see squat. But far off in the distance there's a light I may be able to get focus from. So I can either:
- Switch lens to autofocus, half depress shutter, switch to manual focus, recompose and shoot.
- Hit AF-On, recompose and shoot.

There are many situations in photography where you don't want to AF every time you depress the shutter button. In which case the AF during shutter press can be disabled in the camera. The end result is much less complicated then playing with the auto focus / manual focus switch.

Thanks for the scenario, that helps me to understand. But the thing I still don't understand is when I hit the AF-ON button, the camera focuses but then it loses its focus after a few seconds. So in your nighttime scenario above, are you holding down the AF-ON button while you recompose?

 

[PhotoXopher]

I just set my AF to single instead of continuous or auto, that way it locks the focus and I can recompose.

 

[JClishe]

I just set my AF to single instead of continuous or auto, that way it locks the focus and I can recompose.

That's how I have mine set. When I press the AF-ON button the camera will focus, but it will lose its focus after a few seconds while I'm recomposing. The focus doesn't appear to truly "lock" unless I physically hold down the AF-ON while I'm recomposing. So I'm just wondering if that's what you guys are doing - holding down the AF-ON button while you recompose - or if I have a setting somewhere that I need to adjust so that I don't have to hold mine down.

Here's another question/scenario: Can I use the AF-ON and AE lock (*) buttons individually on the same shot? For example, point at one subject, press the AF-ON button to lock focus, then recompose on another subject, press the AE lock button to lock exposure, and then finally recompose again and take the shot?

 

[KmH]

I just set my AF to single instead of continuous or auto, that way it locks the focus and I can recompose.

That's how I have mine set. When I press the AF-ON button the camera will focus, but it will lose its focus after a few seconds while I'm recomposing. The focus doesn't appear to truly "lock" unless I physically hold down the AF-ON while I'm recomposing. So I'm just wondering if that's what you guys are doing - holding down the AF-ON button while you recompose - or if I have a setting somewhere that I need to adjust so that I don't have to hold mine down.

Here's another question/scenario: Can I use the AF-ON and AE lock (*) buttons individually on the same shot? For example, point at one subject, press the AF-ON button to lock focus, then recompose on another subject, press the AE lock button to lock exposure, and then finally recompose again and take the shot?

What does it say in your camera's users manual?