I have recently discovered, that there was a custom-fuction on the back of my 2 DSLR that I can change the way my camera auto-focuses. This is refered to as Back Button Auto Focus” – Some cameras have a dedicated AF-ON, ie the 50D, This post is for cameras without that button and how to turn it on.
Normally on most DSLR, the shutter release button, has 2 functions. First is to achieve gather metering information and to initiate auto-focus, (We are just discussing the second activity which is initiate auto-focus.) The second function of the shutter realease is obvious and that is to take the photograph.
The way to activate the first function which is initiate auto-focusing from your lens is to press the shutter release button half way. This will get your camera to initiate the focusing mechanism, ready for the photograph to be taken when the button is fully depressed. (The Second Function)
Now, this post is about giving that 1st role, to the Auto-Exposure Lock button which is located as one of the back buttons for your thumb to press. On a Canon DSLR camera this is button is symbolised as a Star. Now here is the exciting part, why would you want to have this??
First, it gives your shutter button an single role of just taking the image. The advanatages that I find is that this allows be to have a more ‘smoother’ pressing action, than to have the auto-focusing and shutter release in 2 separate movements. In a very perdantic world of photography, to minimise camera shake, this little change makes a big difference.
The Auto-Exposure Lock button now has the single role of all of 1st function that was discussed earlier. It is now responsible for collecting meter readings, auto-focus initiation and also activiating the Image Stabaliser of the lens.
I never have to touch the AF-MF switch on my LENS ever again because now, I have full control on when to I want the auto focus to be activated. If I want it on, I press the AE button, if I don’t, I leave it alone and press only the shutter release button.
This is very efficient and practical during product shoots in a controlled location, ie studio. This is because the conentional way, always requires you to press the shutter button, When you press the shutter button, the camera first initiates the AF mechanism. If the camera is locked in a fixed location, and the product is on a table – not moving. There is no need for the camera to Look for Focus and then Shoot for each frame. You will understand what I mean, when you are annoyed that the camera is always struggling to find focus before you shoot. With this mode, Once the focus is set. It will not change until your press that AE button, hence you can keep snapping away at that shutter release button without the annoying, AF searching prior to shooting.
Efficient for Sporting and AI-Servo mode. Ever been annoyed at shooting a sporting event with AI-Servo and tracking your subject, and all of a sudden the referree or bystander walks into frame which disrupts your AI servo tracking. Well with this mode, if you can foresee the bystander about to run into frame, you can release the AE button, which will deactivate your AF, and once the bystander is out of the way, just press the AE button again to initiate it again, and your camera lens would be exactly where you last left it. This is emphasised with Super-Telephoto lenses, where their focusing covers a larger range, Anywhere from 5m to 50m. You don’t want the lens’s auto focus to be disrupted whilst the fast action is happening.
They are the big advantages to changing to this mode: There is only one draw-back I can think of and that is if you give your camera to another person to shoot, and they are not aware of this uncommon way of AF, they are going to get out of focus shots, everytime. :)
This mode can be activated and reverted back in one of the camera’s custom functions:
For the Canon 30D it is Custom Function 04 Shuter/Ae lock button. You want to switch to option 1, which is AE lock/AF
For the Canon 1D Mark II it is Custom Function 04 Shutter button/AE lock button. You want to switch to option 1: AE lock/AF
to revert back to the default settings, on both cameras just go to option 0.
