Back Button Focussing for Wildlife Photography
For wildlife photography, I advocate using Manual settings for Speed & Aperture and Auto ISO. Take the first 'grab shot' then after viewing the Histogram, review the ISO setting. Too dark, increase ISO and vice-versa.
I set centre point focussing and metering for small birds & beasts, because it is the most sensitive. For larger species, increase the area of focussing / metering. Some cameras allow you to set these up in a 'My Menu' setting. I also set AI Servo, to keep track of moving birds & beasts.
Most professional photographers also use 'Back Button Focussing' - and many amateurs, too. Using a separate button to focus has several advantages and once you have practiced with this 'new way' of doing things, I very much doubt if you will return to the previous 'one button does all' method.
Cameras are set up by default to focus with a half press on the shutter button, then take the photograph by fully depressing the button. When you set your camera up yourself, you can assign a different button (on the back of the camera) to focus and keep the shutter button for taking the shot.
Back Button Focussing Advantages:
1. Focus on the subject using your 'back button', release the back button, turn the camera to recompose and take the photograph using the shutter button. The focus point will not have changed.
2. The bird or beast flies or runs and disappears momentarily behind a bush. Your thumb, on the focussing button at the back of the camera, will learn to automatically release the button, stopping the camera from focussing on the bush. And depressing it again once your subject reappears. You will not have lost focus, meantime.
3. Using the shutter button for one purpose only makes life far more simple and you don't need to refocus if you lift your finger - or change the lens to Manual Focus to recompose. There's much less fatigue than attempting to half-way hold down a shutter button.
4. By using AI Servo (AF-C for Nikon) focusing, you can also have the advantage of 'One-Shot' focussing - for birds & beasts that are not moving. Just press the back button focussing button briefly to focus, then let it go. Providing your subject and you don't move, focussing will be constant. Using this system stops the lens constantly 'hunting' and you'll get sharper shots. You can also wait - for that 'special moment' when the bird or beast does something special - without holding the shutter button half-way down, to retain focus.
5. By using AI-Servo and keeping your back button focus depressed, your camera constantly focuses on fast-flying or running birds and beasts. It's a good system to use with a camera set for continuous shooting. No more leaning on the shutter button; you can selectively shoot the best parts of an unfolding scene, saving your camera buffering shots.
6. Macro shots are easier. Press the 'back button' to get within general range, release and then rock backwards and forwards to get focus exactly where you want it, without worrying about the shutter button interfering. Or, if your lens features full-time manual focus, use that, instead of rocking back & forth. Once your thumb is removed from the back button focus, the lens stops focussing.
I urge you to try out this feature and practise it, until it becomes second nature. If, for any reason you simply can't get on with it, you can reset the settings above, or return your camera to 'factory settings'.
In the Menu, look for “Shutter/AE Lock Button”, then “Metering Start / Meter + AF Start”. If in doubt, consult your camera manual. If it isn't there, your camera is pre-1998 (although some camera models featured this customisation previous to this date).
Some Canon Cameras have a dedicated button for the purpose "AF-ON" and others can use the "AE-Lock" button. Select the most appropriate in the Menu.
Nikon have many and various camera models, with the same number of different ways of setting up Back Button Focussing. Here is one example, for the D7000:
Menu > Custom Settings > Controls > F5 "Assign AE-L / AF-L Button" > "AF-On".
Now set the camera up to take a photograph, even when focus has not been achieved:
Menu > Custom Settings > Autofocus > A1 "AF-C Priority Selection" > Release - and - set AF-S Priority Selection to 'Release' as well.