I continually switch between a back-button focus method and having autofocus on the shutter button. It all depends on the situation. For all action photos where you need continuous focus all the time, I find it is simpler to keep the autofocus on the shutter button as then I don't have to fuss with the back-button AF-On button or the button to the right of it as I typically assign that to back button too. I do not have any problem with half-pressing the shutter button to force autofocus and then pressing it all the way down to shoot images.

But, for the harlequin ducks and the sandhill crane in Yellowstone during May, I selected back-button. The harlequins mostly swam and dove, but movement was nothing like flying birds. At times they held still while perched on rocks in the Le Hardy Rapids. For swimming birds, I held the back-button down while keeping the active AF point on its head as best I could - not easy when they move erratically and then dive out of sight. With a duck holding still, I put the active AF point on its head, push the AF-On button down, the lens focuses on the head, let up on the AF-on button to lock focus, recompose and shoot keeping the focus on the bird's head.

When I do have the autofocus assigned to the shutter button, I find using my right thumb to move the active AF point around is effective, but often there is no active AF point where the head of the bird is near the edge of the frame like the sandhill crane. Then back-button works well as the active AF point does not need to be on the target when back-button is used properly. Sometimes I also assign the AF-ON button to stop continuous autofocus when it is held down. I am doing that more and more when using my floating blind from Mr. JanGear. There are numerous other angles to back-button, so perhaps it is time for me to write another article on the topic for Nature Photographer - the magazine I have written a column in for more than 20 years. By the way, I also like to use the Touch screen autofocus in live view with my Canon 5D Mark IV and it does not work unless I have autofocus assigned to the shutter button. And it is convenient to assign back-button to two buttons on the rear of the camera that have a different autofocus point array. Then I can select a single AF point or a small cluster of active AF points just by the back-button option I decide to use. www.gerlachnaturephoto.com

Two digits is pretty thin unless you only have one digit...

My thumb has to be somewhere so it is not a problem to have it on the BBF assigned button.

Third, I find I like the shutter button only operating the shutter and metering. If I want to focus and recompose all I have to do is release the BBF button with my thumb. I find a lot of less experienced shooters try to focus and recompose using shutter button focus and end up with a shot focused on something they didn’t intend because when they half press the shutter button it can refocus. To me BBF makes focus and recompose much easier.

To each their own I guess...

Two digits is pretty thin unless you only have one digit...

My thumb has to be somewhere so it is not a problem to have it on the BBF assigned button.

Third, I find I like the shutter button only operating the shutter and metering. If I want to focus and recompose all I have to do is release the BBF button with my thumb. I find a lot of less experienced shooters try to focus and recompose using shutter button focus and end up with a shot focused on something they didn’t intend because when they half press the shutter button it can refocus. To me BBF makes focus and recompose much easier.

To each their own I guess...

Bill, as the consultants like to say It Probably Depends. Depends on the camera body and the focus mode, focus points, how focus is being activated and metering mode and how metering is being started. It also depends if you have an exposure lock button defined and use it or not. I use BBF on my Canons virtually all the time. I generally use matrix metering because my body used most has a very good matrix metering system. I occasionally use spot metering tied to the focus point. I usually use single or a small group of focus points. Metering is activated by half press of the shutter button.

What I have seen happen too many times is people use shutter button to both focus and start metering. This generally works fine for them, but if they focus and recompose they will likely refocus and remeter when they press the shutter to take the shot. This can result in something else being in focus than what they intended depending on the focus mode and number of points used. If you want to meter, focus and recompose you have to use the exposure lock if you have one and then be aware that you can refocus on something else easily. Therefore I prefer to use BBF to control and even lock focus and the shutter button to start metering. I can still use the Exposure Lock button to hold an exposure if I feel the need to do that...which is rare.

Others may have their own preferred way of controlling their camera. But I have seen inexperienced shooters get confused with poor focus too many times when recomposing. The beauty and or frustration of photography today is there are many different ways to try to get the shot you desire.