EOS R Back Button Focus

[jayjjj1955]

I use an EOS R. My question is about Back button focus. After selecting a point and with servo focus mode how long will the focus remain on the point selected.? For example, a person on a bike riding towards me. I use BBF on them when they are 50 feet away put wait till they are 10 feet away. Will they remain in focus the entire time? Additionally with the same bike rider if I shot multiple shots as the come toward me will they be in focus. The duration of how long BBF is also unknown. If I BB with Servo mode on a duck as it walks along the shore and then 60 seconds later it flies into the sky and lands in the water. Will it still be in focus after it lands on the water? Great tool but I would like more info about its limitations.

[FloridaDrafter]

BBF works exactly like the "default" half shutter press. Provided you have all of your tracking criteria enabled, it will continue to hold focus as long as you have the button pressed and have kept your subject in the focus area you have selected. Stop pressing the button, and you have to reaquire focus. There are other adjustments you can make, like how long to hold focus before shifting that focus to another subject or handing off focus point duties to the next point(s), but that pretty much sums it up.

The key is to keep the button pressed while tracking in servo.

Newton

[jrhoffman75 · Accepted solution]

Back Button Focus (BBF) and AI Servo focus are two different things.

BBF simply means you will activate camera focusing by pressing the button. People sometimes (not always) remove focus action from the shutter button.

AI Servo uses the camera computer to calculate the speed of a subject. then it uses that calculation to predict where the subject will be when the shutter actuates (i.e. the exposure is made). There is a parameter called shutter lag, which is a delay time between when the shutter button is pushed and when the shutter actuates. Depending on the speed of the subject, if there was no AI Servo, the sublject could be in focus when you press the shutter and have moved out of focus when the exposure is made.

If you are using AI Servo (BBF or not) you must keep the focus point on the subject and half press the shutter or hold in the AF-ON button continuously so the camera keeps focusing and calculating.

"The duration of how long BBF is also unknown." Not true. The duration is as long as you hold in the button. If you just press and release the button it will instantaneously focus and then stop. That technique could be used, for example, in a baseball game. Set the camera to BBF and remove focusing from the shutter button. Press and release BBF to focus on first base (for example). Focus will then be locked at that location. Then just keep camera pointed that that location and press shutter button when the runner comes to the base. In that use you aren't trying to track the runner, just freeze his action at the base.

[jayjjj1955]

VERY Helpful! You actually understood my question and gave good examples in your response.  TY

[Pekelo]

Good explanation about BBF. I frequently use my Canon 1Dx for my sports photography usually with my 70-200 f2.8 is ii. I set up BBF with continuous servo using the Custom Control Menu and the AF-On button does the focusing as expected. Taking a photo or turning the camera off results in a reversion to the default shutter button menu setting although the BBF is still active with the AF-On button??? 

[Waddizzle]

It sounds like you might be using a custom shooting mode and need to update the definition.  

[Pekelo]

Thanks! In a nutshell whatʻs puzzling me is that, in spite of selecting AF-On with metering and AF start and pressing set, the camera still reverts to the default Shutter butt half press metering and AF start. Steps to take ?  "need to update the definition" ???

[wkf94025]

Thanks @jrhoffman75.  Good explanation.  I don't have my R5 handy, or I would find the answer to this myself, but how does one decouple autofocus from the shutter half-press operation?

[jrhoffman75]

This is for my 1D X, but the process would be the same for your camera: