gvarner Loc: Central Oregon Coast

After watching Steve Perry, my understanding is that you keep the button engaged with a moving subject. With a static subject, you have the option to release the button in order to recompose and put the subject in a different location of the composition, say from center to a 1/3 location. The focus will stay with the subject. And of course there are other variables involving focus mode and focus point settings.

SuperflyTNT Loc: Manassas VA

That is exactly how BBF works. You hold focus button while you press the shutter button to track your subject.

foathog Loc: Greensboro, NC

I use AI Servo all the time. If the subject is static I push the BBF button and release (and reframe, if necessary) and then shoot. If the subject is moving I continue to hold the BBF button (while holding focus point on subject) and shoot. I have no problems.

47greyfox Loc: on the edge of the Colorado front range

I’m one more person repeating what two pages of responders (so far) have written already. Your instructor (?) should reconsider his advice and learn how BBF really works.

1grumpybear

I was very lucky to learn about BBF here at UHH a few years back. I now use it on my D5, D850 and D6. I wish I had known about BBF when I was shooting sports with my D3. When you push and hold down AF-ON you get continuous AF even when your pressing the shutter release. And when you press and release the AF-ON you get a fixed focus point. The secret in making this work on my Nikons was in the menu and turning AF-ON only so the shutter release did not come into play when focusing.

BurghByrd Loc: Pittsburgh

I've found that when mine starts searching it's because I'm closer than the min focus distance for the lens or there isn't enough contrast in the defined focus point for the system to work properly.

frankraney Loc: Clovis, Ca.

No, Will. When you are in back button focus, you control AFC (continuous) and AFS (single). As long as you hold the button your camera will continuously change focus, good for moving objects like birds. When you press and release the camera will hold that focus when you release the button. This has good for say, recomposing after focus.

Hope this helps.

Edit....
Wow. I was watching the news and replying, and then noticed we're on page 2 already... With several good answers.... What helped me understand was actually trying it.... Hold on the button and move the camera and feel or see the focus change. Then do the same thing, press the button to focus the release it and move the camera and notice no change in hocus.

williejoha

I have to agree with Paul.
WJH

amfoto1 Loc: San Jose, Calif. USA

When using BBF there are times when it doesn't matter if you hold the button, times when you shouldn't keep pressing it, and times when you should.

1. It doesn't matter much of the time. There's certainly no need to lift off the button while using One Shot or AFS (i.e., non-continuous focusing). In that mode the AF system will stop and "lock" as soon as focus is achieved. Of course, one of the key reasons for using BBF at all is so you can leave the camera in AI Servo/AFC/Servo AF continuous focusing mode and use that with both moving and stationary subject. That way you don't need to pause shooting to change focus modes. With continuous focusing, so long as you keep your active AF point right on the subject it doesn't matter if they're moving or not.

2. There are also times when you shouldn't keep pressing that button. If you're using a focus and recompose technique with the camera in a continuous focus mode, you certainly do need to lift off the button and stop focus. This technique is typically only used with stationary subjects, but also might be done to pre-focus on a particular spot anticipating a moving subject will come there. In these cases, before you move the AF point off the subject or pre-focus spot, you need to stop pressing the BBF button so that the camera doesn't move focus somewhere else. (Without BBF, you simply can't do focus and recompose or pre-focus techniques while using continuous focus mode. You must first switch to non-continuous mode that various manufacturers call One Shot, Single Shot or AFS. Some cameras have a Focus Lock button instead.)

Another time you might lift off the button and stop focusing is when a moving subject you're tracking moves behind some obstruction. You can maintain focus on them better by stopping the AF momentarily so that the camera doesn't focus onto the obstruction, then when the subject emerges from behind the obstruction, reapply pressure on the button to pick up focus where you left off.

3. There also are times when you definitely should keep pressing the button. For example, many modern zoom lenses are "varifocal". This means they don't maintain focus when you use the zoom to change the focal length. If using one of those, it can be very helpful to use continuous focus and keep pressing that button, so that focus is re-achieved immediately and seamlessly. You don't even notice it. (If you happen to be using One Shot, Single Shot, AFS non-continuous focus mode with a varifocal zoom, every time you zoom the lens you have to consciously re-focus. That means momentarily lifting off the button and then reapplying pressure, regardless whether using BBF or still using the shutter button half-press. This is a lot more involved than simply leaving the camera in continuous focus mode and continuing to press or half-press the button!)

You can check if your zoom is varifocal or not ("parfocal"). Simply set it up on a tripod aimed at some target, use the AF system to acquire focus, then without activating the AF zoom the lens to a different focal length. Be sure to check different focal lengths and test the lens at different distances. Many zooms are only partially parfocal. They maintain focus at moderate distances and greater, but become varifocal at closer distances. The reason zooms are varifocal is because it's a lot more complex and expensive to build a truly parfocal lens (check the cost of video zooms sometime!) Thanks to auto focus, it's often not a problem for zooms to be varifocal.

OTOH, if it's an extremely critical focusing situation such as shallow depth of field with a large aperture, very long telephoto or very close up shot.... even small changes in distance from your breathing or wind may be counteracted by doing continuous focusing with stationary subjects. Some cameras even are designed with this in mind. When a Canon 7D has a Canon macro lens attached, it quadruples the frequency of focus updates in continuous focus mode. This acts sort of like additional form of image stabilization, correcting for even very slight movements on the near/far axis, while standard IS handles any corrections needed for right/left and up/down (assuming the lens being used has IS).

Another time that continuous focusing with BBF can be useful with some cameras is "trap focusing", where the camera is set so that the shutter won't release until focus is achieved. There are variations of this, such as the 1st Image Priority and 2nd & Subsequent Image Priority (n continuous bursts) settings in some Canon cameras. These let the user select whether to prioritize focus or shutter release. Many people might choose focus priority. After all, who needs unfocused images? Well for example, a photojournalist or sports photog might feel that the image needs to be taken at precisely the right moment even if it's not perfectly focused and set shutter release as their priority.

Note that none of the above is for the reason that "expert" cited. No, that's really not a concern. The AF system won't lose focus if you run it continuously while a subject is stationary. At worst, you're going to use more battery power by keeping the AF running when it's not actually needed or accomplishing anything.

photoman022 Loc: Manchester CT USA

In my experience I can categorically say that it is false (at least with my cameras!); I regularly hold the button when shooting static subjects (I don't know why) and I have never "fluffed" a shot.

JIM H Loc: Broad Channel NY

First the camera needs to be set to autofocus. Seconded the autofocus mode needs to be set to Continuous Autofocus mode. Not single or automatic. Third in the custom Autofocus menu, set your AF-C priority selection to Release Not Focus. Now even if the shutter is pressed it will not affect the focus. Last part you need to set whatever button you chose to AF-ON only Now pressing the BBF will focus and continue to focus as long as you pressing the button. Pressing the shutter release half way will only trigger the the auto exposure mod.

cactuspic Loc: Dallas, TX

I agree with those who said it make no difference. But since I don't know what camera you have, I suggest a simple 30 second test with your camera mounted on a tripod. Take a single back button shot of a static subject releasing the back button before you shoot. Then take a picture of your hand. Then take the same image while holding the back button while you shoot. You should see no difference. (be sure to remove focus from the main button when you set up bbf) You should be able to see the results in less time thatn it took most hogs to read and respond to your question.

Kozan Loc: Trenton Tennessee

The statement is accurate in this respect-- If you keep the BBF button engaged, you are telling the camera to Continuously focus. So if you are using spot focusing area, the camera will move to another area and re-focus. So with static objects AND you want to re-compose the shot, push the BBF button once to focus then release the BBF, then re-compose. As the "expert" alluded to, you keep the BBF button engaged if you are focusing on a moving target like a bird in flight or a football runner.

The "expert" had the right idea. He just didn't know how to say it.

Try this around your room. Keep the BBF engaged and move your camera around to different objects. The camera should re-focus as you move your camera. Now, focus on some object then release the BBF then move the camera around. The camera will not try to re-focus.

Of course, you have to set all this up in the MENU system of your camera. I always disconnect the half-press on the shutter button so that doesn't try to focus the camera.

foathog Loc: Greensboro, NC

I think most people make BBF more complicated than it really is. I understood it right from the start. And I ain't that bright. LOL It's basic. When you set up BBF you are taking the focusing function away from the shutter release (which will now trip the shutter and meter the subject). I have a Canon 7DII and I use the AF-ON button as my focus button. It's basically just to the left of the shutter release on the back of the camera. And I have it set on AI SERVO which will constantly focus until you release the button and then it will hold that focus point. Then you can shoot away if you like. (as long as the subject doesn't move). If the subject DOES move you can hold down your 'new' focus button and as long as the focus point is on the subject it will KEEP focus on the subject continuously. Trip the shutter whenever you like. So I feel that AI Servo is basically all I need for still and moving subjects.