Back-Button Focus, AF Mode & Focal Point Selection?

[Cortian]

With my Canon 40D I believe I can achieve back-button focus by selecting "3" from C.Fn IV -1:

C.Fn IV -1 Shutter button/AF-ON button
3: AE lock/Metering + AF start
Convenient when you want to focus and meter at different parts of the
picture. Press the <AF-ON> button to meter and autofocus, and press
the shutter button halfway to attain AE lock.

If I understand that correctly: The AF-ON button will lock focus and the shutter button will then set only metering.

Assuming I'm correct: Then the next question is: What auto-focus type would one use? I'm normally set at AI Focus, as per Ken Rockwell's suggestion, but I'm wondering if it shouldn't be one-shot when you're doing back-button focus?

I also have the camera set for single-AF-point, rather than automatic AF point selection, as the latter doesn't appear to DTRT for me. (Though I have found it amusing to watch what it "thinks" I want .)

 

[Overread]

A few thoughts that might help

1) Af basically works by looking for the closest point it can find under all active AF points and then focuses upon that point. Typically its looking for a variation in contrast in the scene to help it lock on, which is why aiming at a pure single colour target (eg a sheet of paper) can sometimes confuse the AF as its not got as much contrast variation to work with.
So if you've got all the AF points active the camera will find the closest thing to itself under those points and focus upon that. So yes you are right, letting the camera totally pick whatever it wants might well not get what you want in focus.
As a result most people use single point AF and, where needed, will move the active point around (although the middle point is normally the best, but a good few newer cameras now have very good periphery points as well).

2) With back button AF you are right, with that setting the AF on button will enable the AF whilst a half press of the shutter will only activate the camera metering. This means if you take your finger off the AF On button the AF won't engage at all; you can change any settings, move the camera around, take as many photos as you want and theAF will not start unless you've got that back button pressed down.

3) As a result of point 2 you can leave your camera in continuous AF mode and use it just like one-shot AF by simply releasing the AF On button when the focus is achieved and then not pressing it again.
The ONLY time this will cause problems is if you are using a speedlite flash and in darker conditions and thus wanting to use the flash's built in AF aid. The flash AF aid won't enable unless the camera AF is set to one-shot (ergo it can't work in any continuous situation with the AF).

4) I tend to leave my cameras in AI Servo mode. In that mode the focusing system keeps focusing the whole time the AF is active (ergo the button is pressed).
With AI Focus mode the camera tries to work out if the subject is moving or not and then either picks AI servo or one shot. The problem here is that scenes change. Eg a bird might be very stationary for a while and then it will move suddenly. If you're in AI Focus the camera has to think and detect that motion before the AF enables - whilst if you're in AI Servo the AF is already running and resampling the focus the whole time.

5) If you have any lenses with manual focusing all the time (ergo where you can move the focusing wheel and change the focus even when the lens switch is set to AF). You can use them just like a fully manual lens by simply not pressing on the AF button on the back; you can set the focus manually and take shots and because the AF isn't enabled on the shutter any more it won't mess up your manual setting.

For that reason I use AF Servo the whole time whilst using backbutton AF. Most of my lenses are manual all the time so I can swap between without having to look for the AF on/off switch on the lens. I can use them in one-shot mode or just keep the AF going the whole time. Most of what I shoot is moving subjects more than static so this benefits me to have the AF more ready for action all the time.

The only time it messes me up (or rather confuses for a while) is when I want the AF assist beam from a flash to help and it won't engage. But this is more just remembering to change a setting and isn't a huge issue - a minor niggle.

 

[Garasaki]

Took me some time to get used to back button auto focus. I still screw up the first couple pics I take when I pick up the camera after a long time away, by forgetting it's back button AF.

It's also very difficult to explain to others, as it's a foreign concept to many. However, after having used back button AF for some time, I'd never go back. I usually don't meter (maybe I should) but I definitely like that the "take a picture" button only takes a picture, and the "focus the camera" button only focuses the camera.

I haven't, but should work on it, used the "set the focus and shoot" approach much.

For what it's worth, I use single point focus and I am always moving the focus point around. I usually compose shots with the subject offset to one side or the other (portraiture or action shots) and I find this to be one of, if not, the most powerful things I can accomplish with my camera.

 

[ac12]

I always use single point focus, so that I can choose what to focus on.

Group/zone lets the camera choose the WRONG subject to focus on, then I have to correct it. So to me, group/zone focus is a specialized mode that the conditions have to be right to use.

 

[Cortian]

Thanks for your comments, guys.

Overread: Your "AI Servo" point makes a lot of sense. After all: When you're back-button focusing, you have a specific point/distance at which you want to focus and you want it now. No point in making the camera have to guess.

Garasaki: Right now I plan to reserve back-button focusing for certain limited scenarios. E.g.: Kristofer Rowe mentions using it when shooting birds, IIRC. So far, at least, for me: Focus and metering have nearly always coincided, so the extra button press would be a hindrance. The one time they didn't, AE Lock was the correct solution. (I was metering from the sky surrounding the subject.)

Once I figure out how what I want the base settings to be when I plan to use BBF, I'll assign those settings to one of C1-C3 user settings.

ac12, Garasaki: Took me all of about two New York Heartbeats to figure out I did not want automatic AF point selection

 

[Garasaki]

I don't think you'll like going back and forth. You want the location of the settings you change on the camera to be second nature for you. Like an extension of your body...something that you have programmed into muscle memory.

If you were driving, you wouldn't want to stop and think "hmmmm, did I set this car up with the brake pedal on the right or left today...." when the semi is heading directly for you...

 

[Cortian]

That had occurred to me.

I'll have to experiment, probably have some things that Don't Go As Planned, to figure out which works best for me.

 

[ac12]

Thanks for your comments, guys.

ac12, Garasaki: Took me all of about two New York Heartbeats to figure out I did not want automatic AF point selection

I only figured it out AFTER a bad shoot.
Family party and what was in focus was chair and tables in FRONT of the subjects. GRRRRR
RTFM, Auto mode = closest subject. BOOO hisss.
I want to choose what to focus on, NOT the camera.

At the high school where I help the yearbook, we were puzzled why a bunch of pix were out of focus.
I finally found the SD card with the original files, and check it.
The indicated focus was zone/area mode, and the camera selected subjects was NOT the photographer desired subject.
After that I reconfigured all the cameras to be single point center AF.
KISS principle.

 

[TCampbell]

With my Canon 40D I believe I can achieve back-button focus by selecting "3" from C.Fn IV -1:

C.Fn IV -1 Shutter button/AF-ON button
3: AE lock/Metering + AF start
Convenient when you want to focus and meter at different parts of the
picture. Press the <AF-ON> button to meter and autofocus, and press
the shutter button halfway to attain AE lock.

If I understand that correctly: The AF-ON button will lock focus and the shutter button will then set only metering.

Assuming I'm correct: Then the next question is: What auto-focus type would one use? I'm normally set at AI Focus, as per Ken Rockwell's suggestion, but I'm wondering if it shouldn't be one-shot when you're doing back-button focus?

I also have the camera set for single-AF-point, rather than automatic AF point selection, as the latter doesn't appear to DTRT for me. (Though I have found it amusing to watch what it "thinks" I want .)

In Canon nomenclature, what matters is what they write “before” vs. “after” the slash “/“.

So this says that if you select that option, then “AE Lock” will be performed by the SHUTTER button, and “Metering + AF Start” will happen with the BACK button.

“AE Lock” is something special. Sometimes a photographer will use a technique called “focus & recompose” which means they focus on their subject (say... using the center AF point) but they don’t actually want their subject in the center... maybe they want them off to one side of the frame. So they lock focus and recompose.

AE Lock is similar... but for metering instead of focus. If you use manual exposure this isn’t a problem. But if you use Program, Shutter priority or Aperture priority, the camera will set the exposure based on the metering. So this allows you to “meter”, then lock the meter reading, then re-compose & shoot (knowing that the camera wont change the exposure settings after you metered.)


Back button focus was created in response mostly to requests from sports photographers...

Say you’re following the action down the playing field (American football, soccer, basketball... doesn’t matter) but what you WANT to track is the player controlling the ball. But as you track, there are other people on the sidelines, game officials, etc. and they enter your frame (we’ll call them “distractions”). You don’t want the camera to shift focus to the “distraction”... you want to maintain focus on the person controlling the ball.

With focus on the shutter button, the act of pressing the shutter button forces focus and now you have a nicely focus shot of the game official (not what you want).

With focus on the back-button, you can continue to track the player controlling the ball and when you notice a distraction entering the frame... just stop pressing the back-button (the camera wont change focus). The focus stays at the last focused distance. You can then resume pressing the back-button as soon as the distraction is out of the frame (or at least not on the focus point you selected to track the action).

For most typical photography, the back-button focus isn’t really necessary.

 

[Overread]

I find that it takes a little while to get used to it, but like a lot of actions once you do it enough times it becomes easy and now I don't even think about it. For me I enjoy the freedom it gives me and the only time I disable it is if I'm handing the camera to someone else who isn't a photographer. Otherwise backbutton AF remains on all the time no matter what I'm shooting.

I've become used to it and since my thumb is there anyway its not a huge reach to find it.

 

[Derrel]

BBF is over-rated in many, many situations. Some cameras have a separate AF-ON button and a separate AF Lock button, and an AE-Lock button as well, so exactly how the camera is user-configured can be quite different, based on different users' custom configurations AND on the exact, specific camera model.

BBF may or may not become a massive pain in the butt on "tall" photos, depending on the camera used, and whether it has a battery grip, or an integral grip, and how the grip is set up, control-wise, for "tall" photos.

One thing to consider: in fast-paced shooting scenarios, using BBF, it is possible to accidentally forget to focus after the first shot! There are many,many situations where it is a far better idea to acquire a brand-new focus, for every, single shutter release! There are lenses and shots and exposure settings where a simple thumb-focus with the back button will ruin shots because the depth of field was too shallow, or the subject moved between the focusing step, and the actual shutter release! As I said at the start: BBF is over-rated, especially on the internet and YouTube, where noobs who have discovered this feature do glowing, gushing write-ups about the feature they've just discovered.