Photography seems to go through trends. Camera clubs, and places like Flickr have at some point or the other, been overrun with examples of colour popping, HDR (more accurately, excessive tone mapping), and tilt-shifting (more accurately, tilting). But these trends aren’t always limited to image styles.
One thing which seems to have grabbed the imagination of many photographers is the use of a dedicated button on the back of the camera to autofocus, instead of activating it through a half-press of the shutter button, which is something that can be done in higher end cameras. Many photographers are now of the opinion that this is a better way of doing things. I don’t believe back button focus is inferior, but I certainly don’t think it is a better way of doing things either, aside from one caveat mentioned below.
I believe the trend started when a professional sports photographer or two proclaimed their love of this method, and like many things on the Internet, it soon became the “right” thing to do. I remember first learning about it some years ago and, despite being a professional sports photographer myself, being thoroughly flummoxed as to why people seemed to find this a preferable way of doing things. I still am!
As I understand it, the main advantage of decoupling autofocus from the shutter button is so that the photographer can leave the camera in continuous autofocus, yet still be able to focus and recompose (with the subject off centre) when necessary, by simply letting go of the AF-On button on the back of their camera.
So far, so good. Makes plenty of sense.
Except that my cameras have permanently been in continuous autofocus for over a decade, and I have never used the AF-On buttons to focus with, and I have been perfectly capable of locking focus and recomposing through the years. The “magic” simply comes from the AF-Lock button, or in my case, the AF-On button, that is configured to serve as a dedicated AF-Lock button instead.
Basically, I use the button exactly the opposite to how back button focusing works. Instead of having to press a second button to AF, I press a second button to not AF. On this level, which one works and/or is more convenient for you, depends on which you spend more time needing – autofocus, or no autofocus. Personally when I shoot sport, and wildlife, I find that I need autofocus more often than I don’t. Which means that I’d rather press an extra button on the occasions I don’t need it, than press it on the occasions I do.
Take sport. I estimate that generally I spend at least 90% of the time tracking a subject, and maybe 10% of the time (that’s generous) needing to lock focus and recompose a shot. Why press an extra button for 54 minutes in every hour, when I can just press it for 6 minutes?
I’d argue the percentage is slightly less skewed when shooting wildlife, although generally I still spend comfortably more time tracking animals than locking and recomposing. If you are camping a perch in a hide then back button focus could be useful, but if you are shooting active animals, then you need AF more often than not. Even for an animal on a perch, working with long lenses means depth of field is marginal enough that you periodically have to re-determine focus.
Yet it is apparently in sport and wildlife where switching to back button autofocus is most useful!
Other reasons for using back button focus that I have seen are:
- Optical stabilisation systems do not engage when using the back button to autofocus, so you can track a subject and not waste battery having the stabilisation running while you are doing so.
First point is, if you are worried about battery life, then the answer is to buy a spare battery. In the grand scheme of things under normal usage, I’d be surprised if this saved you more than a couple of percentage points of power. If you’re in danger of running out of power, then you really should have a spare battery or two on hand. If you were in a car and running low on fuel, the real solution is to fill it up, rather than switching off the air conditioning.
More importantly, stabilisation systems need time to work, otherwise they can actually add blur to your pictures. So if you are using back button focus, and haven’t triggered stabilisation, and suddenly need to fire the shutter, then you might actually be compromising the sharpness of your image.
Which is possibly the reason that the latest high end Nikon cameras now also activate VR when using the back button to focus (the D4 and D800 onwards). Which then renders the battery saving argument totally moot.
Note for Nikon users: Generally speaking most action photography is done at high shutter speeds. VR should be switched off altogether at high shutter speeds, as you can end up with softness or additional shutter lag, as the Nikon VR element takes a moment to re-centre itself prior to exposure.
- Avoiding accidentally triggering the shutter, when merely trying to track the subject.
With some skittish subjects, this can pose a problem. But while accidents can and do happen, the solution to this one is to be comfortable enough with your camera so that it doesn’t happen. Most “accidental” shutter actuations seem to come with high frame rate cameras and firing off more frames than intended, as opposed to firing when not intending to fire. In other words, the challenge is stopping the shutter in time, rather than accidental activation.
I would also argue that having to juggle pressing two buttons, makes you similarly liable to you accidentally firing the shutter when you only intended to get ready to shoot. Good shot discipline means that you really shouldn’t be stabbing at the shutter in one motion anyway, so being “ready” with a half press is good practice.
Reasons I would suggest that shutter button focus is better:
None of these are particularly major reasons, but I’ll point them out anyway.
- Long days in the field can result in weary shutter fingers. Having to press the AF-On button as well for an extended duration just adds to that.
I’ve actually seen an argument that AF-On allows the index finger to relax during a long day in the field. Yet if you are shooting action, then your index finger really should be “ready” whether you are using back button focus or not. As mentioned above, proper shutter discipline means you should never be stabbing the shutter, and to react quickly your finger should be ready to go anyway. So really, with back button focus all you are doing is… adding your thumb to the list of things that gets tired.
- Your reactions are ever so slightly faster, as you only have to worry about pressing one button, instead of pressing two buttons in a 1-2 sequence (as the AF-On button has to be activated before you can fully depress the shutter button).
With shutter focus and AF-Lock, when you have to activate two buttons, you are already losing a fair bit of time and an extra split second doesn’t generally matter.
However for when you need to point at something quickly and shoot as fast as possible to capture the moment, then I’d rather be as efficient as possible. With shutter button focus, you…  point,  half press the shutter button, and  take the picture. With back button focus you…  point,  press the AF-On button,  half press the shutter button, and  take the picture. We are talking very fine margins, which is why I’m not trying to suggest that back button focus is inferior, but it’s certainly not better in this situation.
- If you are using a camera that doesn’t activate optical stabilisation with the back button, then you can end up with slightly blurred images if you have to react quickly.
Although as stated above, the newer high end Nikons have taken care of this problem, albeit at the same time removing one of the supposed reasons to use back button focusing. Clearly Nikon think that it’s more important not to ruin the image with a dormant VR system, than saving a bit of battery life.
Notice that both the latter points in favour of shutter button focus are concentrated on responding quickly to action… precisely the area where back button focus is supposed to be “better”.
I have found one reason to use back button autofocus – it makes it a lot easier to use autofocus when doing landscape work on a tripod, for example with 10 stop filters. However I prefer to be in manual focus when doing landscape work to begin with, and really this is not an area where back button focus is supposed to be “better”.
Just a cheeky question for you to mull over: If back button focus is so good, why not back button metering?
Ultimately neither technique is wrong, or inherently worse, but I find it perplexing that photographers have become enamoured with back button focusing. It is significantly better than having your camera in AF-Single or One-Shot AF, but I’d never have the camera in those modes to begin with!