This post is getting some great traffic and I am glad to see that so many people have been inspired to try this technique. One of the benefits of this setup which I forgot to mention is the fact that it simultaneously allows you to switch into manual focus without having to change the AF/MF switch on your lens. This has come in handy for me in situations where I my subject is not easily picked up by the camera’s AF system or when I want to make subtle adjustments to my focal plane.
Something to keep in mind!
I have also completed the follow up post on how to setup back button focus on your camera here.
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The need for this post arose after a Masterclass Presentation which I ran on “Depth of Field in Wildlife Photography” during which I emphasised the importance of achieving correct focus on your subject. If your focus point is not where it should be then you cant expect the resultant depth of field to fall in the correct plane either.
When I asked the question of who has heard of and uses back button focussing on our Facebook Page, the responses ranged from “I have no idea what everyone is talking about…” to “Back button focussing doubled my keeper rate. I can’t live without it now”.
Personally, I think there are a lot more people who have no idea about back button focus and were to afraid to say, so – lets sort that out!
Let me start off by clarifying that back button focus is not to be confused with back focus or rear focus issues. Back focus is when the camera/lens combination focuses behind the intended target, and back button focusing is a technique which separates the focus and shutter activation of the shutter button.
Normally, when you half depress your shutter button, the camera engages the autofocus and achieves focus on your subject, with a bit more downward pressure on the button the shutter is tripped and the image captured. With back button we are taking the autofocus process away from the shutter button and re-assigning it to another button on the back of the camera. No points for guessing why its referred to as back button focus!
The other important consideration in this whole topic is the choice of focal points. Most wildlife photographers spend the majority of their time using only the central AF point – regardless of camera brand or model. This central focus point is almost always the most accurate and sharpest of all of the focus points. The issue with this is that most compositions don’t work with us focusing in the middle of the frame. Yes you can shoot in One Shot/AF-S and recompose or use the AF Lock feature but how much time do you spend in One Shot/AF-S as a wildlife photographer?
What if I told you that you could stay in AI-Servo/AF-C but still focus and recompose as if you were shooting in One Shot/AF-S?
A potential game changer right?
Picture this, you head out on drive and come across this scene:
The pair stand up and start to move down the river bed towards you, so you immediately set your camera into AI Servo or AF-C so that your autofocus is continuously tracking their movement and compensating accordingly.
The pair suddenly stop and start to mate. From a composition point of view, your central focus point is spot on and for all intensive purposes, you should probably switch back to One-Shot/AF-S as the pair are no longer moving. Not ideal because you don’t want to take your eyes off of the action, and they will in all likelihood continue to move once they are finished mating. You stick with your central focus point and AI Servo/AF-C and wait for the critical moment.
In a fraction of a second, the male raises up and twists to the right of frame. Your focus point no longer rests on the subject and falls on the bush behind the pair.
You are rattling off frames as the camera continuously re-adjusts the focus between each shot.
As the moment comes to a dramatic climax, the male snarls and bares his teeth, your focus point is nowhere near his face.
The final images are soft.
The focus point was not positioned accurately and the resultant depth of field was not enough to render the subjects sharp and completely in focus.
Knowing that back button focus separates the engaging of the autofocus function and the activation of the shutter, this scenario could have played out very differently. Lets look at how it would have worked with back button focus setup.
- The Lions are moving in the river bed >> central focus point, AI Servo/AF-C with thumb on back button to continuously track movement
- The pair stops and starts to mate >> Achieve focus on the male and dial in an aperture which gives a bit of extra depth of field, thumb off back button, finger on shutter to pick shots at crucial moments
- Male twists and turns off of central focus point >> thumb still off back button, focus remains as is, as does resultant depth of field, finger on shutter to pick shots
- Pair get up and walk towards us >> thumb returns to back button, AF engaged, finger on shutter to pick shots
Because I now have the ability to decide when to engage or not to engage the AF mechanism in the camera, I can essentially use the camera in both One Shot/AF-S and AI Servo/AF-C at the same time.
Here’s a more simple example.
A lioness is approaching a herd of wildebeest in the Masai Mara.
I track her using my central focus point in AI Servo/AF-C mode and back button focus setup. As she moves between the long grass I keep my thumb on the back button, forcing the autofocus to continuously track her movement and keep her in focus.
As she gets closer to the herd she stops and stares. From a composition point of view I certainly don’t wait her in the middle of my frame.
Using my thumb I achieve focus on her before releasing the back button (note that I am still in AI Sservo/AF-C mode in case she starts to chase or stalk again) and recomposing with her to the left of frame. I trip the shutter with my index finger.
She inevitably starts to move again and, without having to change camera settings at all, I am ready to place my thumb on the back button, track her, and capture any action that takes place.
There are of course other ways to work this situation:
- Changing to One Shot/AF-S (usually requires you to take your eyes off of your subject and you would need to change back to AI Servo/AF-C when your subject moves again)
- Cycling through your selected focus points (This can be time consuming, confusing, and your other focus points may not be as accurate as your central focus point)
- Holding the AF-L, AE-L button (this works for some people but I am personally not a fan)
Hopefully those of you who joined me for my Masterclass presentation will be sitting there knowing that Back button focusing is superior in this instance as long as I was careful not to change the distance between the camera and the subject (which would throw off the focus) as I was using a shallow depth-of-field.
This is one of a handful of things to keep in mind.
Things to keep in Mind:
- If shooting at a shallow depth of field (eg F2.8), or if you are shooting a long lens in very close to your subject, be wary of re-composing a shot as your focus plane can change dramatically. In those cases, you will want to only use an auto-focus point that is DIRECTLY over the area you want in focus, and then do NOT re-compose your shot.
- Spend time practicing this method as you need muscle memory to do this naturally. You DON’T want to try this out for a wedding unless you have used it for while and feel comfortable with it!
- The camera will now take images even if you have NOT achieved focus. So, don’t assume a shot is in focus just because your camera takes the picture.
- Following on from the above, remember that when someone else grabs your camera to take photos, they will more than likely not know how to engage the autofocus!
So hopefully you have a better understanding of what back button focus is and how it can be used in your own photography. Remember that this is by no-means the only way to do shoot. But it certainly has helped me to improve and streamline the settings and though processes before taking an image.
Tomorrow I will follow up on this post with how you can go about setting back button focus up on your camera!
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