A while back I shared a a post on the power of back button focus in wildlife photography and I am planning on recording a video blog on this same topic in the near future. This past weekend I was asked about the benefits of shooting using back button focus and I thought this would make for a great blog post.
1. You can Shoot in AI-Servo (AF-C) and One Shot (AF-S)
Why would you want to do this? More often than not we are working with subjects that will be moving erratically. Whether its a lioness walking across the open plains, or a buffalo grazing in the marshes, the distance between the camera and our subject is almost always changing. In AF-C (or AI-Servo) the camera’s autofocus system is continuously adjusting focus on your behalf in order to render a crisp, in focus image when you trip the shutter.More often than not I’ll be using the central focus point as this is the fastest and most accurate of the focus points available and still leaves some margin around my subject in terms of achieving the desired composition.
What happens when the subject stops or you want to get a bit more creative with your composition?
More often than not you’ll be wanting to compose and leave some negative space to help you tell the story in this scenario. Whilst still being in AI-Servo I have two options to achieve this:
- I can manually adjust my focus point to suite my composition and continue to depress the AF-ON button which engages the autofocus
- I can use the central focus point to achieve focus before releasing the AF-ON button and recomposing
I typically use a combination of both of these methods, especially when a subject is at close range or shooting at large apertures such as F2.8, as even the smallest of changes in distance can effect the accuracy of your focus and resultant depth of field. I will usually select a focus point closest to the centre of the frame which compliments my intended composition before fine tuning my focus points.
In the image below, the red focus block shows where my outermost right-hand focus point was originally engaged in order to achieve focus. The orange dotted line shows the movement of the camera to the left and the final resting place of the same focus point after I had achieved my intended composition. Had a made use of the central focus in this instance, there would have been a good chance that my focus and resultant depth of field would have been off.
2. You can manually focus without needing to adjust settings on your lens
How many times have you been frustrated by your lens “hunting” for focus in low light or busy scenes? Since the Autofocus is only engaged when you press the AF-On button, you can take complete control of the manual focus on most lenses without your camera overriding your focus when you release the shutter.
This is very handy for busy scenes where the camera focusses on obstacles in front of the subject, and in low light where there is not enough contrast for the camera to achieve focus.
3. You save time by not switching between modes
Sometimes the precious time that it takes to pull your head back from the viewfinder and adjust modes means you miss the shot. With back button focus setup you can essentially control all variables without needing to pull your head back from the viewfinder, meaning that you will have a better chance of capturing the action.
4. Other people will think your camera is broken and wont use it!
So this could also be seen as a potential downside but most people who pick up your camera to shoot wont be able to achieve focus. Something to remember when asking someone to take a group shot!