Back-button Focus

For my final article dealing with camera set-up and shooting methods I’d like to draw your attention to something called Back Button Focus. If you recall, in my second article on this site I mentioned some basic camera settings to help us acquire sharp images. “Another simple step to take which can net us a greater number of sharp images is to shoot in 1. AI SERVO (Canon Continuous Autofocus-in the main menu under Autofocus modes or Nikon AF-C) with 2. Continuous Shooting (in the main menu under Drive modes).” I suggested both of these because birds and wildlife tend toward motion. AI Servo is a continuous autofocus system that continuously tracks our subject. As long as you are half-pressing the shutter release the camera's autofocus system will continuously track whatever subject is under your selected focus point even if that subject moves. Continuous Shooting drive mode tells the camera to take one shot after another at the highest speed the camera can manage-as long as we hold the shutter button down. This is helpful because in a series of images you are more likely to get one that is sharp and hopefully one where you are pleased with the behavior of your subject-e.g. eyes open or face visible, etc...

It is the factory set shutter release button that is responsible for these functions. Autofocus is activated when the shutter button is pressed halfway and taking the image occurs when the shutter button is depressed fully. Pressing the shutter halfway also activates the camera’s metering system-the part of the camera that evaluates available light and correct exposure.

Back button focus is a way to decouple the factory set autofocus function only from the shutter release button and to move this autofocus function to a different button on the back of the camera-often the * button. (You can designate an alternate button on the back of the camera for autofocus but for purposes of this article I will refer to the * button as the new autofocus button. Usually folks choose the button that is most comfortable for their right thumb to reach easily and reliably.) With this new setup, pressing the shutter release button halfway activates the camera’s meter as before. Pressing the button fully takes the image as before. But now pressing the * button on the back of the camera is what activates autofocus.

Why might we want to make this change? One of several reasons is that your camera now can switch between single shot autofocus and continuous autofocus without changing your autofocus setting in the main menu setup. Why is this helpful when I have indicated that we want to shoot in continuous autofocus mode? This is because birds and wildlife do tend to stay in motion but often unpredictably. If you have a moment where your subject is stationary you can hit the star button once to focus (then remove your finger from the * button), press the shutter button halfway to evaluate your metering and then acquire the image by pressing the shutter button fully. If the light changes while your subject is stationary you can easily adjust exposure (remember, pressing the shutter halfway still activates your meter) without having to also keep setting focus. You have essentially used single shot autofocus while remaining in your camera’s continuous autofocus mode. If your subject does decide to move you are still in Continuous focus mode and ready! You can therefore move easily between stationary and moving subjects without changing anything in your camera setup.

Similarly, autofocus, while tremendously intelligent, is not infallible. If something moves into your frame in front of your subject, say another bird for example, continuous autofocus will pick up on this object and attempt to acquire focus on it-even though it is not your intended subject. If however, you have locked focus on your subject and are shooting in this modified single shot autofocus (within the continuous focus mode) it doesn’t matter if another object crosses in front of it. The camera will not change focus or try to acquire new focus until you depress the * button again. So you are free to meter and take the image without having to refocus on your intended subject.

Also, many photographers feel that separating these functions is simply more ergonomically efficient and allows for more control. Using the shutter button to obtain focus requires that you continually find that perfect pressure balance of holding the shutter halfway down without 1) releasing the button and losing focus entirely, or 2) pressing the button too firmly and taking the shot before you were ready.

Further, in the millisecond it can take to set focus with the shutter release button and then fully press the button to take the image, your subject may have moved causing a blurry photo. By separating the focus and shutter functions, you can acquire focus and get the shot simultaneously by simply pressing both buttons at the same time. If your subject keeps moving you can maintain your finger pressed to the * button, engaging continuous autofocus, while you fully depress the shutter engaging your Continuous shooting drive. You now have easily managed a series of shots throughout the sequence of your subject’s movement.

This arrangement also allows you to easily focus and recompose to improve your composition. You can set focus but then move the subject’s position in the frame by the following sequence: acquire focus by pressing the * button, then remove your finger from the star button, move the camera to change the composition, check your metering by depressing the shutter button halfway, then fully depress the shutter button to capture the image. This sequence locks focus upon hitting the * button and when you then move the camera slightly to recompose your focus does not change. It will not change until you press the * button again. Therefore as long your subject has not moved it will remain in focus even if you move your camera and then press the shutter to meter and acquire the image. If you are shooting in AI Servo with Continuous shooting as recommended, using the factory preset for the shutter button, as soon as you press the shutter halfway to check metering or fully to take the image-the camera will try to refocus-because that is one of the functions of the factory set shutter button.

Setting back button focus is certainly not universal amongst wildlife photographers. Some folks simply do not like the change, especially if they have been shooting for many years and their muscle memory is built around the traditional setup of the shutter release button. It can indeed take time to learn and get comfortable with. I personally find back button focus to be very helpful and use it exclusively. I would like to suggest that if you haven’t experimented with it that you give it a try-it is a relatively easy thing to change in your camera. Most manuals indicate how it is accomplished and the web abounds with tutorials on how to set it up for your brand/model of camera. If you don’t like it, you can easily reset your shutter button to the traditional factory set method. Good luck and see you out there!


© Copyright 2019 Demayne Murphy


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