Continuous or Single Shot Autofocus?

Focus point selection and focus mode selection on a nikon camera

Hello folks! I got a call this week from an old friend (old as in friends for a long while, not reference to either of our personal tenures). He gets great joy from bird photography and asked for my opinion on focus mode, focus point quantity amongst other minor items because he uses AI Servo (continuous AF) on his Canon camera and determined that his images were not as sharp as he expected.

This is a seriously good question and while I think that I did a decent job in answering him, I thought it might be appropriate for a short piece.

Autofocus Modes

Let’s start with how the two autofocus modes work.

In the simplest user scenario, a half press on the shutter release activates auto focus. We all know this. In Single Shot AF, the half press locks the focus depending on the focus point and holds it without any more AF seeking or self adjusting until the shutter is squeezed or the pressure on the shutter release is removed.

In Continuous AF, absurdly called AF Servo by Canon, so long as a half press is maintained on the shutter release, the autofocus system is continuously searching for what it considers optimum focus and will do so until the shutter is fully pressed or the pressure on the shutter release is removed. This mode, while essential for moving targets may engender some challenges on static subjects as we will see.

How Focusing is Accomplished

There are two predominant focus mechanisms. The most common is called contrast detection. The second is called phase detection. Many cameras use both, contract detection first and phase detection afterwards for very fine adjustments. I confess I am not certain how effective that really is given the fairly sloppy focus helicoids on modern lenses. As has been said about many things, they don’t build ‘em the way they used to. So it goes.

three focus modes on canon. never use AI focus, it’s at best a guess

Whichever method your camera uses is really immaterial in general. Certainly there are folks who love to argue this point, but as I am always honest with you, I follow the science only and really don’t care one way or the other, so long as the image is sharp.

Where we see the subtle potential for micro focus shifts while in continuous autofocus is that the camera is being told to “keep focusing”. Thus so long as the AF system is actuated the camera is continuously seeking what it determines to be best focus. This works well unless we have a stationary subject where we are holding focus for a period of time where the light falling on the subject might change, even very subtly. A change in contrast or change in detected phase will result in ongoing micro adjustment and it is conceivable that the final image storing may be made during a micro movement.

Choosing a focus mode on a wild critter is a challenge. For me, I only use continuous, but approach it using the back button for focus on and off that I will touch on shortly.

Minimum Shutter Speed

Most makers extol their image stabilization capabilities, but the majority of users ignore the first two words of every such claim which are “up to”. Whether stabilization is in the lens or the body shouldn’t matter, in body stabilization data suggests it is more effective, but it only functions in mirrorless bodies. It works by directly reading the image on the sensor and moving the sensor based on gyroscopic data to reduce camera shake. However if there is no camera shake, it may induce some because its job is to counter it. In fairness this happens far more frequently with in lens stabilization than with in body stabilization for myriad reasons that are not important here.

The goal of stabilization is to deal with camera movement that is not deliberate, also known as microshake. When we squeeze a shutter, we create microshake. That is reality. How bad it is depends on the individual. However, image stability is also impacted by other movements. As I have gotten older, I can no longer handhold at the same shutter speeds that I could fifty years ago. This is normal and due to the normal degradation of neuro-muscular fine control. It’s not fixable at this point, so we compensate for it.

If I use my friend’s example, he has a custom setting on his DSLR that locks the shutter speed at 1/2000th of a second, an aperture of f/8 and Auto ISO. His lens setup is a 70-200 with a 1.4x converter. His camera uses a crop sensor.

The lens is commonly at its 200mm setting. Due to the smaller sensor, this delivers to the cropped sensor the same angle of view as a 320mm on a full frame of the same brand. Add in the teleconverter and that delivers an angle of view matching a 448mm lens on a full frame sensor. By our generic standards, we would then say that the minimum handholdable sbutter speed recommended would be 1/500th of a second, for the average person in good health. Averages are both useful and misleading. In the case of a man in his mid-eighties, that might not be a short enough open shutter duration, thus my friend has defined himself to need two more stops of shutter speed to address the potential for microshake.

On inspection, we discover that images made using the same settings using AF-S (single shot) vs AI Servo (continuous AF) are sharper to his eyes. This suggests that the issue is down to microfocus adjustments. That the lens combination is sharp in Single Shot AF suggests that the lens is performing properly.

Potential Resolutions

If AF-S provides more consistent sharp focus than AI Servo aka AF-C, then for static subjects, using AF-S would be optimal. If the subject is moving and you are tracking it visually by keeping your focus point on the subject where you want focus to be, continuous autofocus is your best choice. Many cameras offer “tracking” options. If it works for you, great. I cannot really comment, because it typically requires multiple active focus points and I only ever use one. But there is another potential problem here and that is the number of focus points in use.

Modern cameras can have hundreds of focus points but most people are not aware of how they are programmed to work. In either contrast detect or phase detect, the AF system locks at each focus point and focuses to whichever point it determines by algorithm has the optimal contrast or phase lock AND is closest to the sensor. The algorithm does exactly what it is designed to do, but as a generalization has no idea what is most important to be sharp in the mind of the photographer. Even advanced systems such as face detection, dog detection, bird detection only go as far as the AI algorithm tells it to go. It is doing pattern recognition, not subject recognition and the quality of the recognition is only so good as the sample size used to program the AI. The majority don’t care that most the samples have been stolen without compensation or accreditation from the human creators, but that is a moral problem, not a technical one. In an ideal algorithm, one could say, find the closest eye and focus on it but that’s not really functional at this time. Moreover, it does not change the perfectly functional cameras in use around the globe. My original Minolta SLR built in the 1950s makes tack sharp images when I do my job. Is a newer camera faster? Yes, but if the focus is done well, it doesn’t really matter. Consequently I recommend to my students to treat these kinds of images as if it is 1960 and use the centre focus point only, as it is the most efficient one, and to use it in spot focus mode if available.

Back Button Focus

The application of AF service on a half press of the shutter release is a huge convenience for the masses, but every professional of my awareness disables all focus functions at the shutter release. Instead we all activate the back button focus system, which provides a dedicated on / off focus activation that falls under the thumb of the right hand. By decoupling AF from the shutter release we eliminate the camera doing what it is programmed to do by initiating focus as the shutter release is squeezed. In single shot AF, a press of the back button focus locks focus and when released, the camera holds that focus until the shutter is squeezed or the button is pressed again for a new focus. In continuous focus mode, the cameras is constantly seeking optimally focus while the button is pressed and stops doing so when the button is no longer pressed effectively locking focus.

This ability to lock focus in continuous autofocus is unique to back button focus when the shutter release is decoupled from all focus engagement. You cannot do it if the shutter release activates the focus system.

I teach back button focus, because while I want a specific part of the subject in focus, it is highly improbable that I want that dead center. To quote my friend Rick Sammon, “dead center is deadly”. It’s at minimum boring and uncompelling. By locking the focus in either focus mode, I can then quickly recompose before the squeeze to get the image that I want as correct in camera as I possibly can. Sure I can crop in post and so can you, but doing so means a loss of resolution and everyone spent so darn much money on pixels. I choose not to do so. You do what works for you.

You can learn to activate back button focus and disconnect the shutter release from focus control in the manual for your camera. If you have lost your manual, most makers have them online for free download. The ability to do this dates from at least as far back as the early 2000’s so most cameras in use today that do autofocus can do this.

Wrapping Up

Once we reduce / eliminate human micro-shake as best possible, going to back button focus is the cheapest and fastest way to avoid having the AF system getting involved as you squeeze the shutter release. Yes it is one more step. No the learning curve is neither long nor difficult and done properly it does make a difference.

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NO AI CRAPOLA WAS USED IN THE PRODUCTION OF THIS ARTICLE. THE IMAGE IS LICENSED FROM A HUMAN PHOTOGRAPHER