The Sony A9, A7riii and A7iii have all seen the addition of an “AF-on” button. While it is a fully customizable button, many would naturally consider using it for back button autofocus. Back button autofocus is very popular with professional and enthusiast level photographers. For the reasons below, I’ll share why I have opted NOT to use this button for back button autofocus and I’ll share my personal settings.
WHAT IS BACK BUTTON AUTO FOCUS
Every consumer camera is set to the same default: The camera focuses when you press the shutter button. Half-pressing the shutter button initiates the autofocus and then a full press actually takes the photograph with a shutter release. Some photographers see a benefit in separating the autofocus process from the shutter release process.
For example, if taking multiple photographs of a completely still subject, the photographer may not see any need to re-focus after each frame. While focus is fast especially if unchanged since the prior frame, the focus system would still introduce a bit of lag before the shutter release. Further, if the photographer is confident that focus is absolutely precise on the first frame, the photographer may not want to risk re-auto focusing and potentially getting less precise focus.
A more significant benefit of back button autofocus is it gives the dSLR user the ability to switch between continuous (AF-C) and single (AF-S) autofocus without changing camera modes. In AF-S modes, the camera confirms and then locks focus. In AF-C modes, the camera continuously adjusts focus as long as the focus button is held. With back button autofocus and the camera set to AF-C, the focus will continuously adjust as long as the button is being held down. Then, if the photographer wants to lock focus at any point, he just needs to take his finger off the button. Thus, effectively, the photographer is getting AF-S without switching the mode of the camera to AF-S.
While this is a valuable feature in many shooting scenarios, I find it is much more useful on a dSLR than on a modern Sony A9, A7riii or A7iii camera.
DIFFERENT STYLE OF SHOOTING dSLR and Sony Mirrorless
There are several fundamental differences in how a Sony mirrorless operates in comparison to a dSLR, which eliminate the need for back button autofocus and can even make it a drawback.
First we should address why would anyone ever shoot AF-S? In theory, when shooting AF-C, the focus will adjust with any movement. If there is no movement, the focus won’t adjust. So in theory, you don’t even need AF-S.
In practice, when I was shooting a Nikon D750, I almost always used AF-S. This was due to the comparably limited af point coverage on the Nikon D750:
When shooting portraits or landscapes, I often wanted to place focus in a part of the frame that didn’t have an autofocus point. If lining up a portrait by the rule of thirds, an eye often would not be aligned with an autofocus point. Thus, on a dSLR, I absolutely would need to focus and recompose to get proper framing of my subject. This is a tried and true technique of portrait photography.
Here you can see the benefit of back button autofocus. Assume you are taking a portrait of an active subject. While the subject is moving, hold down the back button and keep the subject in the center area of the frame. When the subject freezes and poses, you can lock focus, take your finger off the back button and recompose.
BUT ON SONY MIRRORLESS I NEVER NEED TO FOCUS AND RECOMPOSE
The autofocus points on the Sony A9, Sony A7riii and Sony A7iii are so generously spread across the frame, I never need to focus and recompose. I can use eye-AF to instantly place my focus point over the subject’s eye, out pretty close to the edges of the frame.
As a result of never having to focus and recompose, I can choose to use AF-C all the time, or at least almost all of the time. This makes one of the big benefits of back button autofocus irrelevant. I don’t need the ability to switch quickly between AF-S and AF-C, I don’t need to focus and recompose, and therefore I can keep the camera in AF-C all the time. EXCEPT…
THERE IS A DIFFERENCE BETWEEN DSLR AF-S and SONY MIRRORLESS AF-S
There is a fundamental difference in how AF-S operates in a dSLR compared to a Sony mirrorless. As shown in the diagram above, the Sony system utilizes phase detect points and contrast detect points. The phase detect points are much better at measuring movement while the contrast detect points are used for more precision and accuracy.
dSLRs don’t use contrast detect in their main viewfinder focus system. Whether using AF-S or AF-C, the camera uses the phase detect system to obtain autofocus. Thus, when a dSLR photographer uses back button autofocus with AF-C, when they remove their finger from the back button, it has the exact same effect as shooting in AF-S mode.
For the Sony shooter, AF-S and AF-C work differently. The AF-C system primarily utilizes the phase detect AF points. When using AF-S, the Sony camera does a contrast detect focus confirmation at the end of the focus process. If you test it yourself on your Sony camera, you will notice that moment of hesitation in AF-S mode which doesn’t exist in AF-C mode. During this moment, the camera is using the contrast detect system to confirm the precision of the focus. Therefore, if I kept the camera in AF-C mode and used back button autofocus, I wouldn’t get true AF-S simply by taking my finger off the button — I wouldn’t get the focus confirmation.
As I said above, I don’t need to focus and recompose on Sony. But if I want AF-S for extra precision, then I want TRUE AF-S. I want that contrast detect confirmation. I don’t want back button quasi AF-S.
So first and foremost, back button autofocus would not give me much benefit. I don’t need it to quickly focus and recompose. If I want real AF-S mode, then I need to switch the mode of the camera anyway. I can’t just save time by using back button autofocus.
So many focus modes and so few customizable buttons
The Sony A9, Sony A7riii and Sony A7iii have plenty of buttons that can be customized but they still have more potential features than buttons. On most traditional cameras, you would only need one button set up for autofocus. A Sony photographer needs at least 2 buttons because 1 of those buttons should be used for eye-AF. The Sony A9, A7riii and Sony A7iii also allow further customized autofocus buttons, and I recommend setting up an instant reaction button.
If I was to also add a traditional back button autofocus button, then I would need to set aside 3 buttons for focus acquisition. Considering all the other things I want to use buttons for, that is throwing away a lot of real estate on focus acquisition.
Given that I wouldn’t really benefit from back button autofocus, I see no reason to lose a button to the feature. Instead, I’m perfectly happy to continue to use the shutter button for my traditional autofocus acquisition. (And also have buttons for eye-AF and instant reaction, that can override my shutter button autofocus).
As promised, my personal settings:
Control wheel: I don’t personally use it but I’ve seen others use it for ISO. It allows adjusting ISO with one dial turn instead of button push + dial turn
Custom buttons 1 & 2: These two buttons are located in front of the shutter button, an ideal location for being able to pick your focus mode and focus area.
Custom button 3: Metering is a function without a labelled button on the current generation of Sony mirrorless cameras. It is a function that photographers should be able to quickly access and move between spot and matrix metering.
Custom button 4: If you want to be able to quickly check critical focus, the ability to magnify in the EVF is a great feature.
Mult-Slc center btn: This is the thumb stick which sits comfortably on the back of the camera. While you will naturally use the thumb stick to move the focus point, pressing the thumb stick inward acts is a customizable button. By default, it is set to re-center the focus point. But I program mine as INSTANT REACTION button. Essentially, I use the thumb stick to move a single focus point but I can hold down the button and get instant wide-AF. See how to program the instant reaction button here.
Center Button: Finder/Monitor selection. Allowing me to manually change between the LCD and the EVF.
Left/Right Buttons: These are the default labelled settings, Drive mode and ISO. Since I need those buttons and they are already labelled on the camera, I’d just confuse myself if I moved them.
Down button: White balance. I’m surprised the Sony cameras do not have a labelled button for white balance. Auto white balance works very well in natural light but can struggle in artificial and mixed light. Fortunately, it’s easy to program a custom white balance but you do need easy access to the WB menu.
AEL Button: While I prefer to leave labelled buttons with their labelled features, I realize I never actually use “AEL” (Exposure lock).
UPDATE 5/16/2018: As shown above, I had been using this button for APS-C-mode. But it led to a problem. The placement of the AEL button led to it sometimes getting pressed by accident. Then I’d be shooting in 1.5 crop mode, without even realizing it, throwing away a ton of resolution. So now I’m using the button to solve another problem:
One annoyance of the Sony system is that while the EVF previews exposure normally, the exposure preview ceases when you use a flash. I shoot often with off-camera flash. I end up turning off the flash, in order to best judge the exposure, set it manually, and then turn back on the flash. With the flash (or trigger) on, you just get a bright viewfinder, no ability to judge the ambient exposure. Photographer Robert Hall found a great workaround, explained in this video. I now set by AEL button to “Shot Result Preview.” This allows me to get an instant preview of the ambient light exposure even with flash, without having to turn off and on the flash. When using a flash or trigger for off-camera flash, I try to have the background just a bit darker than my subject. So I can use this setting to judge my background and ambient light, knowing my flash will properly illuminate my subject.
AF-On Button: If you read this post, you know I don’t use back button AF. And we absolutely need an Eye-AF button. Since I do a lot of portrait shooting, I am making the most use out of that well placed AF-on button. More on eye-AF here.
Focus Hold Button: This button is NOT located on the camera. This is a reference to the focus hold button on many Sony lenses. This is an ideal spot for eye-AF, as your hand should already be comfortably supporting the lens, with your finger in just the right position. So why do I also have an eye-AF button on the camera? Because unfortunately, not every portrait lens has a focus hold button. (For example, the wonderful Sony 55mm F/1.8 does not have the button). Additionally, for lenses with only 1 focus hold button, I find it well positioned when the camera is in landscape orientation but poor in portrait orientation. Thus, it is helpful to me to have eye-af buttons on the lens and on the camera body.
With the great degree of customizability and with a great deal of functionality added to the current generation of Sony mirrorless, I find that Sony has finally caught up to traditional top dSLRs in terms of simple ergonomics and usability. The Sony A7iii is probably the best camera on the market weighing price and quality. Yes, there are cheaper cameras but they are way behind in IQ, performance and features. There are better cameras but they are much much more expensive and don’t blow the A7iii away in any category. The A7iii is priced at $2000 and is probably the best camera you can buy that is under $3000.
