Focus Tip: AF-C/AI-Servo and back button focus

I am often asked “can I not leave my camera on AI-Servo (AF-C if you are a Nikon etc)?

The answer is: not a great idea normally. Because you cannot recompose. The moment you try that, taking your focus spot(s) away from your subject, the camera focuses on whatever is behind the subject!

But there is a trick, and I used it today to photograph these amazing insects:

Namely this:

  1. Set your autofocus mode to AI Servo/AF-C.
  2. Select “back button focus” in your camera’s menu (i.e. focus when you press a button on the back of the camera, not whenever you half-press the shutter button).

Now you focus as follows:

  1. Follow the insect, or hockey player, or whatever you are shooting.
  2. While doing this, keep the back button focus pressed, so your camera adjusts to follow the subject’s distance.
  3. But when the butterfly sits and you want to recompose, let go of the back buttoin focus. You can now move the camera to recompose, yet when you shoot, the camera will not adjust its focus.

Done and done!

A quick note about that amazing insect. Nature knows what many beginning photographers do not: you need a catch light in the eye to make it look real and alive. The butterfly’s owl eye has that catch light (the white circle part ion the “pupil”)! Amazing, eh? So learn from nature and always include a catchlight in your portraits.

 

Back button focus: Why and how.

Some photographers (like me, usually) prefer to use “back button focus”. That means that instead of focusing by pressing the shutter button half way down, they focus by pressing a button on the back of the camera. Usually the “AF-ON” or “*” (asterisk) button top right on a Canon camera like the one below. The shutter button now only operates the light meter (in manual mode) or sets the exposure (in automatic and semi-automatic modes.)

On most Nikon cameras, you can use the AE-L/AF-L button on the back to operate focus.

Why would you want to do this?

You might not, or you might. If you prefer this, it is probably for some of the following reasons:

One is that it is easier to separate the process of determining and setting exposure from the process of determining and setting the correct focus distance. These are separate processes that have nothing to do with one another: why combine them into one button? You may well want to focus on the bride’s eyes, while taking an exposure reading off Uncle Fred’s grey suit. This way I set exposure and ignore focus. When I am done and exposure is good, then I go focus where the image should be sharp, and forget exposure (or vice versa). And the two areas do not need to be the same. And usually they are not the same. Unless, of course, your bride has 18% grey eyes.

Also, it is easier to “set and forget” one or both. If, for instance, your distance to the subject does not change, why should you have to re-focus for every shot? There are plenty of situations where this is the case. Like portrait headshots. Focus once, then concentrate on expression and pose, not on focus. By using a separate button you make it possible to do this: focus once, and then forget it until you or the subject changes position.

The third advantage of focusing like this is that it is now easier to make manual adjustments. I focus using autofocus, but sometimes I want to adjust manually. No problem. I can do that, If my camera is set up for back button focus. Beep focus, then overrule that with manual focus.

How?

How do you set it? That depends on the camera. A few examples/notes:

  • On most Canon cameras it’s a Custom Functions (C.Fn) entry or two. For instance, on the Rebel T3i you use C.Fn 9 (option 1 or 3).
  • On the 60D, use C.Fn IV-1 (option 1, 2, 3, or 4)
  • On my 5D Mark 3 it is done in the Custom Controls section of the Quick (“Q”) Menu: set Shutter Button Half-Press to “Metering Start”, and set AF ON to “Metering and AF Start”. I usually also set the AEL button (the asterisk) to “Metering and AF Start”. That way I can use either the asterisk or the AF-ON button. Less chance I miss it!
  • Exactly the same applies to the Canon 7D.
  • On Nikon cameras, set the AF-L/AE-L button to “AF ON”. To do this, go to the pencil menu, find section “controls”, and use “Assign AE-L/AF-L button.”  Within this menu, choose “AF-On.”

Q: Michael, didn’t you say “back focus” was to be avoided?

A: Yes. But this is “back button focus”. A very different thing altogether. “Back focus” means the focus is inaccurate. “Back button focus” means that we are using a button on the back of the camera to start the focus process.

Q: So should I start using back button focus?

A: No, unless you understand all this, know your camera, and are happy to benefit from the advantages I outlines—and only if those are important to you. It’s no big deal either way; I go back and forth between using it and not using it. But now at least you know what it is all about.

 

Live View—a tip

Live View (seeing the photo on the back of the camera instead of through the viewfinder) is not generally recommended. Use the viewfinder!

Except in a few special circumstances.

Namely, you would use Live View when you need accurate manual focusing. This is often the case when shooting macro, or product, or night pictures, when the camera’s autofocus either will not work well, or is not accurate enough.

On many cameras, like on my Canon bodies, you can:

  1. Set focus to “manual” (slide on the lens goes to “M”);
  2. Put the camera on a tripod;
  3. Activate Live View;
  4. Zoom in on the preview (press the “+” loupe symbol, or just the loupe symbol on many Nikon bodies). Repeatedly: on the Canons, when you press it twice, the preview shows (10x” (i.e. 10x magnification);
  5. Now focus accurately by hand;
  6. Now turn off live view.

You are now ready to shoot. Ensure that your subject and camera do not move, and that you do not zoom in or out after focusing (most lenses will lose focus if you do), and especially, that you do not accidentally move the focus ring.

I am writing this as I prefer to do some night sky shots in the next few days, provided I can find a clear sky without too much light pollution.

By the way, my favourite lens for night sky shots is my 35mm f/1.4, which offers pretty much the best combination of:

  • Large real aperture diameter (means more light gathering) and
  • Wider angle (means longer times are possible without creating star trails. To understand why, imagine a telescope: the longer it is, the more the stars will move).

14 seconds at 1600 ISO at f/1.4 should do it. And at that aperture, accurate focusing is essential—which brings us back to where we started: manual focus using Live View.

 

Back to basics

One thing I notice is that often, even working photographers get focus wrong. So let me go back to basics with you today. Focus basics.

There are two focus “settings”: where to focus and how to focus. Today, where to focus.

In the basic “auto” modes, you let the camera choose. You see all focus areas (all three, or all 9, or all 11, or all 42 – depends on your camera) and when you press the shutter halfway, the camera indicates which ones it uses.

These are the ones that have a close object. So you would get this:

But is that clever? Surely you should choose? Perhaps you want this, instead?

That can only be done if you:

  1. Select one focus point (read your camera manual if you need help doing that)
  2. Aim that focus point at the subject you want to have sharp
  3. Press half way down (you will hear a beep, if you haven’t disabled the beep)
  4. While holding the button half way down, recompose, if you like
  5. Press the shutter all the way down to take the image.

This is how you shoot most of the time. Letting the camera choose where to focus is not a great idea. It does not have a brain, and even if it did, it wouldn’t be your brain.  And no, “more focus points” does not mean “greater depth of field, i.e. more focus”! It merely means “camera gets to choose”.

So go set your camera to one focus point pronto!

 

Challenging Shots

Some shots can be a little tricky. Like this one, from Sunday’s shoot:

Tricky why? Because the model is jumping, making it hard to focus.

The solution? You could try AF-C/AI-Servo, i.e. continuous focus. But often in these cases a better solution is this (and that is what I did): pre-focus, then hold that focus while he jumps. So I had the model stand where he would be when I take the shot; then focused there and held that focus while he moved back and jumped; then I shot when he was once again in the same place.

Why is he not blurry? Because he is substantially lit by the flash, which fires faster than 1/1000th second.

Another, different challenge was presented by this shot:

Why? Because the original plan was to light the aquarium in part from behind. But I was shooting TTL, and guess what? I found out on Sunday that the light-driven TTL does not work through an aquarium. Fancy that. You learn something new every day, even when you have been doing it forever.

The solution was to light it from the sides instead, after removing the aquarium side doors. Sometimes you just have to change your plans a little bit!

Enjoy your speedlighting, everyone. I am off to sleep – finally, at 3AM.