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.


Autofocus point

An important point about autofocus (and forgive the pun).

You have a number of AF points. One in the middle, and then 2 more, or 8 more, or 40 more: whatever. Lots, on my 1Dx:

These “points” are sensors that look for focus by looking at lines and sharpening them. But did you know that some points are sensitive only to horizontal or vertical lines? That’s why, when you select one AF point, sometimes you cannot focus even though you are pointing the AF point at a nice lined surface.

The centre AF point is always sensitive to both horizontal and vertical lines. But many other AF points are sensitive to only horizontal, or only vertical lines.

What’s more, this even depends on:

  • The mode you are in
  • Auto or manual AF point selection
  • The minimum f-number of your lens. Some points are points (sensitive to both) when used with an f/2.8 lens,. but horizontal only, or vertical only, when used with an f/5.6 lens.

So, my strong advice: Read up on how your camera does it. And if in doubt, use the centre AF point, since it is likely more sensitive and a cross-type sensor.



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.


Another tip

One more beginners’ tip today for my readers. (The term “my readers”, it occurs to me, makes me sound a little like col. Qaddafi. whose Green Book I read when I worked in Libya for around a year, a few decades ago).

Anyway. I often see that my images are really, really sharp where others’ are often not quite as sharp. So how do you make your images sharp? I have written about this before, see here, among others. Operation, light, settings, and good (prime, or quality) lenses all come into this.

A student, photographed by Michael Willems.
Student Volunteer, photo Michael Willems. Canon 7D, f/5.6, 400 ISO, 1/125th sec.

When you show that image large (original size), you see it’s sharp.

A student, photographed by Michael Willems. Detail.
Student Volunteer, photo Michael Willems. (Detail).

But today a quick tip about errors I see people make frequently.

Whenever I see people whose images, in spite of good lenses and the right settings, are not as sharp as mine, and when I then watch their technique, I often see these mistakes:

  1. People jam down on the shutter. I hear a beep, and a millisecond later, the click. This means the shooter is not giving himself or herself enough time to verify that focus was achieved on the right object. Instead, you should aim, press half way down, wait for the beep, then wait, and only once you have ensured the focus point activated properly where you wanted it, push down.
  2. The shooter leaves the beep off. This means no confirmation of focus unless you look away from the subject… bad.
  3. The shooter beeps and waits correctly – but then moves forward or backward before pushing down. Even a slight backward or forward movement shifts focus!

Watch yourself carefully when you shoot. If you discover that you are making any of the above mistakes, this will affect your sharp focus.

And that would be a shame. My people deserve sharp focus!

Autofocus assist: will it work?

My Canon speedlites (like the Nikon flashes) have an “Autofocus Assist Mode”, where a red pattern of lines emerges from the red square at the front. I am talking about big flashes, like my 580EX 2:

580ex2 Flash
580ex2 Flash

The flash emits the red beam when needed to help the camera focus. This happens:

  1. When it is dark
  2. When the subject has too little contrast (try focusing on a blank wall).

It is cleverly linked to the focus spot: when you are using a focus spot that can detect horizontal lines, it will emit horizontal lines; when using a focus spot that can detect vertical lines, it will emit vertical lines.

But when does it not work?

  1. When it is not dark and the subject is not low contrast. If the camera can focus without the assist light, it will. Why waste battery power?
  2. When you are not using “One Shot” focus (Nikon calls it AF-S). The red light helps the camera lock focus: when you are in a mode that does not lock (namely manual focus or AI Servo/AF-C focus), it will not try.
  3. When you are using a focus point that cannot be reached by the red pattern (think wide angle lens).

Hope that helps. Flash can seem very complicated, but if you know all these little things, it gets a lot easier to use.