AJ runs through the basics of accurate focusing and choosing the right method for each task. This is not an indepth look at Autofocus, more a simple overview for the more inexperienced photographers among the FotoBuzz ranks...
By Andrew James
Focusing is one of those elements of photography we take for granted. The camera just focuses, right? I know that one or two of you are, like me, long enough in the tooth to have picked up cameras when autofocus (AF from now on) was just an embryonic thought in a clever boffin’s brain. For every shot we took we had to actually turn the focusing barrel. It didn’t matter whether we were focusing on something absolutely still or an animal running at 100mph. Then along came AF. I’ve no intention of writing here about who first invented AF because that’s of no real importance to your everyday photography. Take a look on Wikipedia if you want to understand the history. This basics article, is purely going to look at what you should be using and when you should be using it. It’s going to be incredibly simple too. I also don’t care what kind of a DSLR you use. I’ve used pretty much every make at some point in the last 10 years and on each and every occasion I have used one of three focusing options. I’m sure you are already using the same ones.
There are some hybrid focusing modes out there. For example, some of my Canon lenses also have AI Focus which is designed to switch between One-Shot and AI Servo. Personally I don’t like hybrid modes and I’d suggest you avoid them too. You want a specialist for the right job. Let’s look at each of the three specialist types of focusing modes I’ve highlighted in more detail…
Single Area: When you use One-shot/AF-S the camera locks focus on the point you’ve chosen and once locked, it stays locked until you take the image. It’s that simple. So with this method, you can lock focus by half depressing the shutter button and if you want to, slightly recompose your image before taking the image. The point of focus will not change. This is the focusing method to use on a subject that is pretty much static. You can use it on portraits – as long as you have control over the person or thing you are taking images of – or still life images or landscapes. Basically, any situation where you, or the subject, aren’t moving a lot. It’s easy to use and in almost every case I can think of, you want to use it with just one AF point selected so that your focusing accuracy is absolutely guaranteed. I know people who swear by the basics of using the Centre AF point (invariably the fastest) to lock focus, then recompose slightly and shoot. It works – at least on a static subject.
With this portrait of a glass artists at work, I used One shot with a single AF point selected. This was placed
over his nearest eye. As I was able to control his movement this was the simplest and easiest way of shooting.
With the camera on a tripod, I selected an AF point over the hut while in One-Shot.
Continuous/AI Servo: This AF option, as most of you wildlife fanatics will know, is ‘action’ mode. But don’t just use it when a puffin is flying across you with its little wings beating faster than a Bat of Hell’s, use it with anything that is moving. A fidgety child whose portrait you are taking could well benefit from using continuous focus. Continuous focus re-adjusts to the subject’s position, as long as you keep the shutter button half-depressed. So it’s important to consider what an unpredictable subject might do. For example, a sleeping lion is as static as a teapot and yes, you could use One-shot/AF-S to take a portrait. But what happens if it suddenly decides to move and you have to track it in One-shot? Two things could occur. Firstly, most of your moving images will be out of focus because the AF system can’t track it or secondly, you miss the shot because you are busy trying to switch from One-shot to AI-Servo. So think ahead at all times.
Naturally, for this fast and unpredictable subject of an osprey nipping in for a quick breakfast I
needed to use AI Servo to give me any chance of locking focus and then tracking it as it flew off.
Although relatively static in the long grass, slight movement from the lioness and the fact she could walk off at any moment meant that I used AI Servo here too. The grass made focusing on her eye awkward but the Spot AF mode on my Canon 1DX came to the rescue.
Yep, AI Servo again. Although static at this point, that wasn't going to be for long so it was better to
start with continuous focus and be ready to track them as they moved along.
Manual Focus: The good old-fashioned method of turning the focusing barrel with your fingers to achieve focus. Yes, it’s slow. And yes, unfortunately many modern lenses don’t have especially responsive focusing barrels but it still has its moment. Actually, I’ll often but not exclusively, use manual focus when working in Live View and shooting a landscape or macro image from a tripod. Zooming in on the screen to the area I’ve chosen as my focus point gives me an uber-accurate way of ensuring that the point of focus is precisely where I want it. This can be very useful when taking a macro image where the depth of field is so limited that the slightest mis-focus can count against you.
With the camera on a tripod and pointing directly down at these boulders trapped in ice,
I was able to use manual focus and Live View to ensure very precise focusing.
Working in Live View and on a tripod, I zoomed in on a droplet of water and manually focused before taking the shot.
As with everything in life and photography, there is no perfect blueprint on how to work. However, using the three focusing modes I’ve mentioned here at the right time will help. But be prepared to switch between them when necessary. For example, it low light conditions using a single AF point in One-Shot/AF-S maybe more accurate that manually doing it by eye. This could also be the case in high contrast conditions where it’s actually not that easy to see the Live View image on the screen. Pre focusing on a single point and switching to manual might just be the best way to get an action shot in some circumstances. There have also been times in appalling weather conditions when falling snow has played havoc with the AF system and then, switching to manual has been the only answer, even if that's resulted in near frost-bite! You just have to be prepared to adapt and work it out for each situation you are in.
Driving snow was making accurate AF difficult, so since these two elephant seals were fairly static, I was able to switch to manual focus.
I’ve mentioned AF points a lot already but feel that they need a little more coverage here. I’m often asked when we are out on a workshop – "how many AF points are you using?” It’s a perfectly valid question since for many of us we can choose from one up to 62 Active AF points. The answer is never simple because it’s quite possible I’m switching the number on a reasonably regular basis. If it were always possible to use just one AF point and get the focus where I want it, that’s what I’d do. But it’s not. So again, it’s about adapting to circumstances. This is especially relevant for a moving or unpredictable subject. Sometimes a small group of AF points is what’s needed, but sometimes you need them all. Andy covers some of this in his article Nailing high action daylight bird photography. So rather than me waffling on about it here, refresh yourselves by taking a look at his article.
Finally, there is one thing I want to point out and that is just getting your AF point in the right place doesn't guarantee you a sharp shot. I've lost count of the times I've been handed a camera and asked to look at an image that should be sharp because the AF alert is showing that it locked focus where it was supposed to, yet it isn't. The culprit? Shutter speed! The shutter speed was marginally too slow for the subject to be rendered pinsharp. It might just be the tiniest vibration or sudden shake of an animal's head but the result is the same - an image that's slightly below par. You might get away with it for an image online but if you want to blow it up for a big print - you're in trouble. So good focusing has to be coupled with the right exposure too - that's shutter speed, aperture and ISO. All of which can and will have an affect on the finished image. As of course will the quality of your lens but that's another whole can of worms!
That's a very quick overview of the most useful focusing modes. I've deliberately avoided camera-specific advice and I haven't mentioned Back Button Focusing! If you prefer BBF, great - it works - but it's impossible (in my opinion) to switch between conventional focusing and back button focusing.