An auto-focusing modes primer

Autofocus is such a wonder-tool in our cameras that I'm sure quite a few of us have no idea how we ever managed to take tack-sharp photos without it. But how many of us actually use it to its full potential? There's a bit more to auto-focus than the single or one shot default. Without further ado, here's a quick primer into the different auto-focusing modes you're likely to find on your camera, and when best to make use of them.

One or Single shot

This is probably what you think of as 'auto-focus'. You point your camera at your subject, you line up your auto-focusing points over it, you half-depress the shutter button, and the camera attempts to focus on the subject. When it finds focus, it 'locks' onto it until you complete the depression of the shutter button or release it and re-focus your shot. This mode is great for most subjects that don’t move a lot.

Canon calls this mode 'One shot'; you'll see it marked as 'AF-S' on Nikon, Fujifilm, and Sony cameras; Olympus refers to it as S-AF and Pentax as AF.S.

Continuous or Servo mode

When you're shooting fast-moving subjects and want to keep them in focus, try using continuous or servo mode. While you half-depress the shutter release button, the camera will repeat the auto-focusing operation in order to keep your subject sharp as it moves across the frame.

Canon reckons its servo mode can track subjects approaching or receding at upto 50 kilometres per hour, making it good for capturing plenty of sports.

Nikon, Fujifilm, and Sony refer to this mode as AF-C; it's marked as AI Servo on Canon cameras; on a Pentax you'll see it as AF.C; and it's C-AF on an Olympus camera.

Intelligent or Automatic auto-focus

'Intelligent' focus is a half-way house between single shot and continuous auto-focus. When you half-depress the shutter button, the camera is set to recognise movement in the subject. For subjects that don't move, the camera will use one shot auto-focusing. Should it detect movement from the subject, it will automatically switch to continuous auto-focus and start to track the subject.

While this mode might seem like the best of both worlds, and it can be very handy particularly if you're photographing children or animals, it can sometimes be a bit of a let-down and isn't quite fast enough to deliver the results you want.

Canon calls this mode AI Focus AF. It's AF-A on Nikon and Sony cameras, while Pentax refers to it as AF.A.

Manual focus

It was learning to align the focusing markers in an SLR that first got me hooked on photography, almost 30 years ago. Back in the early 80s, auto-focus was only just beginning to make its way into cameras. Now we often wonder how we cope without it. But still, there are times when auto-focus simply won't do and you need to switch to manual focus, maybe for macro shots or some landscapes. If there's no obvious manual focus option on your camera, try looking on your lens: there's likely a switch to be flicked there.

Often, but not always, your camera will help you by beeping or flashing when it thinks that you might have achieved focus on your subject. If you're using live view rather than through the lens, try using the manual focus assist option that lots of cameras have now. Rather than displaying the full frame, it zooms in on the area where you've focused, making it easier to hone in with precision on your subject. It's a useful tool to demonstrate just how big a difference a small adjustment can make to your focusing, too.

Exploiting your camera's capabilities

Your camera is gifted with heaps of different tools to help you get the shots that you envisage; there's more to it than exposure and metering. Don't forget to make use of them - that's why they're there!

Depth-of-field in greater detail

We're probably all familiar with the notion of aperture controlling the depth-of-field in our photos. By using a faster aperture, you create a shallower depth-of-field. To keep more of your image in focus, you need to use a smaller aperture. But there's a whole lot more to depth-of-field than adjusting your aperture to get more or less of the scene in focus.

'Acceptably sharp'

Let's start with setting out what we mean by depth-of-field. It's the range of distance in a photo that is considered to be 'acceptably sharp', or what we would regard as 'in focus'. Only the actual point of focus in a photograph is definitively sharp and 'in focus'; depth-of-field describes the zone of acceptable sharpness either side of it. A wider band of 'acceptable sharpness' running through an image equates to a greater depth-of-field. To introduce more blur into your photos you would want a shallower depth-of-field with a narrower band that's 'acceptably sharp'.

It's worth remembering that there's no sudden transtition from 'sharp' to 'unsharp': focus falls off gently on either side of the plane of focus, regardless of the aperture you use. It is fair to say, though, that larger apertures have a more rapid transition from in- to out-of-focus than larger apertures.

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Controlling the depth-of-field in an image is achieved primarily by adjusting your aperture—a smaller aperture for a greater depth-of-field; a larger aperture for a shallower depth-of-field—however, there are other factors that affect it, too.

Focal length

You'll often hear people say that telephoto lenses have a shallower depth-of-field than wider angled lenses. This isn’t strictly true. It’s more accurate to say that because telephoto lenses are mostly used to magnify subjects, and the subject will then fill more of the frame relative to the background, the depth-of-field appears to be shallower.

All the same, it's worth capitalising on the magnifying effect from telephoto lenses to pick out your subjects and surround them with blurred foregounds and backgrounds.

Subject-to-lens proximity

If you've ever practised macro photography, you'll appreciate how getting closer to your subject makes it harder to get it all sharp. The closer that you position your subject to your lens, the shallower your depth-of-field will be. Choose a subject further into the background and you'll find that the depth-of-field surrounding it is larger.

Distribution of acceptable sharpness

Depending on the focal length you use, you will find that the depth-of-field isn't divided equally in front of and behind the plane of focus. Instead, the area of acceptable sharpness behind the point of focus is generally larger than that extending in front of the focal plane. As focal length increases, so too does the distribution of the depth-of-field in front of the subject.

When you shoot with a focal length of 15mm about two thirds of the depth-of-field will be behind the subject and one third in front of it. When you get to 400mm it's closer to a fifty-fifty divide.

Depth-of-field: not just about aperture.

Will Toshiba be bringing Lytro-like refocusing to smartphones?

We've grown accustomed to the idea of photos that we can refocus after the fact thanks to Lytro's lightfield camera and to Nokia's Refocus app that takes between two and eight photos that you can play with to your heart is content. Now Toshiba has announced the first dual camera module designed for inclusion in phones and tablets, which is able to record depth data and well as an image simultaneously. The catchily named TCM9518MD comprises two quarter-inch five-megapixel CMOS camera sensors and a dedicated processor. This dual camera module can capture images where the foreground and background, and everywhere in-between, are in focus along with depth data for each object in the picture. Not only can the module allow you to focus, defocus, and refocus the images it produces, but it can generate 13-megapixel images by up-scaling images taken by its two cameras.

Andrew Burt, vice president of the Image Sensor Business Unit, added that: 'Not only does the dual camera module enable these advanced capabilities [e.g. refocusing] with fast digital focus and little shutter lag, the device doesn't require any focus motors, so it can be built much thinner than today's 13-megapixel camera modules.'

Toshiba has made sample modules available to smartphone and tablet manufacturers (they cost $50 a pop), so which devices will be the first to feature them?

(Headsup to the Verge, further information from PR Newswire)