Michael Freeman: Where to focus

Of course, focus is so basic in photography that we tend to take it for granted. All the more so because auto-focus is the default for cameras (you have to choose to switch it off), and manufacturers have develop ped some very sophisticated ways of guessing what exactly you want to have sharply focused in the frame. But there’s more to focus than simply locking onto the main subject, By choosing where you focus you can shape the image and how other people will view it. And focusing always involves another question: how much of the scene will be focused? All of it, some of it, or just a narrow band? That depends on the depth of field, and that’s controlled by the aperture.

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You will need

• Any camera that allows you to control where the focus is (see Tips)
• For deep-focus shots, quite often you’ll need a tripod to avoid camera shake because shutter speeds will be slow

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What’s important about it? We expect the key things in a photograph to be sharp. If only part of a photograph is in focus, our eyes naturally go to that. You can use this difference in focus to control how a viewer’s attention goes. You can also use accurate focus to maximize sharpness across the picture.

The idea Two major things are involved in focusing. One is whether or not the sharp focus is going to be restricted to a small part of the image. The second is where to focus – on what part of the subject. The two are inseparable. To simplify things, let’s think about shallow-focus images and deep-focus images as contrasting types, even though there are, of course, middle-of-the-road kinds. Shallow-focus (or selective focus as it is also known) works at maximum aperture – the widest possible on the lens – and has the strongest effect with a fast lens, such as ƒ1.4 or ƒ1.8. Here’s an example…

Most of us intuitively spot the part of a subject that needs to be sharpest, so it’s not complicated. If the important subject in a picture is a person, it’s natural to focus on that person. If you’re close, focus on the face. If the depth of field is shallow and you’re very close, focus on the eyes. The trick, if there is a trick, is to know what in the frame is most important to you – and within that know what detail of it catches the eye first. With a face, if the eyes are sharp (or at least one eye), people will accept other parts being soft, but not the other way round.

Some scenes you may want as sharp as possible throughout, from front to back, and that means using a small aperture for the best depth of field (see Tech check below). Many landscapes and architectural shots do well with this, as below….

But before moving on to the technical matters, it’s also possible to get away with the unexpected. If there’s a person in the foreground, it would be natural to focus on that person, but what if you did the contrary, as below?

This can sometimes make the image more interesting, although you’ll be taking a risk. In this case, the reason was to get across the idea of the colorful flowers being used in the women’s hair – reducing it to a wash of color without detail.

Tech check Whether you use auto-focus or do it by eye, sharp focus is a visual experience, but it gets technical when depth of field becomes involved, and that happens when you stop down to a smaller aperture. Definitions first…. Depth of field (often abbreviated to DOF) is the part of a scene, from front to back, that is acceptably sharp. ‘Acceptably’ is the key word, because sharpness is a matter of appearance. Meaning, if it looks sharp, it is sharp. The smaller the aperture, the deeper the band of sharpness. Here’s an easy to follow example, a row of mailboxes, photographed with a standard, 55mm lens, first at maximum aperture and three different distances…

None of the above is a ‘wrong’ point at which to focus. Personally I prefer the bottom one, but they all work. Now, however, if you want to get as many as possible sharp, you would need to stop right down. Let’s see which distance gives the deepest focus…

The focusing points and depth of fields are color-coded to make it clearer. As you can see, because the band of sharp focus extends forward and backwards from the point of focus, you would waste half of it by focusing very close or very far. The best place is more central, but nearer rather than farther.

This is typically important in landscapes and other scenes that include the horizon. Focusing on the horizon itself wastes some of the depth of field. The term hyperfocal distance is used to mean the optimum distance for focusing – the one that keeps the horizon sharp and as much as possible closer to the camera. There’s a formula, of course, but who has time for that? The rule of thumb (the best kind of rule), is to stop down the aperture to the smallest possible (eg ƒ16 or ƒ22) and for a standard focal length focus at roughly 5 meters. At half that focal length (wide-angle), focus about 4 times closer (about 1 to 1.5 meters); at twice that focal length (medium telephoto), focus about 4 times further away (about 25 meters). That’s a very rough guide, because it depends on the focal length and the aperture. The short table below gives the accurate distances for a few settings, to give you an idea. The focal lengths are for what used to be called 35mm format, now FX.

If you want to go further into this, and can afford the time when shooting, there are many calculators available. One excellent online resource is at www.nikonians.org

Processing tips

Yes, something can be done about the appearance of focus in post-production. A little to make things sharper, and a lot to make things blurred. Restoring focus is quite complicated, and as far as I know, there is only one retail software, called Focus Magic www.focusmagic.com. It works by a process called deconvolution, and improves actual sharpness. Other programs just simulate it it with some kind of sharpening, which is not the same thing. Blurring, however, is much easier, and programs like Photoshop even have simulated Lens Blur. Select the part of the image that you want to ‘de-focus’ and apply a blur filter. Alternatively, make a duplicate layer in Photoshop, blur the lower layer, then use the eraser on the top layer selectively.

Pitfalls

• Wrong or unexpected focus mode (eg Continuous or Manual when you wanted normal)
• Deeper or shallower focus than you expected because of lack of preview

Written by Michael Freeman
www.michaelfreemanphoto.com