Lesson Six: Taking the Hocus-Pocus out of Focus

So far, all the controls we've looked at have related to exposure. It's high time we (ahem) focused on the other important aspect of photography: focus. The final control is the only one that has to do with focus: namely, the focus ring on the lens barrel. The lens shown in Fig. 2 detail above is an increasingly rare breed, known as a prime lens. Prime lenses do not zoom so only have a single focal distance (in this case, 50mm); because primes have only one focal length they're simpler to understand, which makes their functioning easier to explain.

Prime lenses commonly have a focus distance scale on the lens barrel, which we can see above as two rows of numbers just below the C. The upper/yellow row shows distances in feet; the lower/white row shows distances in meters. By manually turning the knurled black focus ring so the index line matches a given distance in feet or metres we're setting the lens to focus precisely at that distance. Focusing can also be performed (and usually is) by peering through the viewfinder and judging lack of blur by eye.

In Fig. 14 we see three objects – A, B, and C – at increasing distance from the camera; and below are (fictitious) distance settings for those objects, also labelled A, B, and C. According to the settings A is 0.6 metres, B is 1.0 meters away, and C is 2.0 meters away (which of course is not possible given the relative distances shown in the picture).

Inside the barrel of any photographic lens, such as the one shown in Fig. 2 there are several simple curved pieces of glass called elements. The front-most element is on a tube or sleeve that fits snugly inside the main barrel. The focus ring connects to this tube. What actually happens when you move the focus ring left or right is that you're screwing or unscrewing the focus tube, moving it forward or backward, depending on the direction you turn the focus tube. This in fact shortens or lengthens the entire length of what we call the lens and perhaps should call the "lens assembly". So the nominal designation of the lens in Fig. 2 as being a 50mm lens is in fact just an approximation: it actually changes its focal length by several millimeters depending on how much of a turn has been given to the focus tube. That's already more than you ever wanted to know about the nitty-gritty of lens construction – but wait! there's more!

Key fact: Only a single plane in the three-dimensional world can ever be precisely in focus in a lens + camera system with the film or sensor itself on a flat plane.

The edge of the film/sensor plane is shown as a white line in Fig. 15 and the edge of the current plane of focus as a yellow line.

• In scenario A of Fig. 15 light rays from some source outside the picture boucne off the teddy bear, enter the lens, and are bent toward the film plane. But because the teddy bear is closer than the plane of focus, they converge at a point in front of the film plane, and so leave a blur on the film.
• In scenario B of Fig. 15, the plane of focus is on the front face of the six-pack, which is hypothetically 1 metre away from the film plane of the camera. Light "rays" bouncing off the front of the six-pack (red lines) enter the lens, are bent the correct amount, and so converge exactly on the film plane.
• Similarly, in sceanario C of Fig. 15, light bouncing off the cutsey figurines has furthest to travel and so converges somewhere behind the film plane, also leaving a blur on the film.

By extending or retracting the focus tube we change the degree of bending that light rays undergo inside the lens and camera and thus change the mix of which rays converge at the film plane. When the focus tube of the lens is extended as far as possible the plane of focus passes through the teddy bear, when retracted further than is shown in Fig. 15 the plane of focus passes through the figures. (This is illustrated by the three increasingly short red two-headed arrows in Fig. 14.)

Key fact: We can now see that the concept of depth of field is actually a fudge factor. If the mis-convergence of light from objects closer or further from the plane of exact focus is sufficiently small, the amount of blurring will be too small for the human eye to resolve and so appears as if a single point.

The depth of field depends on many factors, but two important ones are

• the focal length of the lens (longer focal lengths have less DOF)
• the degree of enlargement of the print (greater enlargement also enlarges the blur of mis-convergence)
• the aperture set on the lens (the smaller the aperture the greater the DOF).

Zoom lenses complicate matters by having two focus tube extensions and/or two controls for extending a single extension tube. One changes focus through a bigger range of distances and thus changes the focal length by significant amounts, such as from 24mm to 70mm. The other tube is the focus tube we've already discussed, which only changes the extension by a few millimeters,

Your turn: Not a lot I can think of for you to do here. Your camera probably has a zoom lens; change focal lengths by zooming in and out and observe how the lens extends and retracts. Now change focus from near to far and observe how the lens extends and retracts for that. You may find that for zooming your lens does not perform a simple extension/retraction motion as you go from widest to furthest focal lengths. There is a common form of cleverness that reduces the total travel needed for zooming and thus provides greater stability to the design.

Further Reading

...And that, I think, covers the minimum background you'll need to understand the manual and use your camera. Read these pages over five or ten times and it might even begin to make sense. ;)

To explore any of the above topics further, the second editions of The Camera and The Negative by Ansel Adams, after more than half a century, remain the keystone of photography. While their primary focus is black & white film photography, most of the concepts apply to colour and digital. These books are clear and simply written - a joy to read.

• Nikon F2 Series SLR cameras on Mir.com provides as much detail as you could want should you be interested in the camera used to illustrate this tutorial.
• Navy Photography Tutorial excellent and comprehensive, if dated, tutorial on all the topics we covered and more.
• Cambridge in Colour Tutorials by Sean McHugh: covers more advanced topics, particularly relating to the digital in digital photography, but is very lucid.