Bokeh | Unfocussed quality

'Bokeh' is a Japanese word used to describe the quality of out-of-focus areas in a photograph. Points of light which are out of focus appear as discs, and the more perfectly circular and evenly illuminated these discs are, the better the bokeh is considered to be.

Bokeh is dependent on the optical design of the lens and the number of aperture blades in the lens diaphgram mechanism. The closer the aperture is to circular, the better the bokeh. Bokeh becomes particularly important when you're using larger-format cameras, longer focal length lenses and wider lens apertures - in other words, any time when your photographs will have shallow depth of field.

You may never notice any difference in bokeh between one image and another, and you may feel the whole idea is a lot of pretentious rot, but there are instances where differences are obvious. Users of old-fashioned catadioptric telephoto lenses will be familiar with their characteristic doughnut-shaped highlights in out-of-focus areas, and it's possible to spot the hexagonal/pentagonal shapes of defocussed highlights created by many older lenses.

This is why lens makers quote the number of diaphragm blades in their lenses. The greater the number the better, in theory, the bokeh.

Opinions about the correct pronunciation vary, by the way. Most seem to think it should be pronounced with short vowel sounds as in bo-ke ('bo' as in 'box' and 'ke' as in 'kettle').

Pinhole photography | Who needs lenses?

Pinhole cameras use a tiny hole rather than a lens to capture images. Exposures take much longer and the definition is poor, but the pictures have some striking properties, and the fact that you can create images with nothing more than a hole in a lenscap is fascinating.

To create any kind of image you need to focus the rays of light from the subject on to the film or sensor. Normally, this is done with a lens, but a very small hole will do a similar job. The quality is not that good, but the effect is very interesting.

Firstly, there's no need to focus. A lens produces rays of light which converge to a point at a specific distance behind the lens, whereas the hole in a pinhole camera produces rays of light without any convergence.

As a result of this, the depth of field is essentially infinite. Subjects at any distance from the camera will come out with more or less equal sharpness (not that pinhole images are all that sharp anyway, mind).

You can make your own pinhole camera with a digital SLR and a spare body cap (see the illustration above). You need to drill a hole in the body cap, then tape some tinfoil over this hold and use a fine needle to make a 'pinhole' in the foil.

Then you'll need to set the camera to manual exposure mode and experiment with the shutter speeds to find the best exposure. You could try using a handheld meter (illustrated), but unless you know the approximate aperture value of your pinhole, you're always going to be guessing.

Even pinhole cameras have focal length and aperture values. The focal length is the distance between the pinhole and the sensor, while the aperture value is this distance divided by the size of the hole.

Getting the size of the hole right is a bit of an art. If it's too large, the light rays will diverge too much and the photo will be blurred. If it's too small, the image will be blurred again, this time because of diffraction effects. Finding the 'sweet spot' can take a few attempts.

This is an image shot with the setup in the illustration above. It's pretty soft, and this is because of the size of the sensor. Pinhole photography worked well with big, sheet film cameras, but the APS-C sensor in a digital SLR has a fraction of the area.

Despite this, though, pinhole photography has a certain fascination, even when carried out with a modern digital SLR. It's quite uncanny being able to capture a recognisable image without any kind of lens at all, and a reminder that the optical principles behind photography remain at least as fascinating as the latest digital technologies.