Zone Focusing and Pre-Focusing

Focus is not an absolute. There is always a zone either side of the focused point that is acceptably sharp. The size of this zone will depend on how far away the subject is; the focal length and aperture of the lens; the size of the final picture; and what you regard as 'acceptable'. There is more about all this in the free Basics module, focus and depth of field.

 

shops, tooting

 

Shops, Tooting, London

Zone focusing, or its close relative pre-focusing (see below), can allow you to work faster and more unobtrusively, especially if you are using a camera with which you are not familiar. This is an unremarkable picture save for the fact that it was taken with an incredibly rare camera, the Hensoldt Reporter, which Roger borrowed from a kind friend. He loaded it with Ilford HP5 Plus, as far as he recalls, and the print is on Ilford Multigrade IV.

If the lens on your camera is marked with a scale to indicate depth-of-field (hereafter d-o-f) you can use this for zone focusing, as follows:

Choose the minimum and maximum distances at which you are likely to shoot. In the street, for example, this might be 10 feet/3 metres and 30 feet/10 metres or (if you prefer to work closer) 6 feet/2 metres and 20 feet/7 metres.

Choose a middling-to-small aperture, usually f/8 or f/11, and look for the d-o-f marks on your lens (more rarely, on the camera body or focusing knob) corresponding to the aperture chosen.

Set the focusing scale so that the distances in question are within the d-o-f marks. If they won't fit, either narrow your choice of focusing distances or stop the lens down further.

Your lens is now set in a sort of snapshot mode that will require no focusing for any picture taken within the range in question.

 

50-1.2 f11

 

 

50-1.2 f8

 

50mm f/1.2 Canon lens

On the left, the lens is set to f/11 and everything between about 11 feet (call it 3.5 metres) and infinity is in focus. On the right, it is set to f/8 and everything between 10 feet (3 metres) and a bit over 25 feet (around 8 metres) is in focus. We chose this lens as our first illustration, even though it has no metres scale, because the distance and depth-of-field markings are particularly clear.

For exposure, you can either choose aperture priority (if your camera allows it) or set a likely shutter speed. Let's stick with the street shooting example and assume a 'cloudy bright' day and ISO 400 film; the latter is ideal for street photography like the Tooting picture above. Take a couple of readings, one in open shade, one in moderate shade. At f/11 they are likely to be about 1/125 and 1/30. Set the camera to 1/60 and you should get an acceptable exposure even if you have no time to change the shutter speed setting. Usually, though, you can move it one click in the appropriate direction as you are raising the camera to your eye.

This is faster and more accurate than any autofocus, and probably as good as the vast majority of auto-exposure; the latitude of the film will cover up most errors with negative films (not slide films, where exposure is much more critical). As you grow more used to working this way, you will find yourself automatically adjusting the shutter speed dial to give the optimum exposure.

 

50-2.5 f11

 

 

135-3.5 f11

50/2.5 Voigtländer Color-Skopar (left) and 135/3.5 Takumar (right)

The d-o-f markings on the Color Skopar are significantly less precise than those on the old Canon, above, but still just about usable. Again the lens is set to f/11 and again everything from about 12 feet/4 metres to infinity is in focus.

Because the 135mm lens magnifies the image so much more (2.7x as compared with the 50mm lens), the depth of field at f/11 is much smaller: from maybe 80 feet/28 metres to infinity. Actually the infinity mark is set slightly inboard of the f/11 mark because Roger knocked it when he was taking the picture...

 

After a while you may also find yourself 'tweaking' the focus one way or the other, again, without really thinking about it. This is why we prefer lenses with the sort of focusing spars (tabs, finger-grips) that are found on so many rangefinder lenses: you can set the distance by touch, without having to look at the focusing scale. You may not be dead accurate, but it doesn't matter: even at f/5.6, d-o-f will cover up almost all errors.

 

21mm f11

 

 

15mm f11

Voigtländer 21/4 (left) and 15/4.5 (right)

With wide-angle lenses, magnification on the film is small, so depth of field at any given distance is greater. You can see here that at f/11, the 21mm keeps everything in focus from just over 2 feet/60cm to infinity, while the 15mm offers everything from just under 40cm (call it 15 inches) to infinity. Obviously, you could use either at full aperture and still have significant depth of field.

A word of caution

Manufacturers' d-o-f scales are not uniform. They are based on the manufacturers' assumptions of what the camera's users will regard as 'acceptable'. Perhaps the most extreme examples were in the 1930s, where Leica reputedly expected whole-plate enlargements (about 6x) and Contax reputedly expected postcard-size enlargements (about 4x). Contax d-o-f scales were accordingly more generous.

There is however an easy way around this. If you find that your camera is not giving you the d-o-f that you want, just use the next scale down, e.g. if you are shooting at f/11, use the d-o-f scales for f/8, and if you are shooting at f/8, use the d-o-f scales for f/5.6.

 

Pre-Focusing

 

Pre-focusing is no more than a variant on zone focusing. You set your camera to the distance at which you expect to take the picture, and wait until the subject comes into focus. The obvious application is in sports and action photography but you can use it anywhere that someone is certain (or at least likely) to pass a fixed point, including theatre and dance photography. If you are working close up at a large aperture, you do not re-focus to get the final fine focus. Rather, you rock your body to and fro in order to be sure that you are at the right distance.

 

 

Kathkali Dancer, South India

With a 90mm Summicron wide open at f/2, there is not much depth of field. So? If you know where your subject is likely to be, just pre-focus. Roger shot this with his Leica M4-P; film type long forgotten.

 

kathkali dancer

 

The Bottom Line

Some people delight in making everything sound as difficult as possible, and conceal the simplest tricks behind jargon. Presumably this makes them feel superior and clever. We'd rather just take pictures -- and zone focusing and pre-focusing could hardly be easier.

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© 2007 Roger W. Hicks