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NOTE: All images and diagrams unless otherwise stated are copyright W.Brooker 2000 and may not be reproduced in whole or in part without the permission of the author.

The principles of lighting

Almost all cameras, be they digital, film, video or specialist rely on the accurate metering of current light conditions in order to produce pleasing results, and the principle of all light metering hinges on the idea that most scenes will average out to a midtone shade with a reflectance around 18%. What this means is that if you were to thoroughly combine all he shades and colours and brightness levels in a scene, the resulting colour would be equivelent to a colour that sits roughly half way between pure black and pure white in its shade. This system works pretty well for most scenes, but there are certain conditions that can "fool" the meter into making mistakes.

If we take a picture of a piece of white paper, the meter immediately tries to reduce its brightness to that of a midtone.

Does that yucky shade of grey look at all familiar? Well, for most people who have tried to take a picture of snow, particularly on a bright day, that yucky shade of grey is the colour that the snow appears in their pictures.

Now let's take a picture of a piece of black paper

Again, the camera's meter tries to produce an image that is a midtone. In theory, both the image of the white paper and that of the black paper should turn out an identical shade, but this never quite happens because the reflectance varies.

So why am I telling you this? The reason is that there are situations in real life where you should be aware how your camera is viewing a scene, and what you can do to correct it.

The correction you need to make is called "EXPOSURE COMPENSATION" and most digital cameras allow some form of compensation. There are two methods you can use for exposure compensation, one is to "dial in" a compensation amount, the other is to use an "AUTO EXPOSURE LOCK" or AEL.

Exposure compensation

Using a manual exposure compensation, you basically tell the camera how much more or less light to let in than it thinks is necessary. The adjustment is usually set in "stops", and these are F stops as we discussed in part 1. So to get the white paper white again, we would probably set a +1 stop exposure compensation, and if you cast your mind back to part one again, this means you would be telling the camera to let in TWICE as much light as its meter is suggesting. Likewise, to get the black paper black again, you would set an exposure compensation value of -1 stop (half the light suggested by the meter). Let's try.

It looks much better, but they are not perfect, and in this case a setting of plus and minus 2 stops would probably have been better. This is why you need to get to know your camera and how it behaves using certain settings and in certain lighting situations. Although no camera is perfect, there's no substitute for familiarity with the equipment you use.

Auto exposure lock

An auto exposure lock feature does what its name suggests and locks the cameras meter reading. If you know a scene contains an unusually large ammount of light or dark areas, you can point the camera at something that is nearer to a midtone and lock the reading before lining up your shot again. An example whould be a hazy day where the sky is very bright and glaring. If there is enough in the viewfinder, his bright sky would "fool" the meter into thinking it needs to darken the scene in order to create a midtone average. By tilting the camera down towards the ground and excluding the bright sky from the viewfinder, you can then press the AEL button and "lock" the camera's meter at the levels it then sees. Once you tilt the camera back up again and the sky once again comes into the viewfinder, it will have no effect on the meter reading as it is still at the locked values. Likewise if it's a picture of a snow scene, you could get up close to say a tree trunk or a piece of clothing if you're photographing a person, which has no snow on it (a tree trunk isn't far off being a midtone as a rule), lock the exposure, then move back to where you want to take the shot from.

PRO TIP

If you can't find a midtone to meter from, try putting your hand in front of the lens at arms length and meter off this. As a rule, human skin is lighter than a midtone, so if you have it, give an exposure compensation of +1 stop and this should be about right. If you don't have exposure compensation, then just meter off your hand and make your adjustments later on the computer. You can actually buy what's known as a "grey card" which is designed to accurately portray what the majority of camera meters think is a midtone and are calibrated to. You then simply lock your exposure while pointing the camera at this grey card. Remember, the card, tree trunk, piece of clothing or what ever you decide to meter from must be in the same light that your final picture will use. It's no use metering off a tree trunk that is in deep shade then walking out into the sun to photograph a snow scene.

Types of metering (photometry)

By far the most common form of light metering is what's known as "Centre Weighted" metering. With this method, the whole of the viewfinder is measured for light, but the central area is far more sensitive than the edges. this is because as a rule, the main subject is unlikely to be positioned at the edge of the picture.

Centre weighted metering. Green is most senstitive while red is least sensitive.

Because of the way CCD's work, another popular form of metering, and in some cases a more accurate form of metering, is EVALUATIVE. This may also be called MULTI ZONE metering. With this, the viewfinder is split into a number of "zones" and light is measured from each. Once all the information is gathered, the results are then compared against stored information to try to match the conditions to those in the database. Sometimes mathematical logarithms are used to compare and analyze the data in order to produce the best camera setting.

Perhaps the most accurate method of metering, though the one that needs careful mastery, is spot metering. This measures light from only a small central portion of the viewfinder, which means it can be accurately placed over a suitable tone to produce a perfect final result.

 

There is one thing to remember about light. No matter how expensive or carefully used an artificial light may be, there is nothing to compare to narural daylight for taking photographs. Sometimes however you have no option but to use your flash, so how can you make the most of it?.

 

Page 3- Flash photograhy