By Tarak Ch. Sarma, Senior Faculty, PVTI

In our previous lessons we have talked about very basic things which we have to give attention to, to get a good picture. We have also talked about the very basic things about the film, its ISO speeds etc. In all these things we have restricted our discussions to nontechnical aspects; saying that the ISO speed of a film gives us an idea about how much the film is sensitive to light, that an ISO 200/240 film is twice as sensitive to light as an ISO 100/210 film, that the view point from where the picture was taken affects the attraction and / or meaning of two pictures etc.

Our present discussion is on how much light is necessary to get a good picture, because light is the lifeblood of a photograph: photograph means a drawing by light, in actuality when we say expose a film, we mean we allow light to fall on the film-through the lens of the camera, of course. Because the film is sensitive to light, the light will make an impression on the film. This impression on the film is of the image produced by the lens, of the subject at which we have trained our camera lens.

Now to get a good impression of the image on the film, it is of utmost necessity that a proper or correct amount of light falls on the film. If the light falling on the film is of correct amount, we get a good picture (Photograph), and we say that we have given a correct exposure. In this case, the photograph will look live, like the look of a healthy child.

But if the amount of the light falling on the film is less than correct amount, the photograph obtained will look darkish and pale, like the face of an undernourished child. We say that we have given underexposure.

On the other hand, if the amount of light falling on the film is more than correct amount, the photograph will look pale and washed off. We say that we have given overexposure.

So, correct exposure produces a live photograph, underexposure a darkish photograph and an overexposure produces a washed off photograph.
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Underexposed Correctly exposed Overexposed

Then what is correct exposure?

This brings us back to the ISO speed of a film. Higher ISO speed means the film is more sensitive to light, and needs less light for correct exposure. Lower ISO speed means the film is less sensitive to light and needs more amount of light for correct exposure.

This situation necessitates for us to be able to control the amount of light allowed to fall on the film, which is to say to control the exposure.

We have two devices on the camera to control the exposure. One is diaphragm, which is an opaque flat piece of metal with a hole, the size of which can be made larger or smaller. The diaphragm is usually placed inside the lens in such a way that the light coming through the lens passes through the hole, usually called the aperture, before falling on the film. By turning a ring on the lens, which is known as the aperture ring, the size of the aperture of the diaphragm can be made larger or smaller. It is evident that a larger aperture of the diaphragm allows a larger beam of light to fall on the film than a smaller aperture does.

On the aperture ring, a series of numbers, called the f.numbers are etched, the numbers being 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 etc or a part of it. They stand for f.1.4, f.2, f.2.8, etc. The numbers, increasing or decreasing by a factor of 1.4, are so related that at f.8 (say), the aperture allows twice the amount of light than at f.11, and half the amount of light than at f.5.6, i.e. to say the light allowed in by any f.number is twice that allowed in by the next larger f. number, and is half that allowed in by the next smaller f.number. The light passing power by the f.numbers makes certain how much measured amount of light to be increased or decreased to correct an exposure given previously if it has given under exposure or overexposure.

The other device to control exposure is the shutter. Its function is to keep shut the film in the camera from light, until the time we press the shutter button to take a photograph. In most cameras, the shutter is placed just in front of the film, and such shutter is called a Focal Plane shutter. Some other cameras have the shutter built inside the lens and such shutter is called a between-lens shutter.

When the shutter button is pressed or in other words, it is released, the shutter opens and the light through the aperture in the lens falls on the film. Usually the shutter closes automatically after a preset time set on the dial of the shutter speed knob. This duration for which the shutter remains open is the shutter speed. In most cameras now-a-days, shutter speeds marked on the dial are: 1, 2, 4, 8, 15, 30, 60, 125, 250, 500, 1000, 2000, 4000…etc. In practice, the shutter will remain open for a duration which are the reciprocals of the above numbers and are in seconds. That is to say, when the shutter speed is set at, say, 8, the shutter remains open for 1/8 second and then will close automatically, even if you have kept the shutter button pressed. These shutter speeds are usually called ‘Fractional shutter speeds’ as the shutter remains open only for a fraction of a second. There is another setting B, on the shutter speed dial. This is not actually a speed, since the shutter remains open for as much time as you keep the shutter button pressed. B used to mean the shortened form of ‘bulb release’. But now-a-days, many people regard it as the shortening of ‘brief time’. Some present day cameras even have ‘Whole number shutter speeds’ shown beyond B, as 2, 4, 8. 15….up to, say, 30 and marked in a different colour from the fractional speeds, and here 8 means 8 seconds and not 1/8 second.

It can be clearly seen that at a shutter speed setting of, say, 30, the shutter remains open for 1/30 second, and at, say, 60, the shutter remains open for 1/60 second. So, at shutter speed 30, the amount of light falling on the film at a certain f.number will be twice the amount of light falling on the film at the same f.number, and half the amount of light falling on the film at shutter speed 15 at the same f.number. In short, the rule is that the amount of light falling on the film at a certain shutter speed is twice the amount of light at the next higher shutter speed and half the amount of light at the next lower shutter speed, all at the same f.number.

Let us now think of f.number and shutter speed jointly.

f.numbers: 1.4 2 2.8 4 5.6 8 11 16 22 32 45……..

Shutter Speeds: 1 2 4 8 15 30 60 125 250 500 1000…..

Example 1: Last time you took a photograph of a subject under good light conditions on a film of ISO 100/210 and the exposure given was

1/125 second at f.11 (or exposure was 1/125 second at f.11)

(We speak of exposure in terms of shutter speed and f.number. There is no hard-and-fast rule, which number to mention first, as long as you express the numbers correctly)

You got the photograph correctly exposed, i.e. the exposure was correct.

So, this time you will give the same exposure on the same ISO speed film for a similar subject under similar light condition.

But still you have your own choice of the exposure, even for the same correct exposure. You have seen that if instead of shutter speed of 1/125 second, you use 1/250 second (because you know that the faster is the shutter speed, the less is the chances of shake in the photograph), you will allow in half the amount of light at f.11 as used last time. So, to keep the exposure correct, you will have to double the amount of light, and this you will do by using smaller f.number; i.e. using f.8. Such exposures which allow in same amount light but with a different combination of shutter speed and f.number are called equivalent exposure. The principle is: You lessen the exposure using a faster shutter speed, so increase the exposure using a smaller f.number or vice versa. So, from our present example, some of the equivalent exposures are:
1/125s at f.11 or 1/60s at f.16

1/250s at f.8 1/30s at f.22

1/500s at f.5.6 1/15s at f.32

1/1000s at f.4 1/8s at f.45

…………………… ……………

On the other hand, instead of choosing a different shutter speed and correspondingly changing the f.number, you might even choose a different f.number first and correspondingly change the shutter speed for equivalent exposure. For example, in place of the exposure of 1/125s at f.11, you want to use f.16 (larger f.number gives greater depth of field, that means more sharp areas in front and behind your subject). What you want do is that you will half the exposure with the help of f.number; so, double it with the help of shutter speed. So, your equivalent exposure becomes f.16 for 1/60s.

Sometimes we may even have to increase or decrease the exposure. Suppose in the last film you have exposed, one of the photographs came out darkish; that means the photograph was underexposed and you want to retake the photograph. You use the same ISO film and under the same light condition.

Last time the exposure given was say 1/125s at f.8. You guess that if the exposure is doubled, the photograph will come out well. If you want to double exposure with shutter speed of 1/60s at the same f.number of f.8 and your exposure becomes 1/60s at f.8. but might even double the exposure with f.number and keep the shutter speed as the same as before. So, this time your exposure becomes f.5.6 for 1/125s. Have you noticed that exposures 1/60 s at f.8 and 1/125s at f.5.6 are equivalent?

You can double or quadruple the exposure or cut it to half or to one-quarter etc. When you double the exposure, photographically you will say that you have increased the exposure by one stop. (In practice by one step in the f.number or shutter speed series). Increasing the exposure by two stops quadruples the exposure. The same way, decrease the exposures.

This is for today. Next time we will learn to take a photograph by trying to give correct exposure. Happy re-shooting.