This episode is part of a technical series. We will talk about some basic principles that will be revisited in later episodes. Don’t worry if it doesn’t all sink in now as everything will be repeated in a different way later on.

When you take a photograph by pressing the shutter button you are exposing the sensor to light coming from the subject through the lens.

Exposure is all about getting the amount of this light just right. Complete books have been written on the subject. But the truth is there is no magic to it: all you can do is make the image brighter or darker.

The amount of light hitting the sensor is the product of time and intensity. Time is controlled by the shutter, and intensity by the size of the lens opening. Making one bigger or smaller brightens or dims the image. Making one bigger and the other smaller by a maching amount keeps the image the same brightness but has other effects, which we will cover later.

In the early days of photography, the exposure time was controlled by removing and replacing the lens cap. Exposure could take minutes as the photographic plates were not very sensitive to light. So you would remove the lens cap, count a certain number of seconds and then replace it. Victorian portraits look grim in part because the subjects had to stay absolutely still. Often helped by headrests. Ansel Adams, who was still shooting in the late 20th century, started with this technique.

Soon lenses had shutters built into them controlled by a spring mechanism. They would be cocked and then released by a lever, cable, or string for a precisely controlled exposure time. At some point, we started calling this exposure time “shutter speed”.

Originally the opening in the lens was controlled by small brass plates with a hole in them called “Waterhouse stops”. They were slid one at a time into a slot on the side of the lens. They stopped some of the light from getting to the film and so provided another way of controlling exposure.

As an aside, you can buy a brass replica of one of these early lenses, complete with Waterhouse stops, that will fit a modern camera.

Later a mechanical iris was used, which could easily control the size of the aperture formed by its blades. A lever or ring on the lens allows you to alter the aperture easily.

The eye, like the ears, or even the muscles, is not linear. As the light gets brighter the eye becomes less sensitive. This allows it to operate over a wide range of light levels, from moonlight to desert sun.

This phenomenon is even more obvious with hearing. Imagine you were listening to a factory with a thousand machines working away making noise. How many do you think would have to be turned off before you would notice a difference? The answer for many people is half of them.

The consequence of this is that a doubling or halving of light intensity looks to us as a minor change. So the aperture control on a lens is marked in terms of doubling. Moving from one mark to another doubles or halves the light passing through the lens. These marks are called “stops”, after the Waterhouse stop. And the term “stop” has grown to be the unit of measurement when talking about exposure. A one stop change in exposure is a doubling or halving of light. Either by changing the time, or the aperture. A two stop increase would mean a four times increase in light. You would achieve this by moving the aperture lever two marks, or by quadrupling the exposure time.

I mentioned that early photographic plates were not very sensitive. They required lots of light to form an image. But by the late 20th century we had developed roll films that were very much more sensitive to light.

The sensitivity of a film to light is called its “speed”. This word crops up a lot in photography.

The speed of a film was measured using a scale developed by the American Standards Association. Normal film had a speed of 100 ASA, and Fast film was 400 ASA. That’s a difference of only two stops. I often used Kodachrome which was only 64 ASA and the fastest film I ever used was 1000 ASA.

When this standard became global it was named after the organisation that adopted it: ISO.

So now your digital camera, in addition to the traditional exposure controls, has an ISO setting that allows you to adjust the sensitivity of your sensor. So you are no longer stuck with whatever film you put in the camera.

What a time to be alive.

The text of this episode can be found on the website photographywithneil.com

That’s it for now.

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