Part 6 - Exposure
Now we are using Program Mode we need to get to grips with exposure.
Photography became possible when they discovered that silver salts turned black when exposed
to light, moreover the longer they were exposed the blacker they got and we were
off. Now we don't use chemical reactions any more in digital photography, but
in essence it is still the same, produce an image of something according to how much light
is coming from it. Of course we need a lens as well, to focus the image, but
they had those for quite a while before they found out how to record an image.
Before we go any further I need to point out that the use of silver salts leads
to a negative process; the brighter the subject, the darker the record will be.
This is overcome by repeating the process by contact printing or enlargement on to a
photographic sensitive paper. What was dark now becomes light and vice versa;
two negatives make a positive. In the digital world the sensor collects a
positive image (the more photons it counts, the brighter the pixel).
Now it takes some time to record the image, if the exposure is too short you
will have nothing and the final positive image will be black. If the exposure
is too long then the image will be overexposed and burn out to white. So
we need a method to control how long the light is allowed to pass through the
lens. Initially films had poor sensitivity so this was controlled by how
long the lens cap was off for. As the chemists got to work, and invented more
sensitive films, the need for faster exposures lead to the invention of the
It wasn't long before they discovered that if you used only the centre portion of the lens
you got a sharper image than if you used all the lens and the
aperture control was invented,
but this made the image fainter, so they compensated with a longer exposure.
We now have three ways of controlling the amount of light from a given scene;
- the shutter speed
- the aperture
- the sensitivity
There is a fourth method, the amount of light we use to illuminate the
scene, which is important in a studio set up. However outdoors, when the
sun is the light source, we can not control how bright it is, and even in a studio,
although we can turn the lights down we can't turn them up more than we have got.
So I will take the ambient light as our starting point, which we can measure with
an exposure meter or the camera will do this for us.