EXPOSURE 2 -
Hopefully you understood the first page enough to know that the Aperture scale shown again here
controls the amount of light passing through our lens. Reading from left to right each stop will halve the light reaching our sensor. We have a controlled way of adjusting the light which is independent of focal length, manufacturer and all other variables. Even zoom lenses that can alter their focal length will alter the diameter of their aperture as you zoom to keep the light falling on our sensor constant at any aperture setting.
2. Shutter Speed. As we have seen Aperture controls the Amount of light hitting our sensor. The other control we have is Shutter Speed which controls how long we allow the light to act on our sensor.
Shutter speed is easier to understand, again we are using fractions, but this time the numerator is always 1. We get values of say 1/100th of a second or 1/25th of a second.
Here is a scale of shutter speeds, your camera may vary them slightly, but the relationship between them will always be the same. They are all twice as fast or half the speed of the adjacent number. Notice I have put the scale with the fastest speeds to the left, you’ll soon see why.
It’s when we put the two scales together you begin to see the point (I hope).
In the scale above we have assumed that with the light level available (the Ambient Light) a setting of f8 for 1/100th of a second (highlighted yellow) gives us a correct exposure, so will f4 for 1/400th of a second (4 times the amount of light, but for only a quarter of the time. highlighted red) or even f64 for 1 second (highlighted green), in fact any of the vertical pairs give us exactly the correct amount of light (aperture) for the correct length of time (shutter speed), in other words all the vertical pairs will give us a correct Exposure.
Another function of Aperture is that it also controls Depth of Field, the wider the aperture the narrower the Depth of Field is and vice versa, the smaller the Aperture the greater the Depth of Field is. This will be explained in another article. Here.
Note the above scale is only correct for one ambient light level, as we know this varies quite a bit, so we would have to slide the top f number scale to the left or right leaving the shutter speed scale where it is. So in bright conditions f8 might give a good exposure at 1/800th of a second, if we slide the f number scale to the left so it corresponds with 1/800th, then the shutter speed for f4 would be 1/3200, for f64 it would be 1/12th of a second, The relationship holds true.
Our metering system looks after sliding the scales, the important point being that at any light level there are choices you can make, if you select a fast shutter speed for an action shot for instance, you or your camera will have to compensate by having a wider aperture to maintain that critical level of light on the sensor, you can’t have both a small aperture and a fast shutter speed.
Aperture and Shutter Speed work together, like the a set of scales, if you increase one, you have to decrease the other by the same amount to maintain the balance, in Photography this balance is the level of light required by our sensor for a correct Exposure.
The last thing to note is I don’t know of any lens that offers a range of aperture values raging from f1 all the way through to f128. A high quality (read expensive) lens may offer f2.8 to f32, a cheaper lens may only have a range of f3.5 to f22. Your camera on the other hand will almost certainly offer you a wider shutter speed range than shown here. 1/4000th of a second to 30 seconds is common.
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