Water Bucket Analogy
A good analogy for exposure is to compare it to filling a bucket with water. Since a bucket is fixed in size, it needs a certain amount of water to fill it. You can compare that with film or a digital sensor which has a particular sensitivity measured as the ISO number, and, therefore, requires a certain amount of light to capture the image optimally. Too little light and we call it underexposed and shadows are not filled in. Too much light and we say it is overexposed and all the bright areas are washed out. Similarly, not enough water and the bucket is not filled. Too much water and the bucket overflows.
We can compare the size of the bucket to the sensitivity of the film or sensor. Consider a small bucket like a very sensitive sensor, a high ISO number. It is a small bucket because it doesn’t take much water (or light) to fill it. A low sensitivity sensor would be like a big bucket that requires more water (or light) to fill.
To continue the bucket analogy, you can pour a large stream of water for a short time or a small stream of water for a long time to fill the bucket. In both cases, you get the same amount of water. In a camera, the size of the stream of water is analogous to the f/stop and the length of time you “pour” is the shutter speed.
Notice that, with the bucket being filled, it doesn’t matter whether you have the large stream for a short time or the small stream for a longer time. It doesn’t matter as long as the same amount of water is put into the bucket.
There are differences when you are photographing something in motion, but in terms of exposure it doesn’t matter -- it is identical to the bucket analogy. The film or sensor is basically indifferent to the combination of time and the f/stop as long as it is the correct amount of light to obtain a proper exposure. So you can control the exposure, for a given sensitivity of film or sensor, with combinations of shutter speed (how long you pour the water) and the f/stop (the size of the stream).
Shutter speed is the easier of the two variables to explain, so it is a good place to start. On modern cameras, both film and digital, both exposure adjustments typically double or halve the amount of light with each click of the dial or change of the settings to the next value. Compare this to the channel selector on your TV. Each click of the remote, either up or down, changes the amount of light by 50% or 200%.
This may seem, at first, like a rather course amount of change, but it is usually enough fine control for picture taking. There is a very, very large variation of the amount of light in the wide, wide world that you will photograph, and the half/double per “click” of the exposure controls is actually a very fine adjustment amount. Therefore, both exposure controls run through a sequence of settings which involve doubling and halving the amount of light reaching the film.
Shutter speeds are measured in seconds and fractions of a second since it is the amount of time the shutter is open. So the math seems pretty simple. One-eight of a second is twice one-sixteenth and half of one-quarter of a second. One second is twice as long as half a second and half as long as two seconds. Check the shutter speeds on your camera and you will see similar numbers.
On my old Pentax K10000, for instance, the shutter speed sequence is:
8 seconds 4 seconds 2 seconds 1 second 1/2 second
1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000
In general, each of these settings is half or double the length of time of its immediate neighbor. There are some jumps, such as from 1/8 to 1/15 and 1/60 to 1/125. Those values came from older cameras that didn’t have as many shutter speeds and speeds such as 1/60 and 1/125 are very common throughout photographic history. Also, half of 1/15 would not be a simple integer fraction on the lower end, and 1/1000 was typically the fastest shutter in 35 mm film cameras.
My theory is that early cameras had shutter speeds around 1/60. That seems like a good "time" number since there are 60 minutes in an hour and 60 seconds in a minute. When faster shutters were developed, the decimal number 1/1000 probably took hold. If you think about it in our modern, digital age, we are used to the series 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, and 1024. But, considering the accuracy of shutter speed, there really isn't a difference between 1/16 and 1/15 nor between 1/1000 and 1/1024. So the series of shutter speeds can be considered as a series of values X2 or X1/2 even with the small difference in denominator values.
If you check out your modern digital camera, you will see that they’ve obtained much faster shutter speeds. My newest Nikon goes up to 1/8000 of a second on the high end and, on the low end, I think it goes down to somethings around two weeks for the slow shutter speed. I just made that last part up! :-)
(If your current point-and-shoot camera doesn’t allow control of shutter speed or f/stop, then this article isn’t really for you. Or you could go buy a new camera that does allow control of shutter speed and aperture -- just sayin'. Of course, even a point-and-shot camera does control these exposure adjustments, it is just all done automatically and “under the covers.”)
So this simple double or halving is all there is for the the shutter speed control. In the next installment of this article, we will look at the aperture control and f/stops. So, until then, TTFN.