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Category: Hobbies & Recreation : Photography
THE EXPOSURE FORMULA: HOW THE FILM SPEED, SHUTTER SPEED AND f STOP WORK TOGETHER
You may well have been wondering how film speed, shutter speed, and f-stops relate to each other.
As I said earlier, film speed is separate and independent from shutter speed and f-stop, BUT it will 'influence' those other factors in determining which settings to choose and set on the camera. That is, if you use a slower speed film, then you will likely need to use different aperture (f-stop) and/or shutter speed for a good exposure, than would be required for a faster speed film.
Alternatively, with the same speed film, if you decide to change the shutter speed, then you will also need to adjust the f-stop to compensate for the resulting different exposure.
In photography, there is a very basic, but fundamental rule of thumb which applies to MOST normal lighting and exposure conditions, regardless of film speed. Without getting too technical, it is called the Reciprocity Law.
It goes like this:
E (exposure) = T (time) x I (intensity)
Remember, the time is controlled by the shutter speed, and intensity is controlled by the f-stop (aperture) on the lens.
So logically, for every change in shutter speed, there must be a RECIPROCAL and equal amount of change in f-stop.
If you:
Increase the time by using a slower shutter speed, then to adjust to the correct exposure, you Decrease intensity by using a smaller aperture (larger f-number).
 or 
Or in the other direction, if you:
Decrease the time by using a faster shutter speed, then to still get the same amount of exposure on film, you Increase intensity by using a larger aperture (smaller f-number).
Let's use some examples here:
1. With ISO 100 rated film, your light meter might suggest an exposure of 1/125 second shutter speed with an f-stop of f 8.
Instead, for whatever reason, you might wish to use a shutter speed of 1/250 second, which is decreasing the exposure time. To compensate and achieve the same exposure result, you would adjust the f-stop to f 5.6, which increases the exposure intensity.
2. If with the same amount of light, you may wish to use a faster speed film, say, ISO 400. Then one or both of the shutter speed and f-stop will be changed to produce less exposure.
A very effective way of understanding this exposure rule, is to compare it with filling a bucket or basin with water.
Your bucket has a capacity of so many gallons (which would loosely correspond to the film speed) needed to fill it (like film getting enough exposure). If you turn on a tap at full burst so the water pours out (like a lens' maximum aperture), this will require a shorter time (like faster shutter speed) to fill the bucket. Or to fill that same bucket, if you turn on the tap so that the water only trickles out (using a small aperture), then a longer time (slower shutter speed) will be required to fill it.
I hope this article has clarified your understanding of exposure and the basic camera functions.
In my next articles, I will cover such topics as film types, lens focal length and depth-of-field.
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Submitted By
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David Barry |
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Photo enthusiast |
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