Shutter Speed and f-stop settings work together in creating the exposure. The best analogy I’ve seen is to think of getting the desired exposure as a filling a bucket. It takes a certain amount of water to fill the bucket. You can pour water very quickly for a short time, or you can pour a small stream of water for a long time. Either way, you end up with a full bucket. In photography, the size of the bucket would translate to film speed (ASA or ISO rating), the size of the stream of water would be the f-stop, and the amount of time spent pouring is the shutter speed.

If you pour water either too quickly or for too long, you’re going to overfill the bucket (overexpose the film). On the other hand, if you pour too slowly or don’t pour long enough, you’ll have a partially-full bucket (underexposed film).

Here’s a look at shutter speed and f/stops

Shutter Speed

Shutter speed is the amount of time the shutter remains opened and allows light to strike (expose) the film. Each setting for shutter speed is roughly double or half the settings on either side. For example, the shutter speed settings on a typical camera might be:

8 sec, 4 sec, 2 sec, 1 sec, 1/2 sec, 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec, 1/60 sec, 1/125 sec, 1/250 sec, 1/500 sec, 1/1000 sec

You can see, for example, that 2 seconds is half of 4 seconds and twice 1 second. This pattern is typical for most cameras.

F-Stop

Contrary to how it looks, f-stops work the same way – doubling when you pick a larger one and halving when you pick a smaller one.

Now, given that the standard f-stops are:

f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22

It doesn’t seem to make sense for me to say that there’s a halving/doubling thing going on here, does it? Stay with me and I might be able to clear it up for you…

For starters, in the list above. f/1.4 is the largest setting and f/22 is the smallest. This makes perfect sense when you look at the number as a fraction with the variable ‘f’ in the numerator. the number 1/2 is considerably larger than the number 1/22.

But what’s that mean?

The next logical question is, “What does the fraction represent?” The fraction is the focal length of the lens divided by its diameter. For example, a 200mm f/4 lens is 50mm wide.

It’s actually the amount of light allowed through the aperture that’s being doubled (or halved) with each f-stop. To find the amount of light passing through the lens, we need to figure the area of the aperture. How well do you remember your elementary geometry?

Our example lens from above has a diameter of 50mm. That means it has a radius of 25mm. Now, the f/4 means that number is divided by 4, so we end up with a radius of 6.25mm. Since the aperture is circular, we can find its area using the formula πR².

If, however, we change the f-stop from f/4 to f/2.8, here’s what happens:
A 50mm lens has a radius of 25mm divided by 2.8(f-stop) gives us a radius of 8.9mm. This gives us an area of 250.4 mm – roughly twice the previous setting.

For a 50mm lens, the f-stops listed above will give the following aperture values:

If you look down the “Area” column, you’ll see that the area is cut roughly in half with each line.

The numbers are going to change for every different size lens, so, rather than photographers having to stop and go through all sorts of cumbersome math just to get their lens set correctly, the f-stop is a much less complicated abbreviation.

The f-stop setting confused me for a very long time. It wasn’t until I saw an explanation by Matthew Cole that it finally began to make sense. I hope I’ve managed to clarify this for someone else.

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