What Is an F-Stop?
In photography, an ‘f-stop’ or ‘f/stop’is a measurement of the exposure. Exposure is made from three settings, which make up the exposure triangle. These settings are the shutter speed, ISO and aperture settings.
If you were to increase the exposure by one f-stop, you would be doubling the exposure. Decreasing the exposure by one f-stop is halving it.
So, for example, your camera’s aperture is f/4, shutter speed is 1/100 and ISO is 100. If you keep the aperture at f/4 and the shutter speed at 1/100, but you increase the ISO to 200, then you have increased the exposure by one f-stop.
Doubling the ISO makes the exposure twice as amplified; hence the settings jump in single stops.
It can get a little bit confusing, but you must learn this, and here’s why…
As your photography skills improve, you will start to shoot in manual mode more. You will gain more and more control over how the camera exposes the scene. Knowing what one f-stop can do for the shutter speed, ISO and aperture will affect how you change each one.
Let me make this simpler for you
You’re shooting at f/2.8, at 1/100 of a second, with an ISO of 200 but you want a shallower depth of field. You know that widening your aperture opening to f/2 will produce a shallower depth of field.
Perfect! But it will also double the amount of light that’s entering your camera lens. You have jumped up one f-stop with your aperture and made the exposure brighter.
You need to counter this with a change in shutter speed or ISO. To do this, you can halve the ISO to 100 or double the shutter speed from 1/100 to 1/200 of a second.
Let’s start with the easiest to understand: ISO. One stop up from ISO 100 is 200. And one stop up from ISO 200 is 400.
The intervals aren’t equal but, instead, the ISO doubles between stops. Easy enough to understand, so I’ll leave it at that.
Shutter Speed Stops
The majority of the time when you use your digital camera, you’re shooting at a fraction of a second. If you shoot at speeds of 1 second or longer, the same principle as above applies. You double the time from 1 second to 2, then from 2 seconds to 4. Simple.
When shooting at a fraction of a second, such as 1/200, to double this number, halve the denominator (the number on the bottom of the fraction, in this case, 200).
If you’re new to photography, don’t worry; this will soon become second nature.
I’m afraid this is where things get a little bit complicated and somewhat mathematical.
If you use the logic that I’ve explained above, you would probably assume that f/2 is twice the exposure of f/4. Sadly, this is not the case, and f/2 actually allows in four times as much light as f/4.
You might be scratching your head at this but, I promise, it will all become clear if you can just stick with it.
The aperture scale does not take on the same principles as shutter speed or ISO because of how the measurement is taken.