What are F stops?
Imagine you have a window in your room that you can open and close to let in more or less light.
If you open the window all the way, a lot of light comes in and the room becomes bright. If you close the window a bit, less light comes in and the room becomes darker. In the same way, an f-stop controls how much light comes into the camera.
It’ called an F-stop because of it’s relationship to a mathematical formula for calculating exposure. But you don’t need to know the mathematical bit, just know that the f-stop is a number that tells us how much light the camera lens is letting in.
A low F-stop number means a big open window (and lots of light).
A high F-stop number means a more closed window (and less light).
What controls the f-stop?
The cameras aperture is what controls the f-stop.
To use a real world example, an f-stop of 2.8 is a very big aperture hole in the lens and lets in more light than an f-stop of 8, which is a very small aperture hole.
But just like opening a window too wide can let in too much light and make the room too bright, using too low of an f-stop can make your photo too bright and overexposed. On the other hand, using too high of an f-stop can make your photo too dark and underexposed.
In the next example, the shutter speed and ISO remain the same, but if we change the f stop, look what happens.
So, when you’re taking a photo, you need to choose the right f-stop to balance the amount of light coming in with the look you want to achieve in your photo.
A stop of exposure relates to ISO and shutter speed as well
A stop is a doubling or halving of a number associated with exposure
Let’s look at some examples:
Doubling the numbers
ISO: If we change the ISO from 100 to 200, that is one stop. The brightness of the picture has doubled.
Shutter speed: 1/250 to 1/125 is one stop. Amount of light has doubled. One stop difference.
Aperture: f/5.6 to f/4 is one stop. Amount of light has doubled. One stop difference.
Halving the numbers.
Shutter speed: 1/60 to 1/125 is one stop. Amount of light has halved. One stop difference.
Aperture: f/11 to f/16 is one stop. Amount of light has halved. One stop difference.
Why does it go from f/11 to f/16 – that’s not half or double?
An f-stop is a quantitative ratio, go wikipedia if you want to read about the technical and mathematical principles.
Visit the in depth page on what is Aperture to get all the info.
Stops when associated with Aperture
– The scale of f/numbers (or stops) progresses like this: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128
– Opening up or stopping down by one full stop doubles or halves the amount of light getting to the film or sensor.
– Opening up refers to making the aperture bigger (or the number smaller)
– Stopping down refers to making the aperture small (or the number bigger)
Stops when associated with Shutter Speed
– The scale of shutter speeds (or stops) progresses like this: 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250
– Doubling or halving the number, doubles or halves the amount of light getting to the film or sensor.
Stops when associated with ISO
– The scale of ISO (or stops) progresses like this: 100, 200, 400, 800, 1600, 3200, 6400
– Doubling or halving the number, doubles or halves the amount of light getting to the film or sensor.
A third of a stop
Technology allows for minute adjustments in exposure, not just in full stops.
A full stop of aperture would be f/4 to f/5.6
Modern lenses allow you to make changes in increments of a third of a stop.
For example: f/4 – f/4.5 – f/5 – f/5.6
f/4 – f/4.5 is an increase of a third of a stop.
I hope this article on f-stops was useful and you now have a grasp of what it is and how to use it. Let me know in the comments if it could be clearer or if there are areas you’d like covered in more detail.
It’s also useful to know that an f-stop is different to a t-stop – I’ve written all about f stop vs t stop here.