Short answer: F-stop is a setting on your camera lens that directly relates to aperture and the amount of light coming through the lens. A low f-stop, e.g. f/1.4, lets in more light than a higher f-stop, e.g. f/16.
What is Aperture?
First lets quickly cover aperture, the aperture of your camera lens is the diameter of the opening letting light in to your camera. For example, 40mm.
When you adjust the f-stop on your lens you are adjusting the size of the aperture and adjusting the amount of light reaching the sensor. The lower the f-stop the wider the aperture. This is why a low f-stop is more effective in low light and a higher f-stop will prevent your photos from being overexposed on sunny days.
Focal Length Matters Too
F-stops also take the focal length of the lens into account as well.
Broadly speaking, the f-stop is focal length divided by aperture.
F-Stop = Focal Length / Aperture
So a 100mm lens with an aperture of 50mm is on the f/2 setting. And a 50mm with an aperture of 25mm is also on the f/2 setting. In this way both lenses are letting light onto the image sensor at the same luminesce.
This is why a zoom lens, one with a variable focal length, can also have a variable lowest f-stop. For instance, the kit lens for the Sony A6000 has a focal length of 16-50mm and an f-stop range of f3.5-5.6. This is because the aperture cannot open wide enough to provide an f-stop of f3.5 at a focal length of 50mm, this would require an aperture of around 14mm. However at a focal length of 16mm only a 4-5mm aperture is required for the 3.5 f-stop.
So What Are F-Stops?
F-stops are a measure of the amount of light being let on to the image sensor or film. It takes into account both aperture and focal length so that the f-stop is constant across different lenses. A lower number means more light is being let in through a larger aperture and a higher f-stop means less light is being let in through a smaller aperture.
The f-number on your camera can be set at discrete intervals from the following scale:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32
The difference between each f-stop is one stop. A stop is a unit used to measure light exposure. One stop difference is either half or double the amount of light coming in, depending on whether you are stopping down or stopping up.
F-stops are also available between the numbers in the range above and these are fractions of a stop.
Stopping Up and Stopping Down
Going from f/2.8 to f/4 is stopping down, you are halving the amount of light coming in to the camera. Going from f/11 to f/8 is stopping up, you are doubling the amount of light being let in.
Effect on Depth Of Field
The most important points about using f-stops on your camera are to keep in mind that the lower the number the more light being let in. And that a lower number produces a shallower depth of field. This is how you create background blur and the bokeh effect.
A shallow depth of field is a great way to create a photo with a professional feel, by blurring the background in a portrait you remove distraction and the same is true of photographing an object.
Then, of course, if you wish to keep as much as possible in focus, then use a higher f-stop.
What’s Right for My Photo?
This is all about experimenting and getting a feel for the results you can produce. Switch your camera to aperture priority mode, you will be able to control the f-stop and see the impact of depth of field and all other settings will be set automatically.
The minimum f-stop available on a lens is a key feature to look out for when considering a purchase or understanding the lenses you already have. If you want to create background blur or want to take photos in low light you want to get a low f-stop number. More affordable lenses with low f-stops are typically prime lenses where the focal length is not adjustable. So this is going to be a trade off in flexibility of focal length against other priorities.
Check out the kind of photos that are possible in low light with lots of background blur with an f/1.4 from North Borders:
In Depth Video on F-Stops
Here’s a great video from Dylan Bennett on YouTube. He goes into detail and covers why the numbers in the f-stop scale are so specific: