The f-stop is a fundamental measurement every photographer should be well acquainted with. If you’ve ever moved out of AUTO mode, you likely are familiar with the various f-stops possible.
There’s f/5.6, f/11, f/16 – and numerous others.
One thing is certain – these numbers can be quite confusing. What happens when you move from one f-stop to one double the size? Does the light you have to work with increase in double as well?
And how does this affect your shutter speed, depth of field, and other settings?
The Basics of F-Stops
Simply put, f-stop, also referred to as the f-number, or N, refers to the size of the opening of your lens.
This opening is measured in a fraction as a fully opened lens is typically not possible.
The fraction is formatted as the ratio of the focal length to the diameter of the opening. This formula can be expressed as follows:
N = f / D
N stands for the f-number, or f-stop. ‘f’ refers to the focal length of the lens. And D stands for the diameter of the opening of the lens.
Let’s look at a quick example.
You have a prime lens with a 50mm focal length. You open the lens so that the diameter of the opening is 25 mm.
Input the numbers into the formula and you have:
N = 50mm / 25 mm
Do some quick math and you’ll end up with an f-stop of f/2.
In other words, the lens is open halfway.
Note how the f-stop is also written as f/2. The format for writing f-stops is to have the f-number preceded by f/. This shows that the number represents a ratio of the full lens opening size.
This is where things can get a little dicey.
The Smaller the F-Stop, the Larger the Aperture?
It can be quite confusing when first learning about f-stops and aperture. Here’s why…
A large aperture with a big opening that lets lots of light in could be f/2. It could also be f/5.6
Either way, both of these aperture sizes are much bigger than an aperture of f/16.
For many beginners this can be confusing as 16 is bigger than 2. The thing to remember is that when using f-stops, we are dealing with fractions.
½ is bigger than 1/16.
The larger the denominator (the number below the ‘/’ in a fraction), the smaller the aperture opening. This will create an image with more expansive depth of field.
“When using f-stops, we are dealing with fractions.”
A smaller denominator will, conversely, create an image with a more shallow depth of field.
As you familiarize yourself more with f-stop numbers and their effects, this understanding with become quite fluent.
The trickier part to f-stops is the change in available light as you move from one stop to another.
Understanding Stops and Available Light
So we’ve been talking about f-stops, but what is a ‘stop’?
Per Wikipedia, a stop is a unit used to quantify ratios of light or exposure. Often times, for example, you’ll hear photographers say they moved up a stop or down a stop.
A stop is also the same thing as an EV unit (exposure value).
When you use your built-in camera light meter, you’ll see ‘EV’ stops of -2, -1, 0, 1, 2. A move from 0 to 1 or -1 to 0 is one stop.
Each stop up means you double available light. Each stop down means you cut available light in half.
This is where things can get a little bit confusing.
Most lenses have a standard f-stop scale, based on the powers of the square root of 2:
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
A move up or down from one number to the next is one stop.
Any of these stop changes will half or double your light.
So, if you move from f/5.6 to f/8, you will half the amount of light you have to expose with.
If you move from f/11 to f/8, you just doubled your available light.
What about if you move from f/4 to f/8?
While this does look like it’s doubling the amount of light, it’s actually not.
When you truly ‘double’ the f-stop number, you decrease the brightness by a factor of four.
So if you had an exposure at f/2, at f/4 you would have to expose that image four times as long. The reason why is because you moved two stops, doubling your available light two times.
And if you went from an f/16 to an f/8, you could speed up your exposure by four.
F-Stop Numbers Vary By Lens
The typical range of f-stops on cameras is as follows:
- f/4, 5.6, 8, 11, 16
Many newer lenses allow you to select f-stops in half-stop and third-stop increments as well.
- Half-stop: f/4 f/4.8 f/5.6
- Third-stop: f4 f4.5 f4.8
What You Need to Know
The better you are at basic division, the easier it will be to understand what happens when you go from one f-stop to another. This understanding helps you adjust your exposure settings accordingly.
With that said, a basic understanding of what happens when you double and half an f-stop is a great start.
Being aware of the standard stops also helps you know how much light you added or removed as you go from one aperture to another.
For example, going from f/1.4 to f/11 would move you up 6 stops, giving you six times the available light you had previously. This means you can either use a faster shutter speed or bump down your ISO if it’s higher.
As you work more with f-stops, you’ll get a better feel for stops.