An f-stop, or f-number, is the figure that describes how wide the lens aperture is. The scale looks odd at first: f/2 is a larger opening than f/8, and the numbers jump from 2.8 to 4 to 5.6 with no obvious pattern. Both oddities disappear once you see how the number is calculated.
What the number means
The f-number is a ratio:
f-number = focal length ÷ diameter of the aperture opening
The “f” stands for focal length, and the slash is a division sign. So f/4 literally means “the focal length divided by 4”. On a 50 mm lens, f/4 is an opening 12.5 mm across. On a 200 mm lens, f/4 is an opening 50 mm across.
This explains the reversed numbering. Dividing by a small number gives a large opening; dividing by a large number gives a small one. It can help to think of f-numbers as fractions: one half is bigger than one sixteenth.
The ratio is used instead of a plain diameter for a practical reason. A given f-number delivers the same intensity of light to the sensor on any lens. f/4 on a wide-angle lens and f/4 on a telephoto lens give the same exposure, so settings can be transferred between lenses.
The full-stop scale
The standard sequence is:
f/1.4 – f/2 – f/2.8 – f/4 – f/5.6 – f/8 – f/11 – f/16 – f/22
Each step to the right halves the light reaching the sensor. Each step to the left doubles it. This is one stop, the same unit used for shutter speed and ISO.
The numbers multiply by about 1.4 each time because the light admitted depends on the area of the opening, not its diameter. To halve the area of a circle, the diameter must shrink by a factor of the square root of 2, which is roughly 1.4. A useful shortcut follows from this: every second number doubles. From f/2 to f/4 is two stops; from f/4 to f/8 is two more.
| f-stop | Light compared with f/2.8 | Shutter speed for equal exposure |
|---|---|---|
| f/1.4 | 4× | 1/1000 s |
| f/2 | 2× | 1/500 s |
| f/2.8 | 1× | 1/250 s |
| f/4 | 1/2 | 1/125 s |
| f/5.6 | 1/4 | 1/60 s |
| f/8 | 1/8 | 1/30 s |
| f/11 | 1/16 | 1/15 s |
Third stops
Cameras normally change aperture in thirds of a stop, so the dial shows values between the full stops: f/2.8, f/3.2, f/3.5, f/4, f/4.5, f/5, f/5.6 and so on. Three clicks of the dial equal one full stop. Knowing the full-stop sequence by heart makes it easy to see where the in-between values sit.
The language of stops
- Stopping down means choosing a higher f-number: a smaller opening and less light.
- Opening up means choosing a lower f-number: a larger opening and more light.
- Wide open means the lowest f-number the lens offers.
- A fast lens is one with a low minimum f-number, such as f/1.8.
Using stops in practice
Suppose the camera reads f/8 at 1/60 s and a walking subject needs 1/250 s. That is two stops faster, so the aperture must open two stops to compensate: f/8 to f/5.6 to f/4. The exposure stays the same, the motion is frozen, and the depth of field becomes a little shallower.
On a bright sunny day there is a classic rule of thumb known as Sunny 16: at f/16, set the shutter speed to 1 over the ISO. At ISO 100 this gives f/16 at 1/100 s, or the equivalent f/8 at 1/400 s.
Common mistakes
- Saying “a bigger aperture” when meaning a bigger f-number. Be clear whether you mean the opening or the number.
- Assuming f/4 to f/8 is one stop because the number doubled. It is two.
- Thinking f/4 on one lens is brighter than f/4 on another. For exposure purposes they are the same.
- Expecting a variable-aperture zoom to hold f/3.5 when zoomed in.
Practice exercise
- Memorise the sequence 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.
- In Manual mode at ISO 400, set f/4 and adjust the shutter speed until the meter is centred. Take a photo.
- Stop down one full stop at a time to f/11, each time making the shutter speed one stop slower. Take a photo at each step.
- Check that all the frames are equally bright. Count how many dial clicks made one stop on your camera.
Next in this series: What Is Aperture in Photography?, Wide Aperture vs Narrow Aperture Explained and The Exposure Triangle Explained. You can also browse all photography lessons.
