The short answer
An f-number measures the light-gathering capacity of a camera lens. It is calculated by dividing the focal length of the optical system by the diameter of the entrance pupil. You will often see it written with a lower-case hooked f followed by a slash and a number, such as f/2.
How it works
The f-number is also referred to as the focal ratio, f-ratio, or f-stop. It is mathematically the inverse of the relative aperture, which is the aperture diameter divided by the focal length. Because of this inverse relationship, a lower f-number creates a larger relative aperture that lets in more light. Conversely, a higher f-number produces a smaller relative aperture that admits less light.
Inside camera lenses, an adjustable diaphragm alters the size of the aperture stop and the entrance pupil. This mechanical adjustment lets you change the f-number depending on your creative needs. While the f-number indicates light-gathering ability, it remains an approximation because it ignores focusing distance and light transmission. When those factors matter, photographers rely on working f-numbers or T-stops instead.
Lenses with higher f-numbers project darker images onto the film or digital sensor. Specifically, image brightness decreases according to the square of the f-number. For example, a 100 mm lens set to f/2 has an entrance pupil with twice the diameter and four times the area of the same lens set to f/4. That means the f/2 setting admits four times as much light.
Stops serve as a unit for measuring exposure ratios, where each added stop doubles the light intensity and each subtracted stop halves it. Traditional f-stops follow a geometric sequence based on powers of the square root of 2, starting with values like f/1, f/1.4, f/2, f/2.8, f/4, and moving outward. Each step down in this sequence halves the light intensity by reducing the physical diameter of the pupil by a factor of about 0.7071.
How to use it
- Control light intake. Choose a lower f-number to let more light reach your sensor in dim conditions, or a higher f-number to restrict light in bright settings.
- Adjust exposure values. Moving your lens setting by one full stop alters the amount of incoming light by a factor of two.
- Manage your shutter speed. Compensate for a larger aperture opening by shortening your exposure duration to keep overall exposure balanced.
- Use standard sequences. Rely on standard f-stop increments like f/2, f/2.8, f/4, and f/5.6 to predict how much light you are adding or removing.
- Account for focal length. Remember that a longer lens with the same f-number has a larger physical pupil and collects more total light from a given subject.
Examples
If you are shooting a portrait with a 100 mm lens at f/2, the large physical opening admits four times as much light as an f/4 setting on the same lens. To keep your exposure balanced when switching from f/4 to f/2, you must reduce your shutter time by a factor of four.
When comparing different focal lengths, a 200 mm lens set to f/4 has a 50 mm entrance pupil, which is four times the area of a 100 mm lens set to f/4. While the 200 mm lens gathers four times as much light from individual subjects, it also projects an image that covers four times the total area, resulting in the same overall image brightness.
Common mistakes
- Ignoring focal length. Do not assume that identical f-numbers on lenses of different focal lengths result in the same physical entrance pupil size.
- Confusing physical stops. Remember that a physical stop is an opaque barrier inside the lens, whereas an exposure stop is a relative unit of light measurement.
- Miscalculating light ratios. Avoid thinking linear changes equal exposure changes, because light intensity scales with the square of the f-number.
- Neglecting reciprocity. Keep in mind that extremely long or short exposures may suffer from reciprocity failure where standard stop math becomes less exact.
- Overlooking transmission losses. Do not rely solely on the standard f-number if lens glass absorption and light transmission efficiency are critical to your work.
Quick checklist
- Identify your current lens focal length and maximum entrance pupil diameter.
- Determine the required f-number for your target exposure and brightness.
- Check your camera settings for whole, half, or fractional stop increments.
- Adjust your shutter speed to match any changes made to the aperture stop.
- Verify that your chosen aperture fits the standard geometric f-stop sequence.
- Consider working f-numbers if extreme close-up focusing distances alter your light levels.