What is aperture? Aperture is the adjustable opening inside a lens that lets light reach the sensor, and it is measured in f-numbers such as f/2.8 or f/8. A smaller f-number means a larger opening, which gives a brighter image and a blurrier background. A larger f-number means a smaller opening, a darker image and more of the scene in focus.
Why It Matters
Aperture is the one exposure control that also changes how a picture looks. If you do not understand it, you will not know why one lens makes portraits with a soft, creamy background while another keeps every detail sharp, or why your night photos are dark at f/5.6 but usable at f/1.8.
It also trips up almost every beginner because the numbers run backward: f/2 is a bigger opening than f/8. Learning the pattern once makes every lens label and camera display easy to read.
Beginner What Is Aperture And How To Read F-Numbers
Inside every lens is a ring of thin metal blades, called the diaphragm, that opens and closes like the pupil of an eye. The hole they make is the aperture. In bright light it can shrink to a small circle. In dim light it opens wide.
The f-number is written with a slash, such as f/4. Treat it as a fraction: f/2 is a larger opening than f/4, just as one half is more than one quarter. Each standard step is called a stop, and one stop changes the light by a factor of two. The standard full-stop sequence is:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
Moving one step to the right halves the light. Moving one step to the left doubles it. The lesson on stops of light shows how this works across every setting.
What Aperture Does To Your Photo
- Brightness. A wide aperture (small number) lets in more light, which helps indoors, at night and in shade.
- Depth of field. This is the zone of the scene that looks sharp. A wide aperture makes it shallow, so only your subject is sharp and the background melts away. A narrow aperture makes it deep, which suits landscapes.
How To Set The Aperture
- Turn the mode dial to A (Nikon, Sony, Fujifilm, Panasonic) or Av (Canon). This is aperture priority: you pick the f-number and the camera picks the shutter speed.
- Turn the main command dial (usually near the shutter button) until the screen shows the f-number you want.
- Half-press the shutter button and check the shutter speed the camera chose. If it is slower than about 1/60 second for a handheld shot, open the aperture or raise ISO.
- Take the shot, then review how sharp the background looks.
Some lenses, mostly on Fujifilm and many manual lenses, have a physical aperture ring on the barrel. Others, including most Nikon and Canon lenses, are set entirely from the camera body. The mode is covered in more detail in aperture priority mode.
| F-Number | Opening | Typical Use |
|---|---|---|
| f/1.4 to f/2 | Very wide | Low light, portraits with strong background blur |
| f/2.8 to f/4 | Wide | Indoor events, portraits, moving subjects |
| f/5.6 to f/8 | Medium | Everyday shooting, groups, many lenses at their sharpest |
| f/11 to f/16 | Narrow | Landscapes, architecture, long exposures |
| f/22 and beyond | Very narrow | Rarely best; image softens from diffraction |
Common Mistakes
- Thinking a bigger number means a bigger opening. It means a smaller one.
- Shooting a group portrait at f/1.8. Only people at the same distance as the focus point will be sharp.
- Choosing f/22 for maximum sharpness. Very small openings blur the whole image slightly, so f/8 to f/11 is usually crisper.
- Ignoring the lens label. A zoom marked 3.5-5.6 cannot hold f/3.5 when you zoom in; the maximum aperture shrinks with focal length.
Advanced Why F-Numbers Look Odd And What They Measure
The Definition
The f-number N is the focal length f divided by the diameter D of the entrance pupil: N = f / D. A 50 mm lens at f/2 has an entrance pupil 25 mm across. The entrance pupil is not the physical blade opening. It is the apparent size of the opening as seen from the front of the lens, magnified or reduced by the glass in front of it. Photographers use it because it lets one number describe light gathering for any lens design.
Why The Square Root Of Two
Light collected is proportional to the area of the pupil, and area grows with the square of the diameter. Halving the light means multiplying the area by 0.5, so the diameter is multiplied by 0.707 and N is multiplied by 1.414, the square root of 2. The sequence is therefore powers of the square root of 2, rounded for printing: 1, 1.414, 2, 2.83, 4, 5.66, 8, 11.3, 16, 22.6, 32. That is why markings read 1.4, 2.8, 5.6, 11 and 22 rather than clean multiples.
Because the f-number includes focal length, the same f-number gives the same brightness at the sensor on any lens, which is what makes exposure settings transferable.
Aperture And Depth Of Field
For a fixed framing and subject distance, depth of field is roughly proportional to the f-number: doubling N doubles it. The limit is set by the circle of confusion, the largest blur spot you still accept as sharp; the lessons on circle of confusion and controlling depth of field work through the equations.
Diffraction Sets The Narrow Limit
When the opening gets small, light bends around the blade edges and a point of light spreads into a disc, called an Airy disk. Its diameter is about 2.44 x wavelength x N. For green light at 550 nm and f/8, that is 2.44 x 0.55 micrometers x 8, about 10.7 micrometers. Many modern sensors have pixels of 3 to 6 micrometers, so by f/8 the disc already covers several pixels. The result is a gradual softness, not a sudden cliff. See lens diffraction limit for the full picture.
Effective Aperture In Macro Work
The printed f-number assumes focus at infinity. Focused close, the lens extends and the light spreads over a longer path, so the effective f-number becomes N x (1 + m), where m is the magnification. At 1:1 (m = 1), f/8 behaves like f/16 for exposure. Cameras with through-the-lens metering account for this automatically, but a handheld light meter will not.
T-Stops Versus F-Stops
An f-number is a geometric ratio. It ignores light lost to reflection and absorption in the glass. A T-stop (transmission stop) corrects for that: T = N / the square root of the transmittance. A lens at f/2 that transmits 80 percent of the light has T = 2 / 0.894, about T2.2. Cinema lenses are marked in T-stops so that two different lenses set to the same T-number give identical exposure. Photographic lenses are marked in f-numbers, and the real difference between lenses is usually a fraction of a stop.
Try It
- Mount the lens you use most and find its widest aperture on the barrel (for example 1:1.8 means f/1.8).
- Place a subject about 2 meters away with something detailed, such as a bookshelf, 3 meters behind it.
- Switch to aperture priority and take one photo at each of f/1.8 (or your widest), f/4, f/8 and f/16, focusing on the same spot each time.
- Compare the background at full size. Note where it turns from creamy to clearly readable, and check whether f/16 looks slightly softer overall than f/8.
Common Questions
Why do f-numbers go up when the aperture gets smaller?
The f-number is focal length divided by opening diameter. A bigger divisor gives a smaller result, so a large opening gives a small number. It behaves like a fraction: f/2 means a diameter of half the focal length, which is larger than a quarter at f/4.
Is a lower f-number always better?
No. A wide aperture gathers more light and blurs backgrounds, but it also makes focus errors more visible and many lenses are softer wide open. Pick the aperture for the depth of field and sharpness the picture needs, not the lowest number.
Does aperture affect shutter speed?
Indirectly. Opening the aperture one stop lets in twice the light, so the camera or you can use a shutter speed twice as fast. Closing it one stop forces a speed twice as slow for the same brightness, if ISO stays put.
What does a lens marking like 70-200mm f/2.8 mean?
The focal length range is 70 to 200 millimeters, and the maximum aperture is f/2.8 across the entire range. Lenses marked with a range such as f/4-5.6 have a maximum aperture that shrinks as you zoom in.
Related Lessons
- Exposure Triangle Explained to see how aperture trades against shutter speed and ISO.
- Control Depth of Field to turn aperture choices into planned sharpness.
- Bokeh Explained to understand the quality of background blur.
- Fast Lenses to decide when a wide maximum aperture is worth the cost.
- Lens Diffraction Limit to find the narrowest useful aperture on your camera.