Why does a smaller F-number mean a wider aperture?
An F-number is the ratio of focal length to entrance-pupil diameter, so a smaller number usually means a wider opening. Learn about stops, depth of field, diffraction, and choosing an aperture.
An F-number is the ratio of focal length to entrance-pupil diameter, so a smaller number usually means a wider opening. Learn about stops, depth of field, diffraction, and choosing an aperture.
The F-number is the lens focal length divided by the diameter of its entrance pupil, so a smaller value normally describes a larger effective opening. On the same 50mm lens, for example, the entrance pupil is about 25mm at F2 and 12.5mm at F4. A wide aperture gathers more light and reduces depth of field. A narrow aperture extends depth of field, but stopping down too far introduces diffraction.
Why is the F-number a ratio of focal length to entrance pupil?
The physical hole made by the aperture blades cannot by itself tell you the illuminance at the sensor, because lenses of different focal lengths concentrate light differently. The F-number is written as N=f/D, where f is focal length and D is the diameter of the entrance pupil as seen from the front of the lens. Cambridge in Colour's guide to camera-lens aperture also explains the F-number as a ratio between focal length and aperture diameter. It is not a ruler measurement of the opening between the blades, nor is it a function of sensor size.
This is why F1.8 is a “wider” aperture than F4: the smaller ratio corresponds to an entrance pupil that is larger relative to the focal length. To separate actual focal length from angle of view, continue with what lens focal length means.
How do full stops and third stops work?
The area of an opening changes with the square of its diameter. To halve the area, the F-number must be multiplied by about 1.414—the square root of 2. The familiar full-stop sequence is therefore F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, and F16. Cameras also offer half- or third-stop values such as F3.2 and F3.5 for finer control.
Stopping down from F2.8 to F4 loses one stop of light. To keep the amount of light reaching the sensor similar, you can change 1/250 second to 1/125 second, or raise ISO 200 to 400 when the shutter cannot be slowed. See the exposure triangle explained for the complete relationship.
How does aperture affect both exposure and depth of field?
With focal length, shooting distance, and viewing conditions held constant, a wider aperture usually gives shallower depth of field, while a narrower aperture extends the range that appears sharp. But the amount of background blur also depends on subject-to-background distance, focal length, framing, and the lens's out-of-focus rendering. What determines depth of field breaks down all of these variables.
Wider is not automatically better. In a close portrait at F1.2, only one eye may fall within the depth of field; a group portrait often requires stopping down. Conversely, F16 may increase geometric depth of field in a landscape while diffraction softens fine detail across the whole frame.
Why does maximum aperture affect lens size and price?
Maintaining edge quality with a larger entrance pupil usually requires larger glass elements, more complex aberration correction, and tighter mechanical tolerances. The effect is especially obvious with telephoto lenses: the theoretical entrance-pupil diameter of a 400mm F2.8 lens is about 143mm, compared with about 71mm for a 400mm F5.6. The faster lens therefore needs substantially larger elements and barrel, and tends to be heavier and more expensive.
Maximum aperture alone does not guarantee overall sharpness, focusing speed, or build quality. A modern variable-aperture zoom can perform very well across its useful range. The main advantages of a constant F2.8 zoom are low-light capability, depth-of-field control, and consistent exposure while zooming. See constant versus variable-aperture lenses.
How is diffraction related to a lens's “best aperture”?
When light passes through a small opening, it diffracts and a point source forms a finite Airy disk on the image plane. Diffraction spreads more as the aperture gets smaller. This guide to diffraction and Airy disks shows the relationship among aperture, wavelength, and Airy-disk size. A high-pixel-density sensor may reveal pixel-level softening at a wider aperture, although the visible difference also depends on output size, sharpening, lens aberrations, and viewing distance.
At maximum aperture a lens may be limited by spherical aberration, coma, or weak corners, and it often improves after stopping down one or two stops. Stop down further and diffraction gradually becomes dominant. There is no single “best aperture” for every lens and every use. Choose for the final image, not just for peak center sharpness in a laboratory test.
Practical starting points for different subjects
- Individual portraits: start around F2–F4 and check that both eyes and the important facial features are within the depth of field.
- Group portraits: start around F5.6–F8 and place people as close to the same focus plane as practical.
- Street photography: F4–F8 often strikes a useful balance among light, depth of field, and response time.
- Landscapes: try F5.6–F11 first and choose the focus position carefully; do not default to the lens's smallest aperture.
- Night sky: use a wide aperture within the limits of acceptable corner aberrations, then balance shutter speed and ISO.
These are only starting points. Magnify the important parts of each image and confirm that depth of field, shutter speed, and image quality serve the subject in front of you.
How can you shoot an aperture series to find your own usable range?
Mount the camera on a tripod and choose a scene with nearby text, a mid-distance subject, detail in the corners, and distant point lights. Use Aperture Priority or Manual mode and shoot every full stop from the lens's maximum to minimum aperture. Each time you stop down, lengthen the shutter by one stop. Keep ISO, focus, white balance, and in-camera sharpening fixed so brightness remains comparable.
Back at the computer, inspect four things at the size in which you will actually deliver the image: the sharp range in front of and behind the subject, center and corner detail, starbursts around point lights, and any movement introduced by the slower shutter. Record the first aperture that gives enough depth of field and the point where diffraction becomes visible in the final output. If a landscape has a soft foreground at F8, is sharp throughout at F11, and loses a little fine detail at F16, then F11 is the sensible choice for that composition. Test again after changing focal length, distance, or output size; one result is not a permanent “best aperture” for the lens.