Diffraction

Diffraction is the spreading of light waves as they pass the edge of an opening, and in photography it is the reason images become softer when a lens is stopped down to small apertures. In lenses & optics it sets a physical limit on sharpness that no lens design can avoid: even a perfect lens cannot focus a point of light into an infinitely small point.

Instead, each point becomes a small blurred pattern called the Airy disk, whose size grows in direct proportion to the f-number. At wide apertures it is too small to matter; at f/16 or f/22 it can spread across many sensor pixels and visibly reduce fine detail.

How diffraction blurs the image

Light behaves as a wave. When it passes through the circular aperture of the lens diaphragm, waves from different parts of the opening interfere with each other at the sensor. The result for a single point source is a bright central spot surrounded by faint rings, the Airy pattern.

The diameter of the central Airy disk, measured to its first dark ring, is about 2.44 times the wavelength times the f-number. For green light at about 550 nanometers this gives roughly 1.34 micrometers per f-stop number: about 5.4 micrometers at f/4, 10.7 at f/8 and 21.5 at f/16. Because only the f-number appears, diffraction depends on the relative aperture, not on focal length alone.

Diffraction also depends on wavelength: red light diffracts more than blue light, so the effect is slightly stronger in red channels, although in normal photography the difference is small compared with the effect of aperture.

The diffraction-limited aperture and the sweet spot

Every lens has an aperture of best overall sharpness, often called the sweet spot. At wide apertures, optical aberrations such as spherical aberration and coma dominate and soften the image. Stopping down reduces aberrations, but at the same time diffraction grows.

Sharpness therefore rises and then falls as the lens is stopped down. For many full-frame lenses the peak lies around f/4 to f/8; on smaller sensors with smaller pixels it moves to wider apertures. The aperture beyond which diffraction becomes the main limit is the diffraction-limited aperture, which depends strongly on pixel pitch.

These values are guidelines rather than exact limits. Diffraction softening sets in gradually, and higher-resolution sensors always resolve at least as much detail as lower-resolution ones at the same aperture; they just reach their limit sooner.

Diffraction compared with other causes of softness

Softness has many causes, and diffraction is often blamed for problems it did not cause. Distinguishing them matters because each has a different remedy. Diffraction affects the whole frame evenly, unlike most aberrations, which are worse toward the corners.

Diffraction also creates the sunstar effect: bright point lights photographed at small apertures show rays caused by diffraction at the diaphragm blades. A lens with an even number of blades produces as many rays as blades, while an odd number produces twice as many rays.

Managing diffraction in practice

The main practical question is the trade-off between depth of field and diffraction. Stopping down extends depth of field, but beyond a point the gain in zone sharpness is offset by an overall loss of detail. Landscape and macro photographers face this choice most often.

• Use the widest aperture that gives enough depth of field, often combined with hyperfocal focusing.
• Use focus stacking when a scene needs more depth than f/8 to f/11 can provide without softening.
• Apply moderate sharpening in processing; deconvolution-based corrections can recover some diffraction loss.
• Accept f/16 or f/22 when depth of field or a long exposure matters more than maximum fine detail.
• Use an ND filter instead of a tiny aperture when the goal is a slower shutter speed.

In macro photography diffraction is stronger than the marked aperture suggests, because the effective aperture increases with magnification. At 1:1 magnification the effective f-number is roughly double the set value, so f/11 behaves like about f/22.

Common misconceptions about diffraction

Several myths surround diffraction. The most common is that high-resolution sensors suffer more from it. In reality they show diffraction at 100 percent view sooner, but at the same print size they never produce less detail than a lower-resolution sensor.

• Diffraction is not a lens defect; it is a property of light and affects every lens.
• Better lenses do not remove diffraction, although they reduce aberrations at wider apertures.
• Small apertures are not forbidden; they are a deliberate trade-off.
• Focal length alone does not control diffraction; the f-number does.

Understanding it simply

Light spreads out slightly when it squeezes through a small opening, much as water waves spread after passing through a gap in a harbor wall. In a camera, that opening is the aperture. The smaller the opening, the more the light spreads, and the slightly blurrier fine details become.

That is why closing the lens down to f/16 or f/22 gives more of the scene in focus but makes everything a little softer. Photographers choose a balance between depth and crispness for each shot.

Applied in practice

A landscape photographer with a 45 MP full-frame camera photographs a mountain lake with flowers one meter from the lens. At f/22 everything appears in focus, but at 100 percent view the distant peaks lack the fine texture seen in other images from the same lens.

Testing a series of apertures shows the best detail at f/5.6 to f/8, with clear softening from diffraction at f/16 and beyond. The photographer now shoots three frames at f/8, focused on the flowers, the middle distance and the mountains, and blends them with focus stacking. The final image has front-to-back sharpness and noticeably crisper rock texture than the single f/22 frame, which matters for the large prints the photographer sells.