Light spreads out as it passes through any opening. The smaller the aperture, the bigger the blurred spot each point of light becomes. At high f-numbers that spot can be several times wider than a sensor's pixels, so stopping down for depth of field costs fine detail.
f-number drives depth of field and resolution in opposite directions. Stopping down deepens focus but enlarges the diffraction spot, so compromises have to be made.
The smallest spot a lens can make can quickly exceed the size of small pixels. Then the sensor's full resolution can no longer be used with good contrast.
Pick an f-number. The value is the Airy disk diameter for green light (520 nm); the bar shows how much diffraction blur that means.
Theoretical values for a perfect lens, from Edmund Optics' imaging notes. Real lenses also lose detail to aberrations and manufacturing tolerances.
TWO NUMBERS WORTH KNOWING
A perfect lens still has a limit.
One figure on the size of the spot, one on how much detail survives.
≈ 20µm
Diameter of the blur spot at f/16, green light
At f/16 the theoretical Airy disk for 520 nm light is 20.30 µm across, eight times the 2.54 µm it is at f/2. Real lenses can only do worse, never better.
AIRY DISK · EDMUND OPTICS
120lp/mm
Finest detail at f/16, at zero contrast
The cut-off for detail falls from 687 line pairs per millimetre at f/2.8 to 120 at f/16. Sensors generally need 10% contrast or more to record detail at all, so usable detail is lower still.
DIFFRACTION LIMIT · EDMUND OPTICS
OpenStax University Physics (Rice University) calls this limit an inescapable consequence of the wave nature of light: diffraction limits the resolution of any system with a lens or mirror, from eyes to telescopes.
FOUR THINGS TO WEIGH
Choosing an aperture.
There is no single best f-number. These points from optical engineering guides explain the balance.
Know what f-number is
It is the focal length divided by the effective aperture diameter. Higher numbers mean a smaller opening.
Count the light
Raising the f-number by a factor of about 1.4 halves the aperture area and the light reaching the sensor.
Weigh depth against detail
A higher f-number deepens the depth of field but enlarges the diffraction spot. Optical guides say a compromise has to be made.
Remember close-ups
Up close, the working f-number rises: a lens at f/2.8 working at 0.5× magnification behaves like about f/4.2.
AI-generated image, not a photograph of a real person or place.
WHY PIXELS MATTER
Smaller pixels, sooner limits.
AI-generated image, not a photograph of a real person or place.
A sensor can resolve at most one line pair across two pixels, its Nyquist limit. Smaller pixels push that limit higher, which means the lens's diffraction spot overtakes the pixel size at a lower f-number.
Edmund Optics notes that diffraction becomes more of an issue as pixels continue to shrink, and that the theoretical spot size is only a starting point: lens design limits and manufacturing errors reduce real-world resolution further.
Questions
Questions people ask
What is diffraction in photography?
When light passes through any aperture it spreads, forming a bright central spot ringed by fainter circles called the Airy disk. The smaller the aperture, the larger the spot, which softens fine detail.
Does a better lens avoid diffraction?
No. Diffraction sets an upper limit on every lens. Edmund Optics notes that a perfect lens, not limited by its design, will still be diffraction limited.
How big is the blur at f/8?
For green light at 520 nm, the theoretical Airy disk is about 10.15 µm across at f/8, compared with 3.55 µm at f/2.8 and 20.30 µm at f/16.
Why do f-numbers go 2.8, 4, 5.6, 8?
Each step multiplies the f-number by about 1.4, the square root of 2. That halves the aperture area, so each step lets in half as much light.
Why does close-up work need extra care?
The working f-number rises with magnification. A lens set to f/2.8 at 0.5× magnification behaves like about f/4.2, so diffraction and light loss are greater than the ring suggests.
