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What diffraction does to a photo
Diffraction is the bending of light as it passes through a small opening, and in photography that opening is the aperture. As you stop a lens down to a small aperture like f/16 or f/22, the light waves spread slightly after passing through, and each point of the scene becomes a small disc instead of a sharp point. That disc is called the Airy disc, and when it grows larger than a pixel on your sensor, fine detail starts to soften. The photo does not suddenly go blurry. It loses bite gradually, like a slightly soft print, and the effect grows as the aperture gets smaller.
The calculator above puts numbers on that softening. Enter your aperture, your sensor’s pixel pitch in micrometres, and the wavelength of light, with 550 nanometres of green light as the standard choice. It returns the Airy disc diameter and the aperture where the disc equals one pixel, which is the usual definition of the diffraction limit. At f/8 with green light, the Airy disc is about 10.7 micrometres across. On a sensor with 4 micrometre pixels, that disc already covers more than two pixels, so diffraction is softening detail at f/8, even though f/8 feels like a safe, normal aperture.
Why the limit depends on your sensor
Pixel pitch is the key variable, and it shrinks as megapixel counts rise. A 24 megapixel full-frame sensor has pixels near 6 micrometres across, and its diffraction limit sits near f/11. A 60 megapixel full-frame sensor has pixels near 3.8 micrometres, and its limit drops near f/7. A phone sensor with pixels near 1 micrometre hits its limit before f/2, which is why phone makers lean so hard on software sharpening. The same lens at the same aperture is therefore diffraction-limited on one camera and perfectly fine on another. The calculator makes that personal: enter your own pixel pitch, which you can find in your camera’s specifications or work out from the sensor size and megapixel count.
Wavelength matters less but still counts. Blue light, near 450 nanometres, diffracts less than red light near 650 nanometres, so a photo’s blue channel stays sharper than its red channel at small apertures. Green at 550 nanometres is the sensible middle value, and most diffraction tables use it. Real lenses add their own softness on top of diffraction, especially at wide apertures, so the sharpest aperture of a lens usually sits in the middle: wide enough to beat diffraction, stopped down enough to beat lens aberrations. For many lenses that sweet spot is f/5.6 or f/8.
When to ignore the diffraction limit
Diffraction is not a rule to obey blindly. Depth of field often matters more than peak sharpness. A landscape shot at f/16 may lose a little fine detail to diffraction, but it gains a foreground and background that are both acceptably sharp, and that trade is usually worth it. A macro shot at f/16 faces the same choice, and the extra depth often wins again. Diffraction softening is also gentle and even, unlike motion blur or missed focus, and it responds well to careful sharpening in editing. A slightly diffracted but correctly focused photo beats a sharp-aperture photo with the wrong focus every time.
Use the calculator to know where you stand, not to frighten yourself. If your planned aperture sits far past the limit, consider whether a wider aperture, a focus stack, or a tilt movement could solve the depth problem another way. If it sits just past the limit, shoot anyway and check the result at full size before worrying. And if you print small or share online, diffraction at f/16 may be invisible in the final image. The limit is real physics, but the photo is the final judge, and the final judge often forgives a stop or two.
Related calculators
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