Lens diffraction is the bending of light waves as they pass through a small aperture opening, and it softens fine detail once you close the lens down far enough. Most lenses look best at a middle aperture, and pushing to f/16, f/22 or smaller can cost more sharpness than the extra depth of field is worth.
Why Small Apertures Soften the Image
Light does not travel as perfect straight lines when it squeezes through a tiny hole. It spreads, so a single point in the scene turns into a small disc on the sensor instead of a sharp dot. At wide apertures, lens aberrations are the main source of softness. As you stop down, aberrations shrink but diffraction grows, and at some point the second effect takes over.
That crossover is why sharpness usually rises as you close down from the maximum aperture, peaks in the middle, and then falls again.
Sensor Size and Pixel Density Matter
Diffraction becomes visible sooner on cameras with small pixels, because the spread disc covers more of them. Smaller sensors also show it at wider f-numbers than larger ones. A compact camera may show softness around f/8, while a full frame body may hold up to f/11 or so before it becomes obvious. These figures are general guidance, so run a quick check with your own gear.
Finding Your Sweet Spot
- Put the camera on a tripod facing a detailed, flat subject such as a page of small text or a brick wall.
- Use base ISO, a timer or remote release, and focus carefully with live view.
- Take a frame at each full stop from the widest aperture to the smallest.
- Open each file at 100 percent and compare the centre and the corners.
- Note the one or two apertures that look best and use them as your default for landscapes and product work.
When Stopping Down Still Makes Sense
- You need depth of field that cannot be reached any other way, and a little softness is acceptable.
- You want a longer shutter speed for moving water in bright light, where a small aperture is one tool among several.
- You are making small prints or web images, where slight loss of detail is hard to see.
If you need both depth and sharpness, consider focus stacking at a mid aperture, or use hyperfocal focusing to make the most of the depth you have. Some editing programs also offer deconvolution sharpening that partly reduces diffraction blur, though it cannot restore lost detail fully.
FAQ
Is f/22 always bad?
Not always. It costs fine detail, but for small output sizes or scenes where depth matters most, it can still be the right choice.
Does diffraction affect all lenses?
Yes, it comes from physics rather than lens quality. What differs is how good the lens is at its sweet spot and how the corners behave.
The Short Version
Diffraction is the softening that appears when the aperture gets very small. Test your lens to find its best apertures, use mid settings by default, and reach for stacking when you need more depth without losing detail.
