One point of light on the pixels
Airy disk
A 36 µm patch of the sensor. Each square is one pixel; brighter means more light. Dashed ring: the Airy disk.
Chapter 11Part 2: Focus and depth
Diffraction and the
sharpest aperture
Stopping down gives more depth of field, but past a point the whole picture gets softer. Watch one point of light spread across the pixels as you close the aperture, and find where your lens is sharpest.
Illustrative sharpness curve
Model score relative to this illustrative lens's best aperture. Click the chart to set the aperture.
1/3 stop per step, f/2.8 to f/32.
Sensor
- Airy disk
- Pixel size
- Best in this model
- Airy disk reaches 2 pixels
Show the chart as a table
| Aperture | Airy disk | Model score |
|---|
Why light spreads
Light passing the edge of an opening bends slightly, like water waves spreading through a gap in a harbour wall. The smaller the gap, the more it spreads. So even a perfect lens can't focus a point of light to a point: it makes a small bright disk with faint rings, the Airy disk.
Airy disk ≈ 2.44 × wavelength × f-number
For green light (0.00055 mm), that is about 1.34 µm per f-number. It depends only on the f-number, not on the lens or the camera.
| Aperture | Airy disk | Compared with |
|---|---|---|
| f/4 | 5.4 µm | about 1 to 2 pixels in this lab |
| f/8 | 10.7 µm | 2 to 3 pixels |
| f/11 | 14.8 µm | about 2.5 to 4.5 pixels in this lab |
| f/16 | 21.5 µm | about 3.5 to 6.5 pixels in this lab |
| f/22 | 29.5 µm | about 5 to 9 pixels in this lab |
Your lens's sharpest aperture
Two kinds of blur pull in opposite directions. Wide open, a lens's aberrations may blur fine detail, and stopping down often helps. Stopped down far enough, diffraction takes over. The sharpest aperture depends on the lens, focus distance and where in the frame you look. Test your own lens; Nikon's focus-stacking guide suggests two or three stops down as a starting point when sharp edges matter.
Where to start
Start by comparing wide open with one and two stops down. Some lenses are already very sharp wide open; others improve as you close the aperture.
Smaller sensors are enlarged more for the same output size, so the same diffraction blur can show sooner. As a rule of thumb for equally framed pictures, diffraction on Micro Four Thirds at f/8 looks like full frame at f/16. See the diffraction explanation at Cambridge in Colour.
Pixels decide when you see it
At 100% zoom, diffraction may become noticeable when the Airy disk spans roughly two pixels, but there is no sharp cutoff. Smaller pixels reveal it sooner at that magnification; the effect on a same-size print or screen view can be much less obvious.
Test your own lens: photograph a detailed, flat subject on a tripod at every full stop. Keep focus fixed, adjust shutter speed to maintain brightness, then compare the centre and corners at 100% and at the intended output size.
The Airy disk diameter follows the diffraction formula. Pixel brightness and the lens's aberration curve are illustrative, so the model scores are not measured lens performance.
Depth of field or sharpness?
Landscapes pull both ways: you want the rock and the mountain sharp, so you stop down, and stopping down softens everything a little. The softness from diffraction is even across the frame and small, while missing depth of field is obvious. So depth of field usually wins, up to a point.
A sensible order
- Try f/8 to f/11 at the hyperfocal distance. It covers most landscapes.
- If the foreground is still soft, f/16 is fine for screens and small prints.
- Very close foreground? Focus stack at the sharpest aperture instead of going to f/22.
A common mistake
"f/22 gives the sharpest landscape." It gives more depth of field, but diffraction softens details at the focus plane compared with f/8. Foreground or background objects outside the f/8 sharp zone may still look sharper at f/22. Compare the whole picture at the size you will use.
Sharpening in editing can recover some diffraction softness, but not detail that was never recorded.