Optics May 4, 2026 5 min read

Diffraction: Why Stopping Down Too Far Makes Your Photos Softer

Smaller apertures give more depth of field. Past a certain point they also give a softer picture, everywhere, including the parts that are in focus. Where the trade turns against you depends mostly on how many pixels your sensor has.

What is happening

Light is a wave, and when it passes through a small opening it spreads out a little at the edges. In a lens the opening is the aperture. Each point of light in the scene lands on the sensor not as a point but as a small bright disc with faint rings around it, called the Airy disc. Its diameter is about 2.44 times the wavelength of the light times the f-number, which for green light works out at roughly 1.3 micrometres for each unit of f-number. At f/8 the disc is about 11 micrometres across, at f/16 about 21, and at f/22 about 29. No lens design gets round this. It is physics, not a flaw in the glass.

Whether that matters depends on how big a pixel is. A disc a pixel or two wide does no visible harm. Once it covers several pixels, neighbouring detail starts to blend. Here is the disc's width in pixels on common sensors, worked out from sensor width and pixel count:

Sensor	Pixel size	f/8	f/11	f/16	f/22
24 MP full frame	6.0 µm	1.8	2.5	3.6	4.9
33 MP full frame (Sony A7 IV)	5.1 µm	2.1	2.9	4.2	5.7
45 MP full frame (Canon R5, Nikon Z8)	4.4 µm	2.4	3.4	4.9	6.7
61 MP full frame (Sony A7R V)	3.8 µm	2.8	3.9	5.7	7.8
26 MP APS-C	3.8 µm	2.9	3.9	5.7	7.8
40 MP APS-C (Fuji X-T5)	3.0 µm	3.5	4.9	7.1	9.7
20 MP Micro Four Thirds	3.3 µm	3.2	4.4	6.4	8.8

As a rough guide, when the disc reaches 2 pixels you can start to measure a loss, and at 3 pixels you begin to see it at 100 percent. That works out at about f/9 to f/13 on 24 MP full frame, f/6.5 to f/10 on 45 MP, and f/5.6 to f/8.5 on 61 MP full frame or 26 MP APS-C. These are rules of thumb, not thresholds. The softening grows gradually, and at a normal size on a screen you may never notice it.

It is not the same problem as depth of field

Depth of field describes which distances look sharp. Diffraction softens everything, including what you focused on. You can have a deep zone and a soft picture (f/22 on a high-resolution camera), or a thin zone and a sharp picture (f/1.8 with careful focus). The sensible aperture is usually the one that gives enough depth of field without letting the disc grow past a few pixels.

Your lens has a sharpest aperture too

Lens aberrations push the other way. Wide open, most lenses are a little soft, and they sharpen up as you stop down, then diffraction takes over. For many lenses the peak lands somewhere around f/4 to f/8, with the corners often peaking a stop later than the centre. It varies from lens to lens, so a quick test is more reliable than a rule. Shoot a flat, detailed subject on a tripod at each stop from wide open to f/22 and compare at 100 percent.

Where it bites

In landscapes, f/16 on a 45 MP camera puts a 4.9 pixel disc on every point. If the shot needs more depth than f/8 to f/11 can give, a second frame focused on the foreground and blended in usually beats one frame at f/16 or f/22. There are good reasons to stop down anyway, such as sunstars or a slow shutter on water in daylight, and the landscape aperture post covers those.

In macro work it is worse, because the aperture you set is not the aperture you get. The effective f-number is the marked one multiplied by one plus the magnification. At 1:1 that doubles it, so f/11 behaves like f/22 and f/16 like f/32. And at 1:1 and f/8 the depth of field is only about 0.9 mm, so the pull toward small apertures is strong. The usual answer is to stack several frames at f/5.6 to f/8 (an effective f/11 to f/16 at 1:1). The focus stacking post explains how, and there is a companion piece on macro working distance.

For product shots on a tripod there is rarely a reason to go past f/8 to f/11, since you can stack if you need more. The product photography post goes into that.

Does it matter for your output?

It depends on how the picture will be seen. For social media or a small print, the softening at f/16 is invisible and the extra depth is a real benefit. It matters when you print large or look at 100 percent. Lower-resolution cameras also tolerate small apertures better: a 12 MP full-frame sensor has pixels of about 8.4 micrometres, so f/16 gives a disc of about 2.6 pixels, similar to f/11 on 24 MP.

Can software fix it?

Partly. Sharpening and deconvolution can win back some of the lost contrast, and some raw converters, Capture One and DxO among them, include tools aimed at diffraction. They cannot restore detail that was never recorded, so treat them as a way to soften the loss rather than remove it.

To check what a given aperture does to depth of field before you decide, put your lens and distance into the calculator.