Diffraction is the slight spreading of light as it passes through the opening in your lens, and it is the reason photos get softer when you stop down to small apertures like f/16 and f/22. It matters because it sets a ceiling on sharpness that no lens, camera or technique can lift, so choosing an aperture is always a trade between more depth of field and less fine detail.
The butte above was shot at f/8 with a 90mm lens. The subject is one distant object, so it did not need the depth of f/16, and f/8 kept the fine ridges in the rock crisp. This page shows why that choice works: what diffraction is, how big the blur really is in micrometers and in pixels, when you will see it and when you will not, and when stopping down is still the right call.
What diffraction is, and why every lens has it
Light travels as a wave. When a wave passes through an opening, it does not keep a perfectly straight edge; it fans out a little past the rim. You can see the same thing with water waves spreading after they pass through a gap in a harbor wall. In a camera, the opening is the aperture, so every beam of light that reaches the sensor has been spread slightly by it. This is physics, not a flaw in the glass. A perfect lens would still have it.
The amount of spread on the sensor depends on the f-number, not on the physical size of the hole. That surprises people, because the hole in a 600mm lens at f/8 is 75mm across while the hole in a 24mm lens at f/8 is only 3mm. The smaller hole spreads light at a wider angle, but the shorter lens puts the sensor closer, so the light has less distance to spread before it lands. The two effects cancel exactly. At the same f-number, every lens makes the same size of diffraction blur on the sensor.
What diffraction looks like in a photo
Diffraction does not soften one part of the frame the way missed focus or a moving subject does. It dims fine detail and small-scale contrast everywhere at once, including on the exact plane you focused on. Foliage, fabric, hair and distant brickwork lose their bite and look slightly mushy at 100%, while the picture as a whole still looks fine at small sizes. The clearest way to recognize it is by comparison: shoot the same detailed scene from a tripod at f/8 and at f/22 and view both at 100%. If the f/22 frame is softer even where both are perfectly in focus, you are looking at diffraction.
The Airy disc: how a point of light becomes a spot
Point a lens at a single star. Because of diffraction, the lens cannot focus it to a true point. It forms a small bright spot surrounded by faint rings, a pattern called the Airy disc after the astronomer George Biddell Airy, who described it in 1835. About 84% of the light lands in the central spot, so that spot is what counts for sharpness.
Its diameter is simple to work out: 2.44 times the wavelength of the light times the f-number. For green light, about 0.55 micrometers (µm, thousandths of a millimeter), that is about 1.34 µm for every unit of f-number. Every point in a scene becomes one of these spots, and fine detail is the spots of neighboring points sitting close together. When the spots get big enough to blur into each other, that detail is lost.
| f-number | Airy disc (green light) | Width on 6.0 µm pixels | Width on 3.9 µm pixels |
|---|---|---|---|
| f/2.8 | 3.8 µm | 0.6 pixels | 1.0 pixels |
| f/4 | 5.4 µm | 0.9 pixels | 1.4 pixels |
| f/5.6 | 7.5 µm | 1.3 pixels | 1.9 pixels |
| f/8 | 10.7 µm | 1.8 pixels | 2.8 pixels |
| f/11 | 14.8 µm | 2.5 pixels | 3.8 pixels |
| f/16 | 21.5 µm | 3.6 pixels | 5.5 pixels |
| f/22 | 29.5 µm | 4.9 pixels | 7.6 pixels |
| f/32 | 42.9 µm | 7.2 pixels | 11.0 pixels |
Each full stop multiplies the f-number by about 1.4, so each stop makes the disc about 1.4 times wider, and two stops double it. The color of the light matters too: at f/16, blue light (450 nm) makes a disc about 17.6 µm wide and red light (650 nm) one about 25.4 µm wide. Green sits in the middle and is the color the eye is most sensitive to, which is why it is the usual reference.
Not sure why f/16 is a smaller opening than f/4, or what the numbers mean? F-stops explained shows how the f-number is worked out and why the scale runs the way it does.
When the spot outgrows the pixels
A disc smaller than a pixel does no visible harm: the pixel grid is the coarsest part of the system. Once the disc spreads across several pixels, neighboring pixels start to record a blend of the same points, and fine detail loses contrast. A color sensor records each color on only some of its pixels, through a mosaic of color filters, and then works out the rest (how image sensors work explains the mosaic), so it already blurs at the scale of a pixel or two. In practice, diffraction starts to show at 100% view when the disc is about 2 pixels wide and becomes plain by about 3 pixels. The f-number where it starts to show is often called the camera’s diffraction-limited aperture, or DLA.
That gives a rule you can use for any camera. Work out the pixel pitch: sensor width in millimeters divided by the number of pixels across, times 1,000 for micrometers. Then:
- Diffraction starts to show at 100% at about 1.5 times the pixel pitch, as an f-number. (A disc 2 pixels wide: 2 x pitch / 1.34.)
- It is plain at 100% at about 2.2 times the pixel pitch. (A disc 3 pixels wide: 3 x pitch / 1.34.)
| Sensor | Pixel pitch | Starts to show at 100% | Plain at 100% |
|---|---|---|---|
| Full frame, 24 MP (6,000 pixels across 36 mm) | 6.0 µm | about f/9 | about f/13 |
| Full frame, 45 MP (8,192 across) | 4.4 µm | about f/6.6 | about f/10 |
| Full frame, 60 MP (9,504 across) | 3.8 µm | about f/5.6 | about f/8.5 |
| APS-C, 24 MP (6,000 across 23.5 mm) | 3.9 µm | about f/5.8 | about f/8.7 |
| Micro Four Thirds, 20 MP (5,184 across 17.3 mm) | 3.3 µm | about f/5 | about f/7.5 |
These are the apertures where you will notice a change at 100% on screen, not the apertures where photos go bad. The “diffraction limit” is a gentle slope, not a cliff. The next table shows how gentle. It gives the contrast a perfect lens can still deliver in the finest detail each sensor can record (one line pair across two pixels), from diffraction alone, in green light.
| f-number | Finest detail on 6.0 µm pixels (83 line pairs per mm) | Finest detail on 3.9 µm pixels (128 line pairs per mm) |
|---|---|---|
| f/4 | 77% | 65% |
| f/5.6 | 68% | 51% |
| f/8 | 54% | 32% |
| f/11 | 39% | 12% |
| f/16 | 16% | 0% |
| f/22 | 0% | 0% |
Read it slowly, because it holds two lessons. First, contrast fades step by step as you stop down; there is no aperture where it suddenly collapses. Second, the denser sensor’s column looks worse only because it is measuring finer detail. At 83 line pairs per mm, the 3.9 µm sensor gets exactly the same contrast from the lens as the 6.0 µm one, and it can still record coarser detail at every aperture. More pixels never give you less detail. At f/16 the high-resolution camera has simply run out of extra detail to add, so its advantage shrinks toward nothing.
Wondering how pixel size and sensor size differ, and what each one changes? Camera sensor sizes sets pixel pitch beside the size of the sensor for the common formats.
Pixels decide what you see at 100%; sensor size decides what you see in a print
Looking at a file at 100% compares the blur to one pixel. Looking at a print, or a photo filling a screen, compares the blur to the whole frame. For that, the number that matters is how big the disc is compared to the sensor, and that depends on sensor size, not megapixels.
At f/16 the disc is 21.5 µm on every sensor. On full frame (43.3 mm corner to corner) that is about 1/2,000 of the diagonal. On Micro Four Thirds (21.6 mm) it is about 1/1,000, so in two prints of the same size the blur is twice as large on the smaller sensor. A common yardstick for “sharp in a print viewed at normal distance” is a blur no bigger than the diagonal divided by 1,500, the same circle of confusion used for depth of field. Here is where the disc reaches it:
| Format | Diagonal | Crop factor | f/16 disc as a share of the diagonal | Disc reaches the print yardstick at | Full frame f-number that matches f/16 here |
|---|---|---|---|---|---|
| Full frame | 43.3 mm | 1.0 | 1/2,000 | about f/22 | f/16 |
| APS-C | 28.2 mm | 1.5 | 1/1,300 | about f/14 | about f/24 |
| Micro Four Thirds | 21.6 mm | 2.0 | 1/1,000 | about f/11 | f/32 |
The last column is the useful one. Multiply the f-number by the crop factor and you get the full frame f-number that gives the same depth of field and the same diffraction blur at the same framing. So a smaller sensor does not escape the trade. It reaches the same depth of field at a wider f-number, and at that matching f-number its diffraction is the same as full frame’s. Diffraction and depth of field move together; only the labels on the aperture ring change.
Phones show this from the other end. Their lenses are so short that they get deep focus at a fixed wide aperture, around f/1.8, and they do not stop down. Even so, the disc at f/1.8 is about 2.4 µm, which on the 1 to 1.2 µm pixels of a typical phone sensor is already about 2 pixels wide. That is one reason most phones have no adjustable aperture: with pixels that small, stopping down would add mostly diffraction. A phone on a typical 1/1.3 inch sensor at f/1.8 behaves, for depth of field and diffraction, like a full frame camera at about f/6.5.
Want the full picture of what a smaller sensor changes and what it does not? Crop factor covers field of view, depth of field, light and noise at matched framing.
The sweet spot: aberrations against diffraction
If diffraction grows as you stop down, why are most lenses not sharpest wide open? Because a second force pulls the other way. At wide apertures, lens flaws such as spherical aberration, coma and chromatic aberration blur the image, and stopping down cuts them by blocking the edge rays that cause the most trouble. Sharpness peaks where the shrinking aberrations and the growing diffraction cross. That aperture is the sweet spot.
For many lenses the sweet spot is one to three stops down from the widest aperture, often around f/5.6 to f/8 on full frame, but it varies with the lens, and the corners usually peak a stop or so later than the center. A very good lens moves the sweet spot wider because it has less aberration to correct. It cannot move the diffraction line at all; by f/16 every lens is close to the same. The only way to know your own lens is to shoot a test, which how to test a lens sets out step by step. Understanding lens optics explains the aberrations in more depth.
When stopping down is still the right call
Diffraction is a cost, not a rule against small apertures. If the scene runs from a few meters away to the horizon, a frame where everything is a little soft looks far better than one where the foreground is a smear. The road above was shot at f/14 for exactly that reason. At normal viewing size the slight loss from diffraction is invisible, while a foreground out of focus would be obvious at any size.
The smart move is to use as little aperture as the depth needs, not as much as the lens allows:
- Find the depth you need first. Focus at the hyperfocal distance and you get the most depth any f-number can give. Often that means f/8 or f/11 will do what you thought needed f/22.
- Stop at the aperture that just covers it. If f/11 holds the scene, f/16 adds diffraction and nothing else. A depth of field calculator shows the near and far limits at each aperture, so you can find the widest one that still covers your scene.
- Need more depth than f/11 or f/16 gives? Shoot several frames at the sweet spot, each focused at a different distance, and merge them. That is focus stacking, and it gives both depth and peak sharpness for subjects that stay still. A tilt-shift lens is another route: it tilts the plane of focus to lie along the ground.
Try the trade-off here: in this simulated canyon landscape, get the nearest rocks and the far horizon both sharp without going past f/16, then push to f/22 and use Magnify to see diffraction soften the whole frame.
Want to check you can pick the right aperture for the depth a scene needs? The aperture and depth of field quiz asks you to choose, and explains each answer.
Macro: the f-number you set is not the one you get
Close up, the lens sits farther from the sensor than its focal length, so light spreads over a longer path and the effective f-number rises. Roughly, effective f-number = marked f-number x (1 + magnification). At life size (1:1), a marked f/16 works like f/32, with a disc about 43 µm wide, 7 pixels on 6.0 µm pixels. At half life size it works like f/24. This is why some cameras show a larger f-number than you set when you focus close, and why macro photography hits diffraction much sooner than landscape work does. It is also why macro shooters lean on stacking.
Need more depth than one frame can give without the softness of f/22? Focus stacking shows how to shoot and merge the frames.
Sunstars: diffraction you can see and use
Diffraction is not only a cost. A bright point of light shot at a small aperture spreads into a star, because each straight edge of the aperture blades diffracts light into a line at right angles to it. In the photo above, f/16 turned the sun into a star of sharp rays at the edge of the rock.
- Blade count sets the points. An even number of blades gives that many points (8 blades, 8 points), because opposite blades share a pair of rays. An odd number gives twice as many (9 blades, 18 points).
- Smaller apertures give crisper rays. At f/11 to f/16 the star is well defined. Wide open there is usually no star at all, because the opening is a circle with no straight edges to make rays.
- Straight blades give sharper stars than rounded ones. Lenses with rounded blades for smooth background blur often make softer stars.
- Clip the source. Let the sun peek past an edge, as here, so only a small point of it is in the frame. A full sun disc tends to flare instead.
Want a star around the sun or a streetlight on purpose? How to shoot a star-shaped lens flare covers aperture, placement and what makes the rays sharp.
What sharpening can and cannot bring back
Diffraction blur is regular and predictable, so sharpening works on it better than on camera shake or missed focus. Capture sharpening, and the deconvolution tools in some raw editors, lift the contrast of fine detail that diffraction has dimmed and can noticeably recover a file shot at f/11 or f/16. Some cameras and raw converters offer a diffraction correction setting, which is sharpening tuned to the aperture recorded with the shot.
The limit is hard, though. Above a certain fineness, called the cutoff, a lens at a given f-number passes no contrast at all: about 227 line pairs per mm at f/8, 114 at f/16 and 83 at f/22. Detail finer than that never reaches the sensor, so there is nothing for sharpening to find. Detail just under the cutoff is so faint that sharpening it mostly lifts noise and draws halos. Sharpening restores what was dimmed; it cannot invent what was erased.
Common mistakes with diffraction
- Shooting everything at f/16 or f/22 “for sharpness”. Small apertures give depth, not detail. For a flat or distant subject, f/5.6 to f/8 is sharper. Use the smallest aperture the depth needs and no smaller.
- Treating the diffraction limit as a wall. Contrast fades a little with each stop. Past the onset aperture, photos get gradually softer at 100%; they are not ruined.
- Thinking more megapixels make diffraction worse. A denser sensor shows diffraction sooner at 100% because each pixel is smaller, but at the same print size it records at least as much detail as a lower-resolution one at every aperture.
- Blaming the lens, or hoping a better one will fix it. Diffraction depends only on the f-number and the light. A better lens has its sweet spot at a wider aperture; at f/22 every lens is limited by the same physics.
- Thinking a smaller sensor dodges it. At the same framing and the same depth of field, every format has the same diffraction. A smaller sensor just reaches that depth at a smaller-looking f-number.
- Forgetting the macro penalty. At 1:1, f/16 behaves like f/32. If your close-ups are soft everywhere, open up a stop or two and stack.
- Judging only at 100%. Look at the photo at the size it will be seen. A landscape at f/14 that is sharp front to back will beat an f/8 one with a soft foreground at every print size.
Try this: find where your own lens peaks and fades
Ten minutes, a camera on a tripod (or set firmly on a wall), and a flat subject full of fine detail a few meters away, such as a brick wall, tree bark or a page of print.
- Use aperture priority, your lowest ISO and the 2-second self-timer so pressing the button does not shake the camera.
- Focus once on the subject, then switch to manual focus so the focus cannot move between frames.
- Shoot one frame at every full stop from wide open to the smallest aperture: for example f/4, f/5.6, f/8, f/11, f/16, f/22.
- On a computer, view each frame at 100% at the same spot in the center, then the same spot in a corner, flicking between them.
How to judge it. Find the frame where the finest texture is crispest; that is your sweet spot. Then find the first frame that is clearly softer than it; compare that f-number with the “starts to show” figure for your pixel pitch from the table above. Finally, view f/8 and f/16 side by side at the size you normally share or print. If you cannot tell them apart there, you have learned how much depth you can buy before diffraction costs you anything you would see. With a phone, which cannot change its aperture, do the sunstar version instead: frame the sun just clipped by a roof edge or tree trunk and compare how the rays look against a frame with the full sun showing.
Questions about diffraction
At what aperture does diffraction start to matter?
At 100% view, it starts to show at an f-number of about 1.5 times your pixel pitch in micrometers: around f/9 on a 24 MP full frame camera, around f/6 on a 24 MP APS-C one. In a print viewed at normal distance, it takes about f/22 on full frame, f/14 on APS-C and f/11 on Micro Four Thirds to reach the usual sharpness yardstick.
Does a better lens reduce diffraction?
No. At the same f-number every lens makes the same diffraction disc. A better lens has fewer aberrations, so it is sharp at wider apertures and its sweet spot sits wider, but from about f/11 onward the gap between good and great lenses narrows sharply.
Is a high-resolution camera wasted at f/11 or f/16?
No. It will show diffraction sooner at 100%, but it still records at least as much detail as a lower-resolution camera at any aperture. What shrinks is its advantage: at f/16 a high-resolution sensor gains little over a 24 MP one.
Why does my camera show f/32 when I set f/16 for a close-up?
Close focusing moves the lens away from the sensor, which raises the effective f-number by roughly (1 + magnification). At life size f/16 works like f/32, and some cameras display that real value. Diffraction follows that real value, which is why close-ups soften sooner.
Can sharpening fix diffraction?
Partly. Sharpening restores contrast to detail that diffraction dimmed, and it works well on files shot at f/11 to f/16. It cannot bring back detail finer than the lens’s cutoff at that f-number, because that detail never reached the sensor.
Does diffraction happen at wide apertures?
It is there at every aperture, but at f/2.8 the disc is about 3.8 µm, about one pixel wide or less on most cameras, so you cannot see it. Softness wide open comes from lens aberrations and shallow depth of field, not from diffraction.
Should I ever use f/22?
Yes, when the scene needs the depth and nothing else will give it, when you need a slow shutter speed for long exposure and have no filter, or when you want strong star points on lights. Make it a choice for the picture, not a default for sharpness.
Is diffraction the same thing as lens flare?
No. Flare is light bouncing inside the lens and shows as haze, streaks or colored blobs. Diffraction is the spreading of light at the aperture edge, and the sunstar is the visible form of it. Both can appear in the same shot of the sun; lens flare covers the other one.