Be careful when selecting a small lens aperture - it could cost you dear!
The left half of this magnified portion of our test image was shot at f/22 and the right hand side at f/5.6
I occasionally come across photos in the gallery or on the forum here on DPNow that have been taken with a very small aperture setting. Depending on the camera being used and the type of picture being taken, setting a very small aperture may result in a picture that could be noticeably sharper if a larger aperture was used instead. While there are tangible benefits in using a fairly small aperture in order to achieve greater depth of field, for example, using a very small aperture setting runs the risk of something both nasty in name and effect: diffraction limiting. The good news is that it's very easy to avoid diffraction limiting and this article will explain how and why.
What is diffraction limiting and how does it spoil photos?
The simplest explanation of diffraction limiting is a loss in resolving power when an aperture smaller than a diffraction limit threshold setting is used. In other words, if you use an aperture that is smaller than ideal for your camera and lens, your photos will be unnecessarily soft and the smaller the aperture gets past the threshold, the softer the image gets.
The red area near the centre of the frame (above) represents the cropped area reproduced in the sequence of magnified views at different apertures below.
To demonstrate this effect, have a look at the following magnified portions of the scene at the top. The sharpness and resolution from f/2 through to f/8 is reasonably similar. But at f/11 the sharpness has begun to be eroded. f/16 and f/22 progressively worsen the image quality. With this knowledge, you would probably aim to keep the aperture no smaller than f/8. Of course there are some types of photography that demand smaller apertures in order to maximise depth of field, or the distance in front of the lens that is in focus. An obvious example is macro photography, when the use of f/16 or f/22 is routine. But for the vast majority of photographs you should be able to keep the aperture above the diffraction limit aperture threshold.
(Above) Shot at f/2.0, the lens used (Olympus Zuiko Digital 35-100 f/2.0 on a Four Thirds sensor Olympus E-5 DSLR) is very sharp even at the widest aperture.
(Above) f/2.8
(Above) f/4
(Above) f/5.6
(Above) f/8 At this point there is a very slight degradation in sharpness as diffraction limitation begins to take effect.
(Above) f/11 At this stage the diffraction softening is quite obvious.
(Above) f/16
(Above) f/22 At the smallest aperture you can see that the image has become quite fuzzy.
To complicate matters a bit, the threshold varies from camera to camera and is dependent on the size of the sensor and its pixel count, in other words the pixel pitch, or area covered by each pixel on the surface of the sensor. In the table below are some examples for cameras with different sensor sizes and pixel counts of the smallest recommended aperture before diffraction softening comes into play:
| Sensor size (Cropping factor) |
Megapixels |
Aperture before diffraction limit |
| Full frame |
24 |
f/11 |
| Full frame |
12 |
f/13 |
| APS-H (1.3x) |
16 |
f/8 |
| APS-H (1.3x) |
8 |
f/13 |
| APS-C (1.5x) |
16 |
f/8 |
| APS-C (1.5x) |
12 |
f/8 |
| APS-C (1.5x) |
10 |
f/11 |
| APS-C (1.6x) |
18 |
f/8 |
| APS-C (1.6x) |
15 |
f/8 |
| APS-C (1.6x) |
10 |
f/8 |
| Sigma SD Foveon 14.5M pixels (4.8M pixel array) (1.7x) |
4.8 |
f/13 |
| Four Thirds (2x) |
16 |
f/5.6 |
| Four Thirds (2x) |
12 |
f/8 |
| Four Thirds (2x) |
10 |
f/8 |
| Compact camera 2/3rds inch |
10 |
f/4 |
| Compact camera 1/1.8 inch |
10 |
f/2.8 |
| Compact camera 1/2.7 inch |
10 |
f/2 |
What causes diffraction softening?
We could indulge in some serious geekery here, but I will try to keep this explanation as simple as possible. In a perfect world, it would be great for an optical lens to resolve, on the sensor or film plane, a tiny and perfect point of light from the subject that the lens is focused on. However, in practice it's impossible, even with the highest quality lenses, to achieve the projection and resolution of a perfect point of light. Instead, a circular spot results. The very smallest spot that the lens can resolve is called the circle of least confusion. But once again reality has another trick to play. Light is transmitted as a waveform and when waves pass through gaps you get diffraction.
The aperture of a lens will diffract the light to varying degrees. The smaller the aperture gets, the more diffraction occurs. The result of diffraction on a circle of confusion spot is described as an Airy disc (named after the Victorian scientist, Sir George Airy). Instead of sold circular spot, an Airy disc can exhibit rings around a central bright spot. An Airy disc gets larger because of increased diffraction caused by progressively smaller aperture sizes. Airy discs can also interfere with each other and form non-circular spots. When this happens, two points of light that represent two points of detail will be partially or wholly merged and so resolution is lost.
The main limiting factor for digital cameras is when Airy discs exceed the size of individual pixels on the sensor. At this stage an individual pixel is no longer able to record a single Airy disc or minimum point of detail from the subject, and so resolution progressively falls off. You may also notice a reduction in contrast.
Small cameras and small sensors
You may have wondered why compact cameras have relatively small aperture ranges. While a digital SLR with its relatively large sensor, and so large pixels, may offer aperture selections down to f/22 or even f/32, compact cameras may limit you to f/5.6. It's all explained by diffraction limitation. The size of individual pixels in tiny compact camera sensors is so small that diffraction limitations start at relatively bright apertures. If you had a compact camera that let you set an aperture of f/8, f/11, etc.loss in resolution would result in very poor sharpness.
In the old days lenses were not always efficiently designed and sharpness at full aperture (the brightest or widest aperture) tended to be poor. It was routine to stop down to a mid-range aperture for optimum sharpness. With compact cameras this is not possible because diffraction limiting can, as the table earlier in this article demonstrates, start as high as f/2 or f/2.8. This has forced lens designers to up their game, with the aid of advanced computer-aided design tools, to produce very efficient lens designs that are sharp even wide open. So don't be afraid to use wider apertures when using good quality modern lenses.
So be careful when setting up your picture
If you want extra depth of field, of course you need to use a smaller aperture. But try to remember what the smallest aperture is for your camera before diffraction limitation starts to erode resolution, and combine this knowledge with the use of the hyperfocal distance technique I covered earlier to make best use of the available depth of field without suffering from resolution-sapping diffraction limiting by using excessively small apertures..
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