What it looks like
The above calculations and diagrams are quite useful for getting a feel for the concept of diffraction, however only real-world photography can show its visual impact. The following series of images were taken on the Canon EOS 20D, which often begins to exhibit softening from diffraction at about f/11. Move your mouse over each f-number and notice the differences in the
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No Overlap of Airy Disks |
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Select Aperture: |
f/8.0 |
f/11 |
f/16 |
f/22 |
Partial Overlap of Airy Disks |
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Note how most of the lines in the fabric are still resolved at f/11, but they are shown with slightly lower small-scale contrast or acutance (particularly where the fabric lines are very close). This is because the airy disks are only partially overlapping, similar to the effect on adjacent rows of alternating black and white airy disks (as shown on the right). By f/22, almost all fine lines have been smoothed out because the airy disks are larger than this detail.
Calculating the diffraction limit
The form below calculates the size of the airy disk and assesses whether the system has become diffraction limited. Sections in dark grey are optional and have the ability to define a custom circle of confusion (CoC).
Начало формы
Note: CF = "crop factor" (commonly referred to as the focal length multiplier); assumes square pixels, 4:3 aspect ratio for compact digital and 3:2 for SLR. *Calculator assumes that your camera sensor uses the typical bayer array.
Конец формы
This calculator decides that the system has become diffraction limited when the diameter of the airy disk exceeds that of the CoC. For a further explanation on each input setting, please see their use in the flexible depth of field calculator.
The "set circle of confusion based on pixels" checkbox is intended to give you an indication of when diffraction will become visible when viewing your digital image at 100% on a computer screen. Understand that the diffraction limit is only a rough limit; there is actually a gradual transition between when diffraction is and is not visible. Real-world results will also depend on the lens being used; this limit is only a best-case scenario for the sharpest lenses at optimal apertures.
Notes on real-world use in photography
Even when a camera system is near or just past its diffraction limit, other factors such as focus accuracy, motion blur and imperfect lenses are likely to be more significant. Softening due to diffraction only becomes a limiting factor for total sharpness when using a sturdy tripod, mirror lock-up and a very high quality lens.
Some diffraction is often ok if you are willing to sacrifice some sharpness at the focal plane, in exchange for a little better sharpness at the extremities of the depth of field. Alternatively, very small apertures may be required to achieve a long exposure where needed, such as to create motion blur in flowing water for waterfall photography. Diffraction is just something to be aware of when choosing your exposure settings, similar to how one would balance other trade-offs such as noise (ISO) vs shutter speed.
This should not lead you to think that "larger apertures are better," even though very small apertures create a soft image. Most lenses are also quite soft when used wide open (at the largest aperture available), and so there is an optimal aperture in between the largest and smallest settings — usually located right at or near the diffraction limit, depending on the lens. Alternatively, the optimum sharpness may even be below the diffraction limit for some lenses. These calculations only show when diffraction becomes significant, not necessarily the location of optimum sharpness (although both often "camera lens quality: MTF, resolution & contrast"for more on this.
Are smaller pixels somehow worse? Not necessarily. Just because the diffraction limit has been reached with large pixels does not mean the final photo will be any worse than if there were instead smaller pixels and the limit was surpassed; both scenarios still have the same total resolution (although one will produce a larger file). Even though the resolution is the same, the camera with the smaller pixels will render the photo with fewer artifacts (such as color moiré and aliasing). Smaller pixels also provide the flexibility of having better resolution with larger apertures, in situations where the depth of field can be more shallow. When other factors such as noise and depth of field are considered, the answer as to which is better becomes more complicated.
Technical Note: Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out. Therefore the size of the airy disk only depends on the f-stop, which describes both focal length and aperture size. The term used to universally describe the lens opening is the "numerical aperture" (inverse of twice the f-stop). There is some variation between lenses though, but this is mostly due more to the different design and distance between the focal plane and "entrance pupil."
For additional reading on this topic, also see the addendum: Digital Camera Diffraction, Part 2: Resolution, Color & Micro-Contrast