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Diffraction is when the regular pattern of waves we see as light gets disrupted and behaves erratically. Most photographers understand depth-of-field and how aperture affects sharpness. But there is a point of diminishing return, and the reason for that is diffraction.
When taking landscape or architecture photos, it's natural to try to maximize detail by increasing the depth of field. This is done by making your aperture smaller. It's easy to get carried away and stop down too much "just to be safe" in trying to to create a depth of field which is large enough.
Be careful while doing this though. Although a smaller aperture will provide a larger depth of field, the effects of diffraction will become increasingly noticeable at extremely small apertures, reducing the overall sharpness of the image.
This is counterintuitive to the purpose of using small apertures in the first place, which is capturing sharp detail. Knowing the limits of your lenses is very important to avoid this phenomenon, as well as mitigating the high ISO or long exposure times required for using unnecessarily small apertures.
The Science of Light Diffraction.
Although it may not appear so, light actually travels as a wave. So all the properties which can be identified in other waves such as sound or water ripples can also be identified in light.
Huygens' Principle states that "every point of a wave front may be considered the source of secondary wavelets that spread out in all directions with a speed equal to the speed of propagation of the waves."
This means that the light passing through the aperture creates new waves of light.
The tiny aperture hole of a lens, or more specifically the aperture blades, has the effect of bending parallel light rays. Think of an opaque object placed in front of a light source. The mass of the object blocks the light, creating a shadow.
Look closely, however, at the edges of that shadow. You may notice that even though the object has a sharp edge, the edges of the shadow are always fuzzy.
Notice the the difference in sharpness between top of the actual blade and the shadow it casts.
I used a photo of a pocketknife to demonstrate the effects of diffraction on a straight line. I took this image in a completely dark room where the only source of illumination was my flash. I also adjusted the contrast on this image in Photoshop to further highlight this effect. Notice that the top of the knife is very straight, and in this image it is rendered very sharply.
However, while looking at the shadow cast by this blade, we notice that the shadow is somewhat fuzzy, even in the presence of a strong, unidirectional light source. This effect that my knife edge has on the light is also observed as light enters a lens, where it interacts with the edges of your aperture blades.
As light bends, it now must travel different distances and begins to interfere with other sources of light created by the aperture blades. This creates brighter areas where the light compounds and dark areas where light is absent.
This is a phenomenon which can be observed not just in light, but in all waves. It is this uneven distribution of light which eventually leads to diffraction.
The effects of diffraction on your camera can be simulated by squinting your eyes. As you squint, the image of your world will become fuzzier as you reduce the size of your pupils decreases.
How light bends differently with differently sized apertures.
Assume we have a physically perfect lens with a perfectly circular aperture, the lens would then be called "diffraction limited." This is because the only limitation to the maximum resolution of an image created by that lens is the physical phenomenon of light diffraction rather than any imperfections, misalignment, or sensor resolution.
The interference pattern produced by a circular lens, given uniform lighting, is called an Airy disk, named after Sir George Biddell Airy. More specifically, the center of the image is called the Airy disk, while the collection of encompassing rings is called the Airy pattern.
The Airy disk: the effects of diffraction through a perfect circle.
The size of the Airy disks in your image depend only on the aperture, and can be approximated by taking the f-stop number and dividing it by 1500. This roughly gives the diameter of the Airy disk in milimeters. For example, when we use f/22, each Airy disk is around 0.0015mm.
If the diameter of the Airy disk's central peak becomes too large in relation to the pixel size, the image will appear blurred. This becomes the ultimate limiting factor in the pursuit of sharp images, and is determined through the choice of aperture.
Now that we are done with the boring stuff, let's look at a real life application of this principle. Testing the effects of diffraction for yourself is a very simple process.
Simply take a set of photos of a static object while keeping the focus and exposure constant, while varying apertures through aperture-priority mode. For the demonstrations to be meaningful, avoiding any change in subject is crucial.
Use a good tripod, a remote shutter, lock up your mirror, and do whatever else is necessary to reduce camera shake and keep focus constant. Taking the photo indoors is important to reduce the effects of wind and other outdoor variables.
The following sets of images are side-by-side crops at 100% of a Crown Royal bottle label. These shots were taken indoors with my camera on the ground.
Every consideration was taken to ensure the most comparable images. (Note: even though everything is controlled and there is plenty of light, my camera could not autofocus at f/36. This is another drawback of using extremely small apertures.)
Extremely small apertures are physically limited from being "sharp" through the physics of diffraction.
From this set of images we can observe that the images start to lose their sharpness around f/11, but is tolerable even up to f/16. From f/22 onwards though, the sharpness images worsen dramatically until f/36 which is quite unusable.
Don't forget that using some lenses wide open also reduces sharpness. It's important to find the aperture which is optimal for your lens. I tend to use f/8 or f/11 at most if I can.
The main reason for limiting aperture is to provide a larger depth of field, so it's important to know how much depth of field you need and use an appropriate aperture. There are many ways to calculate depth of field, and many resources online to assist doing so.
The distance and focal length do not require a small aperture to maintain a large depth of field. There is no need to tighten the aperture further than necessary.
Let's look at the example of this tree and evaluate the depth of field required to sharply take this shot. This photo was taken using a crop frame camera at 18mm and the subject, the tree, was roughly 20m away. Because this subject is so far away and the lens is wide angle, even a moderately large aperture of f/6.3 provides a depth of field from 2.26m to infinity.
This is more than enough to capture all the detail I need. In fact, with this focal length and subject distance, even an f-stop of f/1 will give me a depth of field from 8.95m to infinity, enough to capture this tree with full clarity and anything behind it.
Because the situation provided me with such a large depth of field, I had no need to use a smaller aperture, allowing me to capture with a higher shutter speed and lower ISO, both contributing to the overall sharpness.
It's good to remember that while using a smaller aperture will provide a larger depth of field, there are other factors with arguably much larger impact.
For example, with a subject 25m away and using f/8, a lens with focal length of 100mm will only give a depth of field from 17.9m to 41.6m, with a total length of 23.7m.
However, if you change to a lens with focal length of 75mm, the depth of field grows to cover distances from 14.6m to 85.9m, giving it a total length of 71.3m, almost three times as much as using the 100mm focal length.
Contrast that to stopping down one f-stop to f/11, this will give you a depth of field from 16m to 57.3m, for a total of 41.3m.
For shots which require long exposure times during the day, it's natural to first choose the shutter speed which will give you the effect you desire and use an aperture to match. However, with the effects of diffraction in mind, it's best not to use any aperture below f/8 or at most f/11.
Using ND filters or waiting for less light will result in an image with much more clarity than one taken with the appropriate shutter speed, but using f/32.
I hope that people have found this article useful. Knowledge of diffraction is effortless to apply (in fact, it won't be the main issue in most circumstances) but can have terrible consequences for those who are not aware of it.
Diffraction effects are easy enough to avoid, simply keeping the aperture larger than f/8 should do the trick!