What is Diffraction at Small Apertures? Do I Care?
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Single Aperture Diffraction in a Ripple Tank
Hello neighbours! Gordon and I recently did a podcast discussing the whys of depth of field reality and once area that we did not touch on was the subject of lens diffraction at small apertures as this is sometimes incorrectly aligned with depth of field.
We had considered doing this as a podcast but as I reviewed the materials,. I determined that a podcast is not the optimum vehicle for this topic, so an article it is.
Diffraction Definition
As noted in the podcast, light is very special. What we think of as a ray of light is both a wave and a stream of particles. We have heard the phraseology light wave and the term photon before. While it may seem odd that something can in fact behave as two very different things, light does. Why? As this is not a physics classroom, my cheap answer is; Because Physics.
Light rays, if you will pardon the simplifcation, move in straight lines unless interfered with. Light can be reflected When light passes through a medium change, such as from air to water, the medium change can cause a change in the direction of the light ray. This is called refraction. This principle is critical to lens design. When light waves must pass through a small slit, think small aperture, some never make it through, but since light waves are not all parallel, when constrained by a small opening, we are more likely to see a loss of sharpness as the waves passing through the slit tend to bend around the edges of the slit.
To make a simple allegory to this, drop a pebble into a circular birdbath. A set of waves is created. Each wave has a crest and a trough. We all see this. Now introduce a barrier with a very small opening in the path of these waves. Not all pass through the opening. Those that do have a primary crest and trough, but the opening creates an interference whereby the opening creates a reflection of the original wave that passes through. The interference patterns now create intersections where the crests and troughs interfere with each other. If you ever did experiments with ripple tanks in High School physics this is probably coming back to you now.
Here is a simple example of the wave pattern as light passes through a small opening. This image is using a laser as the source which is much more constrained than a typical light wave, for the purpose of illustration. As photographers we are not imaging based on structured sources.
Laser Interference pattern courtesy Wikipedia
Impact on Photography
Now we understand the basic concept that diffraction introduces interference exposing the crests and troughs of a wave. To see how this interference impacts us as photographers, we now need to look at an example of a wave passing through a small opening such as a lens aperture.
In this scenario. there is a light wave inbound that strikes the aperture of the lens. The smaller the aperture, the more interference in the incoming wave happens.. We can see that in this next image.
A light wave hitting a small aperture. Image courtesy Wikipedia
Think of the wave on the left as coming from such a distant source that the wavefronts are effectively parallel to each other. That wave hits the tiny opening (our lens aperture. Part of the wave passes through the aperture of course but we see that the aperture itself creates interference to the passage of the wave. This is evidenced in the bending of the wavefront in general and that the edges of the aperture also create reflections such that we see wave portions that are darker and portions that are lighter, yet none have the intensity of the centre portion.
The smaller the aperture, the more the change of the wave is apparent. Some people even sense movement in the wave image on the right side. It’s not moving in the still image, but your amazing eyeball is trying to make sense of the interference pattern.
If we use a larger aperture, there is still diffraction happening. Diffraction is ALWAYS happening regardless of the size of the aperture, however the point at which it becomes impactful will always be more apparent as the size of the aperture decreases.
How This Manifests on the Sensor
So far we have looked at images of the wavefronts from the side as it were, but not as they would impact the sensor.. To do this, we bring our observation point to be viewing the incoming wave as it strikes the sensor. Imagine that you are super tiny and the sensor is enormously huge. What you would see on the face of the sensor is called an Airy Disk.
Computer generated Airy Disk, courtesy of Wikipedia
This Airy Disk shows the area of falling sharpness. What is sharp is the bright dot at the centre. However the other bright areas are also impacting the sensor, creating a lack of sharpness. The smaller the aperture, the greater the lack of sharpness.
We have to remember the power of scale though. Remember that light wavelengths are measured in micrometres or Angstroms. An Angstrom is 10 to the -10 metres. Really really small, with visible light wavelengths in the range of 4000 to 7000 Angstroms, or 0.4 to 0.7 micrometres. When we look at the interference patterns they are also very very small.
Depth of field is often bound to the concept of Circle of Confusion. We talked about this on the podcast. but very quickly, the Airy Disk is representative of circle of confusion. It is widely accepted that any circle (it is shaped thus because the aperture is circular) of diameter of 0.25 millimetres (one quarter of a millimetre) and greater when viewed at a distance of 25 centimetres will be perceived as going unsharp. I want you to consider holding an 8x10 print about a foot away from your eyes and considering if you can at that distance resolve 0.25mm.
If we can now comprehend how an Airy Disk is similar in concept to the photographic Circle of Confusion, aka visible blur spot, we can now apply concepts of visible scale to things.
As documented by experimentation, the sharper the edge of the aperture, the more intense the diffraction. This is not surprisingly called the Knife Edge effect and supports the Huygens-Fresnel posit on the wave nature of light. You may have heard of Fresnel lenses used on spot lights to concentrate a beam. Look at a fresnel lens and you see a wave pattern in the glass design. As a student of science, I have many heroes of science who made incredible leaps in their work. One of the greatest, in my opinion, is Christiaan Huygens.
And Now to Reality
We choose small apertures for maximum depth of field. In most all cases we see no difference between the sharpness of a focused subject shot at f/32 and a focused subject shot at f/8. Diffraction is happening in both cases, but the effect is so small, you cannot see it in a high resolution print at a proper viewing distance. Don’t talk to me about seeing it on your display, the display resolution is so low, that any diffraction you think you are seeing is in your imagination. I know people claim that they can see it. Ok fine. Your display cannot possible show it, but if it makes you happy, crazy go nuts.
This concern only comes into its own when we think about serious close up photography, ie 1/2 life size or larger magnifications shot with ideal optics in a perfectly stable shooting situation at very small apertures.
There is definitely more diffraction happening at f/32 on your macro lens than at f/16, which is exhibiting more diffraction than at f/5.6. We make a trade off between depth of field and diffraction with each shot. and while the math may fascinate some, most folks are more concerned with being happy or not with the image.
For myself, I have made close up images at f/32 and smaller apertures. On a nice large print at a proper viewing distance, I cannot see the impact of diffraction and if I compare that to the same image shot at f/16, I will see the difference in depth of field much more obviously than I see the impact of diffraction. That’s me, you should do what you prefer.
When I need more depth of field than the camera to subject distance, aperture and angle of view can deliver, I will resort to focus stacking. In this scenario, I have committed to multiple images already so I may choose a larger aperture, not because of diffraction concerns but to give me shorter exposure times if not using flash so I spend less time on the photographic exercise. Then I will bring all the images into my editor of choice and feed them all to Helicon Focus to do the masking and stacking for me. I could do it myself, but it does not interest me to do so, and I find the output from Helicon Focus to be superb.
Summary
I was trained in Physics a long time ago. I am a scientific person by choice. I don’t care for assertions without evidence or foo foo dust. I know that diffraction is real and I understand reasonably well I think the impact of it on my photography. Photography uses the principles of science to make art with me being the artist. I like close up photography of some things such as plants and rocks and crystals. While I respect bugs, I’m not going to be going out of my way to make images of them. Whether I was working with a bellows on a frame, an old style fixed lens on an SLR bellows system, a macro lens, extension tubes or focus stacking, I have yet to make an image that I was unhappy with because of diffraction. Your mileage may vary and now you at least can understand the reality and scaled of diffraction versus the fictional tales so prevalent on the Internet.
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I'm Ross Chevalier, thanks for reading, watching and listening and until next time, peace.
