Diffraction

Today I would like to try to help us understand a little about what diffraction is. Not getting too deep in the theory. Just enough to demystify it a bit.

Scary

Diffraction is probably a scary word to most of us. Even if we don’t know what it really means, we have heard of it and have been taught that it is a “bad thing”.

Have you been taught to avoid using apertures smaller than f/11? Note that when I say a “small” aperture I am referring to the physical size. Remember that as the aperture numbers get bigger the actual opening in the lens gets smaller. This simple graphic illustrates that:

The lore is that very small apertures (large f-numbers, like f/22) make an image too blurry to be useful. Don’t believe everything you hear without testing it.

Light theory

I’m going very light on theory (yes, pun intended). We’re just going to graze the surface without taking a deep dive in. (Here is a source to start at if you want to go deeper. Abandon all hope ye who enter…)

Light behaves as waves (most of the time). Actually, a number of things are waves: light, water waves, sound waves, gravity waves. Quantum mechanics theorizes that even matter is waves. Too deep for me.

We tend to visualize light going through our lens as rays. That is, straight lines. Yes and no. That is one useful model of looking at it. But light also behaves as waves. An interesting and important property of waves is that every point on a wave is a wave. So if the wave is blocked by a small opening, the wave spreads on the other side of the opening.

This picture by Verbcatcher does a marvelous job of illustrating that for waves in water:

See how the waves spread after going through the small opening to the sea? The smaller the opening (aperture) the more pronounced the effect. That is, a small aperture opening causes waves to spread out more.

What does it really mean

This is the basis of the recommendation to use physically large apertures (small f-numbers). Apertures that are large relative to the wavelengths of light do not cause much “bend” of the waves. Small apertures (large f-numbers) “bend” the light more.

What we can actually see in practice is that using small apertures causes our images to have a mildly “fuzzy” look. Because the waves spread more after going through a small aperture, the individual waves cover a larger pixel area. This slight spreading of the light causes the image to appear less sharp.

The best discussion of diffraction for photographers I have found is from this article by Spencer Cox. But even this gets too deep into theory.

I borrowed this image from it to illustrate the practical effects of diffraction as we change aperture:

See how the larger apertures (small f-numbers) are sharper than the smaller ones?

This illustration below, also from Spencer Cox) gives a great conceptual representation of what is happening. Take that the grid represents pixels in your sensor. At f/4, the point of light only strikes one pixel. It will be seen as very sharp. But at small apertures, the waves spread some onto adjacent pixels and create a kind of fog.

Should you fear it?

Should you fear it and always shun small apertures? No, it is just a reality of physics. It is no more to be feared than gravity. As one of my sons would say, it is what it is. Be aware of what is going to happen and consciously decide how far you need to go.

All of the exposure determinations we make daily are tradeoffs. How much to stop motion? How much depth of field do we need? Is there enough light for a good exposure? What ISO setting should I use? All of these things and more have to be balanced in the moment of shooting, besides composition and esthetic issues.

Each setting costs something. As experienced photographers we must understand the tradeoffs and be able to judge what is right for us at the moment.

Diffraction is one of those tradeoffs. Know what it is going to do and how to use it or avoid it.

Sometimes you need more

But why would we ever intentionally make our image less sharp? We seldom actually choose to make it less sharp, but sometimes we need other things. I can give 2 easy examples.

The first and most common one is to increase depth of field (DOF). It is counter intuitive, but making the aperture smaller increases the perceived depth of field. So on the one hand we are making the image less sharp, but on the other hand we are making it appear sharper throughout. When we need to make a certain range of the field of view acceptably sharp we stop down the aperture until we achieve our goal. A tradeoff.

© Ed Schlotzhauer

The second case that comes to mind is to reduce the shutter speed. I often intentionally shoot motion blur. But I usually forget to bring a neutral density filter for the lens I am using at the time. I can generally achieve the effect I want by using my polarizer, reducing the ISO to the lowest setting, and cranking the aperture down to the smallest possible one. This will probably give me a shutter speed in the range I want to use. Yes, the small aperture increases diffraction and makes the image less sharp. But it is handheld at a long shutter speed. It is already intentionally blurred.

© Ed Schlotzhauer

But maybe more importantly, in a great video on Lumminous Landscape, Charles Cramer said “sharpness is something we have to get over.” He explained that if we take a picture just because it is sharp, it probably won’t be very interesting. We have to forget about how sharp is it and instead react to the scene before us on an emotional level.

Shoot the picture

Diffraction is a side effect of physics and our photographic technology. Don’t be afraid of it. Don’t blindly follow some rule you learned in the past about what you can or can’t do. Understand enough about it to recognize it and know how to use it to your advantage.

Look at the image above of the woman’s face. Even at f/32 – an extreme case – it is acceptable. Extra sharpening can be applied in your editing tool to compensate for it.

So diffraction is just there. Allow it to happen if that is the tradeoff you need to make. Just like using a high ISO adds noise, that is acceptable most of the time and better than missing the shot.

I know many of us don’t want to deal with what we perceive as increased complexity or too much technical detail. We just want to go take great pictures. My hope is that topics like this will actually make your photography life simpler by providing some grounding for information you may have heard in the past. Rather than trying to remember rules for how to use your equipment, you now have a model for what diffraction is doing and how strong its effect is. I hope you will be able to stop fearing it and accept it is just part of the tradeoffs of the technology.

Today’s image

This is a great old WWII era truck I found in my town. It is a Coleman. This was actually a Colorado company. It was designed and manufactured in the Denver area.

I needed enough depth of field to span from the great rust and paint patterns on the near outside through most of the cab. So it is shot at f/22. Diffraction? Works for me.

What do you think?