by Brent Bergherm
Diffraction in a lens is not something we commonly think about when we're starting out in photography. However, if your images are softer than you think they should be, it may not be your technique, but quite possibly your lens/camera combination.
My point here is to give you the basic details in simplified terms. I believe that learning the general principles is way more important (and more applicable) than learning the science behind what's happening. However, if you're interested in a more in-depth article, I've provided a link at the bottom of this post for your ultimate geek-out pleasure.
Let's dive in and take a look at this phenomena to understand it a little better.
Setting up the shot
For this test I'm using one of Canon's finest lenses ever made. The 11-24 f/4. It's an amazingly sharp lens. But it does suffer from diffraction, like all lenses, and it seems that wide lenses tend to suffer the most.
This image was created at the Boardman Poplar Tree Farm, in Boardman, OR. The camera is tripod mounted and the aperture is at f/8. When you click on the image you'll be shown a detail shot of the trunk area. This is a 1/1 view in Lightroom. Nice and sharp.
This lens is at its best performance between f/5.6 and f/8. If you go beyond that you'll start to notice the diffraction starting to be an issue. However, I must say, in real-world shooting, I would be very comfortable with this lens at f/11. Even f/16 on many subjects, but f/22, as seen next, gets too soft for me.
Here's a shot at f/22. Click on the image and see the same trunk details. It's a bit softer. But this is at f/22. There's more depth-of-field therefore it should be sharper, right?
Unfortunately, diffraction is happening. This is not usually an issue when we're shooting wider open because the diffraction gets worse when the opening (aperture) is smaller. You probably already know that shooting wide open often gives you a softer image as well, so what's going on here?
When you're shooting with the lens wide open, the diffraction is barely detectible. But the other imperfections of the lens are showing up. When you stop down, you start to effectively "hide" those other lens imperfections, but diffraction starts to creep in.
That's why each lens has what we call the "sweet spot" where it's performing at its ultimate best. You get maximum sharpness and minimum diffraction at the lens' sweet spot.
So, back to this example. This lens costs about $3,000. For that kind of money you'd expect it to be a perfect lens. But unfortunately, the rules of optical physics don't change when you toss more money at it. I do like how the trees in the background are in sharper focus thanks to the longer depth-of-field. But the softer trunk stinks. If I really need everything in focus and to be nice and sharp, I need to take multiple images with the lens focused at different depths in the scene. With those images I can then do a focus stack in Photoshop.
There's a lot of things we didn't address in this article. Such as vignetting when wide open vs. at f/8 and beyond. Nor did we measure anything related to sharpness at the various apertures. This is a preview only of diffraction and how it behaves with this lens.
If you're using a longer lens you'll still have diffraction. There's a lot of factors that go in to what makes diffraction. Check out this article if you want to know just about all there is to know about diffraction.
If interested, check out this review of Sigma's competition, the 12-24 f/4. They compare it directly to this fine lens by Canon. The Canon option, while more expensive, seems to be worth the money.
Download the RAW images
For your enjoyment I'm making the RAW DNG files available to you. I do this in good faith that you won't attempt to claim this image as your own. I just want you to see exactly what I'm seeing when I inspect these images and you can interpret it on your own computer without any JPG compression.
F8 raw image.
F22 Raw image.