[Fstoppers Original] What Is Lens Diffraction And When Does Diffraction Happen?

If you are like me then you might not always get caught up in some of the super technical aspects of photography. One aspect of photography I recently investigated was the loss of sharpness caused by Diffraction. Last night while playing with the new Nikon D800 camera I examined lens diffraction and how diffraction can seriously affect the sharpness of your photography.

One thing that always blew my mind in science class was when my professors told me light can act both as a particle and as a wave (quantum physics was never my strong suit). When described as a wave, light usually travels in a straight line but can also bend when passing through or around objects. When light travels through your camera's lens and through a large aperture, light doesn't bend or disperse all that much. But something strange happens when light passes through a tiny hole such as a small camera aperture: it bends and interferes with itself. This interference is called diffraction.

Diffraction in simple terms is a phenomena that occurs with light when it interacts with an obstacle. Most of us are familiar with light diffraction patterns found on the backs of CDs, in water molecules in the air, or on spider webs when looked at the right angle. Diffraction can also occur in your DSLR camera which can become a major problem and cause your images to lose their sharpness. Here is a simple diagram that shows how light particles hit your camera's digital sensor when going through large apertures and smaller apertures.

So what does this all mean for your photography, and how does diffraction make images soft? I recently did a few tests with a Nikon D800 and a Nikkor 60mm lens to see exactly what the real world effects of diffraction were on macro photography. The first test was my own eye. We setup an Alien Bee R800 Ringflash and took a few photographs at different apertures. The resulting 36 megapixel images revealed that stopping down the macro lens past f22 resulted in a lack of sharpness. I would have loved to have taken test shots with an even wider aperture like F8 but the ringflash was too powerful at the lowest setting for anything below f22 at ISO 100. Here are the resulting photos (click each image to view full res).

After reviewing the results and seeing first hand how super small apertures can decrease sharpness, I decided to test something more static and remove as many variables as possible such as camera and subject movement and lens focusing distances. Lee and I decided to see how much detail we could resolve in a $50 bill. We used the same Nikon D800 and Nikkor 60mm lens but instead of a ring flash we backlit the currency with a strip box fitted to a Profoto Air 1000. Now with the camera on a tripod we were able to see exactly how different apertures were affected by diffraction. Here are the results:

As you can see, the images from F8-F16 look the sharpest and then diffraction really starts showing up in the smaller apertures. However, with macro photography, sometimes you are more concerned with overall depth of field more than overall sharpness. You can clearly see how shallow the depth of field is on the first 3 images and how the entire bill becomes in focus as we decrease the aperture. Below are the full Nikon D800 files at the widest aperture and the smallest aperture so you can examine the effects of diffraction more carefully.

Obviously lens diffraction only becomes concerning at super small apertures. If you are shooting portraits then you will probably never stop your lens down far enough to see the effects of lens diffraction. However, when you are shooting with very powerful studio strobes, you might have to shoot at much smaller apertures if you cannot get your lights far enough away from your subject. Diffraction affects each lens differently and as pixel size decreases diffraction usually shows up earlier. So my best advice is to run a few tests with your different cameras and lenses and find the sweet spots where diffraction is tolerable. For you super scientific thinking photographers out there, feel free to leave your thoughts in the comments below as we would love to hear your explanations as well.