Last night I did a webinar for Singh-Ray Filters, and the audience had some terrific questions. One question brought up a point you don’t hear mentioned a lot when talking about when to use filters; lens diffraction.
What is lens diffraction? Light waves enter the camera through the aperture opening reaching the sensor, resulting in your photograph. When the aperture is wide open, say F2.8, the aperture doesn’t constrict much and the light waves are relatively unaffected. However, as you close down to smaller apertures, the opening for light to enter gets smaller and smaller. At F22 the opening is very small. When the light waves pass through this tiny opening (at F22), they interfere with each other which results in loss of sharpness, especially at the edges and corners of your frame. It is almost counter intuitive; you get better depth of field (more areas sharp) at smaller apertures, but you start to lose sharpness due to diffraction. But do you really have to worry about this?
First, it will depend on the lens. Some lenses do an excellent job of managing diffraction, where others don’t. My rule of thumb is I try to avoid shooting at the smallest aperture opening on any lens. I regularly shoot at F16 and don’t worry about diffraction. If I need that amount of depth of field, or to reduce light in my exposure, I will use F16. Sure, there might be a tiny bit of diffraction on my 14-24mm, but it would be overshadowed by the need for more depth of field. You can do a test with your own lens. Set it up on a tripod, focus on a subject, and shoot the full range of apertures for that lens. Then compare them on your computer, and blow up to 100 percent in the corners to look for diffraction.
How does all this relate to filters? Think about it…if I need to reduce light for a slow shutter speed, but I don’t want to use F22 because of diffraction, I could put on a 5 stop ND and that would put my aperture at F4. Little or no diffraction there, and I now can use that slow shutter speed to transform the fast moving river into a silky blue ribbon of color in the final image.