Lens Diffraction and Image Sharpness

This video by Steve Perry from Back Country Gallery does a nice job of explaining lens diffraction. Which has to do with the way waves of light bend around edges, especially when traveling through a small opening. This causes a loss of sharpness in the overall image, as you stop down your aperture (i.e. you use a small opening). Now sharpness is a different quality of an image than the area that appears in focus. Aperture also controls the depth of field, and so to get the highest quality image, you need to make some trade-offs, which involves knowing the limits of your particular camera and lens combinations to find the “sweet spot”.

A bit long and without a high quality and high resolution monitor, it maybe difficult to see the differences in the sample images1. Overall the video contains good information about lens diffraction, sharpness, f/stop and area of focus (depth of field). The Q&A part provides a good treatment of understanding the trade-offs.

What ever you do, do not use this video as an excuse to go out and buy new stuff, first learn how to use what you already have to make the best image possible.


An Analogy

My two cents on understanding this is to make an analogy to the analog system. When we are trying to focus our eyes on something, we squint, particularly those of a certain demographic. Squinting makes more things appear to be in focus, looking through a narrower opening increases depth of field. But it also creates a bit of starburst around the edges of our eyelids, as the light defracts around them. 2


Another observation: to get a starburst pattern of the sun or street lights in a photo, we stop down to the smallest opening. The starburst comes from the light bending or defracting around the tiny opening created by using a large f/number. Well this is happening to all of the photons coming through the lens aperture to the sensor, you only notice the really bright light defracting; at least until you do some nerdy pixel peeping.

  1. I could see the differences on my iPad; they are there, but I had to look really hard. If you are adventurous, you can reproduce what Mr. Perry did with your own camera and lens. In fact, you may want to at least shoot a target at the same distance with different f/stops to determine the sweet spot for your camera and lens combination 
  2. Squinting also compresses the eyeball which changes the focal length. That does not change how much appears in focus, just where the point of focus is located; that’s like holding the page further away to read it.