Lens Diffraction

A simple version

Overview

In physics diffraction is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Diffraction occurs with all waves, including sound, water electromagnetic, radio, and light waves. In order to understand lens diffraction, we need to understand a little about light. For the most part, light travels in straight lines. However, when light goes through a small hole, like a lens aperture, it begins to disperse or diffract. Figure 1.

The larger the opening, the less diffraction occurs. Very small openings can have a dramatic effect. Figure 2.

Rays of light that pass through a small opening begin to diverge and interfere with one another. The smaller the opening the more divergence occurs. Light that passes straight through the aperture travels a shorter distance than light that diverges and travels to the edge of a sensor. Because the light is now traveling at different distances they will fall out of phase and interfere with one another. In some cases phases of the light wave will cancel out other wavelengths and in some cases it will intensify other wavelengths. This causes symptoms other than diffraction.

When light passes through a perfect circle aperture it creates a diffraction pattern called “airy disks” (named after George Biddell Airy), The picture shows a representation of airy disks. Figure 3 The width of an airy disk is used to determine the theoretical maximum resolution for a lens. Based on these airy circles two things can happen that will degrade an image. 1st when the diameter of the airy disk’s central region becomes large, relative to the size of the pixels in the camera, or 2nd if two airy disks become closer than ½ their width they are no longer resolvable. That means that diffraction sets the resolution limit, not how many pixels the camera has.

What have we learned, and how do we apply it?

We now know that small apertures of say f/16, f/22, or f/32 will diffract light more than big apertures of around f/2.8, f/4 or f/5.6. And the more diffraction the more potential for loss of quality in our photos. We also now know that when the diameter of the airy disk’s central region becomes large, relative to the size of the pixels in the camera it will start to have an effect on your photo! I bring all this up because today we have cameras with a large number of pixels on the sensor. The Canon 7d has 18mp on an APS-C sensor and now the NEW Nikon D800 has 34MP on full size sensor. The more pixels a chip has the smaller the pixels have to be. In the Nikon D800 Technical Guide, it states The effects of diffraction are partly influenced by the size of the pixels in the camera image sensor, but with the D800/D800E’s high resolution the effects generally become noticeable around f/11. NOW, chances are camera shake, improper focus, motion blur or bad lenses will have more impact on you photo’s than diffraction. But if you want the very best picture you can get you had better understand this principle. The best thing you can do is grab your camera and favorite lens, stick them on a good sturdy tripod, set mirror lock up on, use a remote shutter release, and shoot a subject with a ton on fine detail. Start shooting wide open and work your way down to the smallest aperture you can. Then jump on your computer and look closely at the results. It would also be a good idea to shoot different color test subjects. Get to know your cameras limits and how to work within them.

I hope this brief explanation will help you become a better photographer.

This entry was posted on Friday, February 24th, 2012 at 9:37 am. It is filed under Tutorials and tagged with lens diffraction. You can follow any responses to this entry through the RSS 2.0 feed.

←

→