Confusing Concepts - Diffraction

OF THE 3 different concepts in this 3-part series, this one is probably the most technical, the most difficult to understand, and the most difficult for sure to explain. Sensor resolution, we saw, was really just a matter of the relative size of the "container" (the sensor size), the size of the individual photo sites (pixels), and the number of photo sites within a given sensor dimension. We also said, however, that these things don't work in a vacuum. A sharp, detailed (resolution) image is affected by not only the pixel number and size, but also by diffraction. The creation and presentation of a digital photographic image is not a precise science. In fact, I am going to introduce a term that will be central to the final post about image sharpness: "appearance." We probably really ought to refer to image sharpness in a digital photo as "apparent sharpness." The reality is that there is no absolute sharpness - only apparent sharpness). What do I mean by that? Stay tuned for the third and final installment of this series: "Confusing Concepts - Image Sharpness."

The creation and presentation of a digital photographic image is not a precise science

THE EFFECT of diffraction on a digital image is a strongly related concept, however. To make a photographic image with virtually any camera and/or medium, we must focus rays of light through a lens. We mentioned in the previous installment that almost all lenses are circular. A primary reason for this is that the circular coverage provides the most consistent coverage of the rays of light smoothly from side to center. The round lens, however, "bends" the light rays, which generally requires a series of glass elements to - if you will - "unbend" them. 

WHY DOES any of this matter? Diffraction occurs during the process of  "bending" the light through the lens. What causes diffraction is the light waves that diverge from parallel. As a general rule, diffraction is effected by the size of the opening that the light waves pass through, and the length of the light wave. Let's address opening size first. There are going to be two mechanical factors: First, the  physical size of the lens circle at its widest aperture (which is what brings relevance to the above "coverage" discussion) is constrained by design. It follows that we should experience less diffraction from the larger openings of lenses designed to cover larger sensors. Confoundingly, as our apertures get larger, the depth of field of an image gets more shallow, so from front to back the apparent sharpness of the image seems less. Somewhere, "the twain shall meet," creating the "sweet spot" I talk about below.

THE OTHER mechanical factor is lens aperture (within a given system). Generally, the smaller the aperture (for the same reasons as the size of the physical lens circle matters), the more diffraction, and vice versa. Note that I have referenced lens "size," and lens "aperture." I did not say f-stop number. Why? Because a given f-stop varies in physical size between different lenses. This is true both in terms of focal length within system, and different system lenses (i.e., an M4/3 lens f8 will be physically smaller than a "full frame" lens at f8).

ANOTHER THING that effects diffraction is the length of the light waves. Again, as a general proposition, shorter waves diffract much sooner than longer waves do. Think about the spectrum of light. Blue light waves are among the very shortest (those who understand polarizing filters are probably familiar with this). This explains what certain light conditions demonstrate the effects of diffraction more than others.

Every Lens has its own "sweet spot"

EVERY LENS has what we sometimes refer to as its "sweet spot." That is where it is at its absolute sharpest performance. Most of us have an awareness that  many lenses are not sharp across the frame at their most wide open apertures. We also have a general awareness that as we stop down the aperture, we tend to get increasingly (apparently) sharp images. Some of us have been aware, over the years, though, that there is a point of no return, where not only does the lens no longer render an increasingly sharp image, but the image might even degrade some. This degradation is due to diffraction. Recall above that we said diffraction increases as the lens opening gets smaller. This is why it is important to keep that "sweet spot" in mind. Generally, a "full frame" (35mm equivalent) lens will be at its sharpest at f8 - maybe f11. An M4/3 lens will probably be at somewhere between f4 and f5.6. We will talk about why there is a difference shortly. All of this is, of course, also limited by lens design and overall quality. So-called "cheap glass," or zoom lenses trying to encompass too much zoom range, will mechanically and optically also negatively effect image quality, sometimes introducing optical and color aberation, and lack of contrast.

THERE IS another factor in the diffraction discussion other than lenses. Perhaps the most significant factor is sensor and pixel size. Once again, smaller pixels will be more susceptible to the effects of diffraction. That is the primary reason we find that "sweet spot" in M4/3 lenses to be at a wider aperture (f4- 5.6).

THE CONTRIBUTORS to diffraction mean that there is an aperture on each lens that is that "sweet spot." While we have generalized, each lens has its own "spot" and you may need to do some empirical testing of each of your lenses to arrive at that spot. It is important to acknowledge that there will always be some diffraction at every lens aperture. That point where it becomes visibly deleterious to image qualilty is referred to as the point where the lens is "diffraction-limited." My definition here is, of course, overly simplified. The simplest "technical" definition I could find was: "The diffraction limit is the maximum resolution possible for a theoretically perfect, or ideal, optical system." Think back to our discussion of "resolution." They are interdependent, and this "technical" definition feels awfully circular to me. The ultimate conclusion for me is that diffraction is one of the primary factors which effect image quality (without regard to the quality of the equipment being used), along with resolution and sharpness.

We shouldn't let all this technical jargon get in the way of our creativity

DOES THIS all mean that you should always and only shoot at the "sweet spot" aperture of your lens? Of course not. As I am fond of saying here, all of photography is a compromise. The artistic part of composition means that we must work with the limitations of the tools. Sometimes we want very shallow depth of field. Sometimes we want the image to be crisp from front to back (one of the ways photographers have been dealing with this issue in still photographs, by the way, is a phenom called "focus stacking"). But we shouldn't let all this technical (sometimes pixel peeping) jargon get in the way of our creativity. It is just useful to know the limits of our equipment when applying it to our craft. Next time we will address that third factor: Image Sharpness.