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.

Confusing Concepts - Resolution

VOLUMES UPON volumes have been written about this topic. I am not for a moment trying to convince you that I am either an expert or the proverbial "last word." In recent weeks, I have read a few comments here, and online in other blogs (mostly in the discussion and comments) that seems to me to underscore a lack of complete understanding of the terminology. What motivates this blog (and a couple more to follow) is the thought the maybe I can shed some - albeit elementary - light on these topics. This is the first of a 3-part series.

PART OF the confusion probably stems from the fact that there are actually different kinds of "resolution" when we apply it to photography. It is a rather broad term, which is often used imprecisely. The making of a photographic image involves a lens, a medium - these days mostly a digital sensor and resulting file, and a manner of display. Each of these components applies a different "spin" on the word, "resolution." Consequently, when we are addressing resolution, we need to understand what kind of resolution we mean.

Sensor Size Comparison

THE RESOLUTION of a particular camera lens (or its "resolving power), simply refers to its ability to resolve detail. There are numerous factors that effect this ability, including lens design, size and quality of the glass elements, coatings, etc., 

IN THE case of digital cameras, resolution refers to the sensor used to record the digital image. This component of the optical "system" is perhaps the most difficult to get one's arms around. Sensors are intricate mechanisms. On a rudimentary level, they seem simple enough. They are just a collection of electronic recording sites (known as photo sites) grouped together on the camera sensor surface. They are, of course, microscopic in size. A more in-depth look at sensors leads us to realize that things aren't as simple as that sounds. Two significant factors are the size and number of individual photo sites. It seems evident enough that a smaller sensor will not be able to hold as many of the same-sized photo sites (or photo cells) as a larger sensor. Sensor size is functionally related to the lens circle. Smaller lenses will only "cover" a smaller sensor area. As the sensor gets larger, in order to cover the sensor area, lenses must be designed with larger circles. The reason lenses are circular is really beyond the scope of this article (and my expertise, 😰), but it is a matter of physics, and the desire to balance the light being directed by the lens. If you use an image sensor that is larger than the image circle, the image will show up framed as a circle encompassed by a black area outside the circle. As a general rule (we will see as we go on, that these things don't work independently), smaller photo sites will have less "resolving" power than larger ones. Coupled with the concept of an optical occurrence known as diffraction (stay tuned), conventional wisdom has it that smaller sensor - based cameras will generally have less resolving power than larger ones. While not precisely correct, it is a valid consideration when using such equipment. I have only recently empirically tested (and concluded) that this applies to my m4/3 camera setup as compared to my "full frame" sensor gear. The rationale for this line of thinking is that it is difficult to match photo sites in terms of both number and size on a smaller sensor. My Olympus m4/3 sensor is nearly 1/4 the size of my Sony a7Rii "full (35mm equivalent) frame" sensor. At only 20 megapixels (a measure of the number of photo sites on the sensor), it is the largest m4/3 sensor available, to the best of my knowledge. My Sony, on the other hand, is 46 megapixels (and the newest iteration - the a7Rv - is 61 mp). Not only are there from 2 -3 times the sites, but each individual photo site is also significantly physically larger. That phenomenon creates conditions for increased sensor resolution.

Bayer Color Filter Array

THERE IS more to the sensor story than photo sites and sizes though. Diffraction plays a significant part in this equation, too. I will cover diffraction all by itself in the next post. During the early years of digital sensors, one of the concerns that designers (and users, of course) had was the phenoma of "aliasing." As we have discussed here in the past, the basis of a digital image is the "lego-like" stacking of rectangular pixels to produce the shapes found in images. Because of these individual pixels, there are always straight line transitions between pixels (to continue the analogy, each lego block). At some level - particularly in lower "resolution" (in this case meaning smaller and less megapixels) images will have the appearance of jagged edges (or "jaggies"). In order to address this concern, camera manufacturers put an anti-aliasing filter in front of the sensor (known as a "low pass" filter), that was designed to introduce a bit of blur. Obviously, my explanation is hopelessly oversimplified and the process is/was complex, if not consistent. As they added megapixels (my first Nikon D100 was a 6mp camera), and processing software (especially raw conversion engines) got better and better, the aliasing issue has become less important. Indeed, I have personally looked for camera specifications that do not have the low pass filter, reasoning that I don't want anything I don't absolutely need to introduce softness. On the contrary, I am looking for the maximum sharpness I can get. In my view, the presence of an AA filter - though perhaps only very marginally - effects resolution. Neither of my current cameras (Sony a7rii and Olympus EM10iv) have AA filters on their sensor.

Why are the pictures square if the lens is round? - (Steven Wright)

 A SECOND filter (or filter array) known as a "Bayer Color Filter Array" is placed in front of the almost every digital sensor. Through a process called digital sampling, the sensor creates the digital image. The Bayer filter involves additional color sampling, which produces the colors in our images, using primary colors of red, green and blue. The sharp observer will note that there are (many) more green sensors than red or blue (see the illustration above). This is because our visual system is the most sensitive to the green light spectrum, which is where the sun emits the largest amount of light. Green light contributes much more to our perception of luminance. Color filter arrays are designed to capture twice as much green light as either of the other two colors. The takeaway here is that the Bayer filter is yet another path of interference between the light rays and the sensor sites. This introduces softness and therefore, effects resolution. This also explains why most raw processing software has a "default" amount of sharpening (often referred to as "capture sharpening") that is applied automatically to a raw digital file. Most software (Adobe ACR does) allows the user to adjust, or even eliminate, that default sharpening.

all of these individual measures of resolution work together to create the end product

THE LAST of my three resolution considerations is the manner of display. For many years, the primary method of display was the print, on a photographic fiber medium. This involved some kind of pigmentation process from being embedded into the medium (traditional photographic darkroom printing) to printing press ink printing methods to the more modern digital inkjet printing. By the time of the latter, we were also commonly projecting images onto a cathode ray type tube (CRT), and eventually, LCD screens. Prior to the emergence of digital, another method of displaying images was through what was called a color-transparency system (or simply slides). Each of these presentation methods react differently in terms of resolution. The medium itself has its own "resolution," which - once an image is put in the form of the presentation, becomes the predominant factor. With the ascendancy of social media, smart phones and tablets, it is probably safe to say that digital projection is the most common manner of presentation today. Resolution in the context of presentation media, has begotten perhaps one of the most confusing terminology puzzles in the realm of resolution. Resolution of an image when projected on a CRT/LCD screen is purely electronic and is often measured as pixels per inch (PPI), a measure of the size and density of the displayed image. When we speak of an inkjet print however, the printer uses colored pigments to create a microscopic dot-based pattern on the medium. The correct resolution terminology here is "dots per inch" (DPI). DPI is also used for traditional printing-press type media presentations. The two (PPI/DPI) are often - confusingly - interchanged.

AS I said earlier all of these individual measures of resolution work together to create the end product. When choosing and using camera gear, an understanding of these factors will make more sense out of your choices. When the hype from the seller, or the specifications from one of the testers out there emphasizes the particular component's "resolution," or "resolving power," it is important to think about the other components. The highest quality (think Leica or Zeiss) lens, with a "medium - format" sensor (or larger) camera (and yes, confoundingly, in the digital world, MF is bigger than "Full Frame"), that is going to be only seen on your FB or Instagram page is extreme overkill. The final digital resting spot for the image cannot begin to match the resolution of the other two components.

Resolution in the context of presentation media, has begotten perhaps one of the most confusing terminology puzzles in the realm of resolution

I  AM not saying you shouldn't have high quality or high resolution equipment. I am saying that an understanding of resolution and its significant variability will help put your photography - and your gear needs/wants in perspective. "Pixel peeping" is a (sometimes pejorative) description that is given to a lot of photographers these days who tend to place an over-emphasis on technical factors, like resolution, noise (see, What's All the Noise about Noise), and diffraction (another term I will cover in an upcoming blog), over the more artistic part of photography. To be sure, some fundamental skills and reasonably good quality equipment are required to make sure the image is going to be viewable as intended. But beyond that, in many cases, the technical issues tend to be overblown, in my opinion.

WE STILL haven't told the whole story though! Stay tuned for upcoming blogs on Diffraction and Image Sharpness.

GEAR WARS: The Sensor Strikes Back

THE STAR Wars story couldn't be a better example of the age-old plot: the good guys against the bad guys. The white hats against the black hats. Canon against Nikon. Ironically enough, back in those days, most of the Canon lenses were white and the Nikons were black. If you are old like me, you probably shot 35mm Single Lens Reflex (SLR) cameras. During those years, I used to see arguments about "which one is better" all the time. Many of them were good natured, but sometimes it got down into the trenches. Then: grab some popcorn and sit back for the show. Almost always a dark comedy. 😏

I SHOT both over the years (though mostly Nikon). I never looked at them with a "one-is-better-than-the-other" attitude. In my toolbox, I have two hammers. One is the Estwing brand and the other is Stanley. I kind of favor the look and feel of my Estwing - but really, both have the identical function. They drive (and pull) nails. And both work just fine for the task. Perhaps an overly simplistic comparison, but I have done so purposely - hopefully to make a point. Probably the two "best seller" hammers. There are many others, though, that perform equally well.

There ain't no good guy; there ain't no bad guy . . . 

THESE DAYS the "Gear Wars," are no longer between best sellers Canon and Nikon (in the mirrorless camera world the two best sellers are now Sony and Canon). Today the war has moved to a different battlefield: sensors. Still just as much a waste of time in my book, internet camera sites, blogs and social media pages/groups are often mired in arguments about whether certain M4/3 systems are as good or better than the so-called "full frame" (FF = 35mm equivalent) systems. There are, of course, other sensors that are smaller (1" and below - as in P&S cameras and smartphones), larger (so-called "medium format" which is - nonsensically - larger than "full frame"), and in between (the very popular APS-C sensors). The two major camps in the skirmish, though, appear to be the M4/3 against the FF.

TRYING TO keep my multiple metaphors under control, 😓 I want to repeat what I have harped on here before: (Dave Mason said it better than I can): There ain't no good guy; there ain't no bad guy, there's only you and me and we just disgree. As they always have been and always will be, these things are just tools!

The operative word here is compromise

THE RIGHT tool for the job, was something my dad and grandfather (both consummate craftsmen and engineers) repeated time and again to me over my youthful years (think of that time when you grabbed a butter knife from the kitchen to unscrew something). Sometimes there is more than one tool for the job. Sometimes there is only one. Surely there are poorly-made copies and brands out there, but when we are talking about the primary camera market, for the most part they are of equivalent quality manufacture. Because of that, the brand of screwdriver doesn't  matter as much as some of us might think it does. They aren't "better or worse." They are just different.

The two major camps in the skirmish . . . appear to be the M4/3 against the FF

I AM not for a moment saying there aren't differences, or that those differences are useless and unimportant. I am saying that we place an inordinantly heavy emphasis on them. Let me stay with the tool example for a bit. Same toolbox, different drawer. I have two full sets of ratchet drive socket wrenches (full disclosure: I actually have several more sets). One is 1/4 inch drive and one is 1/2 inch drive. There are many of the same size sockets for both drives. In that way, they both work (mostly) equally well for the job of tightening or loosening hex-head nuts and bolts. They are tools. There are also some larger sockets that are only 1/2 inch drive and some much smaller ones that are 1/4 inch drive. There are times when you need the larger one for more leverage and torque and times when the smaller one fits the space better. 

FOR THOSE not as into tools, you will probably have only one set. Much like choosing a single camera system, there are going to be compromises. As you can see from the above, sometimes one system will do things the other won't. The choice of a single system is going to be a compromise -something I also harp on here (for the tool-confused, there is also a 3/8 inch drive ratchet setup which is probably going to be the best compromise in most cases).

The right tool for the job

I ACKNOWLEDGE that we are talking about a sophisticated camera system costing thousands of dollars and not a mostly single purpose simple wrench or hammer. The oversimplification is my attempt to make a stark point. When it comes to a camera system, there are, of course, multiple functions that must be addressed. They vary from camera to camera, and it is the manufacturers, and not us, who decide which model has which features (I have often thought about how nice the now-ancient Gateway Computer model would be for cameras; where we pick our base model and then add or delete features we want or do not, ala carte). Unfortunately, that is not even slightly feasible. Instead the operative word here is compromise. When we look at the M4/3 system against the FF system, each is going to do things the other will not. I have talked in some detail about these differences in my series about image quality (Resolution, Diffraction, and Image Sharpness). I won't publish another "comparison" laundry list here. My primary point is that we buy camera systems for a variety of reasons. Some of them are well thought out and some aren't. I suggest we all look more toward the "well thought out" approach. What kind of photography will we be doing? Where will we be doing it (will airline or other baggage-dependent travel be in the mix)? What kinds of technology will we need? What will we be doing with the images once recorded (do we want to make/sell large, detailed prints, or sell our digital images to commercial third parties)? How much does size and weight matter to us?

FOR SOME of us, we just couldn't make the one-system compromise work for us. If you know me, or have read the "What's In My Bag," page here, you know that I shoot both FF and M4/3 systems, depending on some of the factors mentioned above. For travel that is not dedicated to photography, I carry and use the M4/3 system 99% of the time. For outings dedicated to photography (especially landscape stuff), I use the FF gear 99% of the time. I have a good friend and talented photographer in his own right, who has FF gear for shooting wildlife and "Medium Format" system for landscape. That is how we deal with too narrow a compromise. The key is that we have chosen these systems for thought out reasons, and do not care a bit that anyone else thinks their gear is "better." It's not. Its just different.