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Learn these in the following order and some of my posts will begin to make sense!

• Light: Well…it’s a wave, because it interferes with itself and disperses exiting an aperture. No…wait…it’s a particle, because it responds to gravity and travels in a vacuum (except for a Kirby, perhaps?). Anyway, it travels very fast and has electromagnetic wavelengths that range from about 400 nanometers to 800 nanometers (billionths of a meter). The fact is that Max Planck and Neils Bohr were both smarter than Albert Einstein.

• Modulation Transfer Function: MTF is thrown about because simply referring to it makes you very, very serious photographer. It is very important to understand MTF before selecting your lenses for various types of work. You should commit MTF to memory, and Norman Koren is willing to help you do so. “MTF50″ and “lppm” (”line pairs per millimeter”) are particularly important concepts: The ability to resolve, for example, 40lppm doesn’t really tell you how well they’re resolved, just that they can be distinguished given extraordinary scrutiny by a certain observer, and we’ve already covered the issue of variable acuity. MTF standardizes and quantifies the lppm testing results. Advanced topic: Check the ~10% criteria for MTF in the determination of Rayleigh Criterion Limits. For digital photographic applications in the 35mm format I prefer ~20%, but my explanation-as usual-is confused and rambling (Airy Disks should be measured, at a minimum, to or through the first Diffraction interspace).

• Diffraction: There are three primary colors: Red, Green, and Blue. If you want to sound particularly bright, refer to them as the “fundamental constituents…the irreducible components of white light.” Red, Green, and Blue all bend at different angles when the enter or exit mediums of different density, such as glass, air, water, or Edgar Cayce. That bending is Diffraction. Incidentally, Isaac Newton figured all of this out before he was 30. “Spectrum” actually comes from the Latin word for “ghost.” Happy Halloween.

• Abberation: In optics, a corrupted light path. Light would ideally travel in a perfectly straight line as it went from air to glass media, and the primary colors of Red, Green, and Blue would bend at the same angle if and when they did bend. Fat chance. Because lens elements used curved surfaces (if they don’t, we refer to them as “windows”), and because Refraction is variable within any concentric annular zone on the lens surface…well…you get the idea. It’s a mess. All you probably need to know is that Abberations are typically caused by curved surfaces.

• Distortion: Distortion is distinguished from Abberations in causation, and is produced by the position of the aperture in certain lensing systems. Distortion is typically divided into two classes: Barrel and Pincushion. The Amish homecrafting taxonomy is a bit goofy, but beer comes in barrels, so I tolerate it.

• Seidel Aberrations: The technically correct term is “von Seidel Aberrations,” which sounds more exotic than just “Seidel Aberrations.” Mary Shelley would have used the “von” precedent, I’m sure…had she lived another 200 years. There are 5 such, and here’s a great explanation. Note that Distortion is a Seidel Aberration. It’s all starting to tie together, ain’t it?

• Chromatic Aberration: CA is not a Seidel Aberration, which probably breaks your heart. There are two species of CA: Longitudinal (”LCA”) and Transverse (”TCA”). Here’s a very good resource on CA. It’s one of the spatial and tonal resolution serial killers and is not accounted for in MTF (to the best of my admittedly limited knowledge). LCA is sometimes called Axial CA (erroneously, because it occurs off-axis as well), and TCA is sometimes called Lateral Color.
  • LCA is nothing other than various colors focusing at different distances, and varies with the aperture (think “Depth of Field”). Canon’s L and Nikon’s ED glass have all but resolved LCA.
  • TCA occurs when varying colors are magnified at different rates, manifests as color fringing in the periphery and is not mitigated by stopping down. TCA is common.
  • Primary Chromatic Aberrations occur when the Red and Blue margins of the Red-Green-Blue Spectrum are incoincidental. If you notice purple fringing, you’ll think to yourself, “Purple is a combination of Red and Blue!”
  • Secondary Chromatic Aberrations occur in the CA that remains after the Primary CA’s have been corrected…meaning misbehavior by Green-you’ll notice green-magenta artifacting and think to yourself, “Green, and its complement Magenta are deviant! I sure wish I had an apochromatic lens!”

• Curvature of Field: One of the Seidel Aberrations is Curvature of Field, which simply describes the fact that your image is forming within the camera not on a plane coincident with the sensor, but as a bit of a hemisphere. This may explain why our eyes are spherically designed-so that all elements of the sensing surface are equidistant from the lens node. Thus, no light will have to travel further (and thus magnify disproportionately) than any other light in a single image. It is related to Elliptical Distortion, which makes people gain weight in the periphery of an image…just in case you’re into that sort of thing. Astigmatism makes the curved field sort of lumpy.

• Airy Disk: If you’ve been developing an understanding by familiarizing yourself with the concepts in this list sequentially, Sean T. McHugh’s superb post on Diffraction and the Airy Disk is the logical next step. An important concept is the Rayleigh Criterion, which is more comprehensively explained here. The Rayleigh Criterion is fundamental to understanding why a 6 micron pixel pitch is difficult to lens for.

• Rayleigh Scattering: This guy Rayleigh, whose actual name was John William Strutt, 3rd Baron Rayleigh, was an extremely intelligent fellow. As if the Rayleigh Criterion weren’t enough, he also figured out why the sky is blue (insert Los Angeles joke here): Light “scatters” off of the molecules of Nitrogen, Oxygen, Argon, Carbon Dioxide, and the copious amounts of methane produced by cow flatulence in the atmosphere. Blue light (the shortest wavelength class in the visible light spectrum) is scattered “down” (and all other directions, but most importantly…) toward the earth more aggressively than other light. The reason clouds aren’t blue is because their molecules (water) are so much larger than the molecules of air gasses that the scattering effect is much more evenly distributed over the visible light spectrum. The “dark” portions of clouds are simply their bottoms, which are shaded. This is why Britney Spears isn’t a meteorologist.

• Nyquist Frequency: This is the tough one. First, think of two screens placed on top of each other. If they perfectly register (line up), the path through the screen holes will be maximized, otherwise they will be constricted. That’s a tangential analogy, but it gives you something to work with. Next, imagine taking a photograph of one of the screens with your camera. The screen is a rectilinear raster form…very regular square shapes at very regular intervals. So is your camera’s sensor (with respect to your Bayer Array’s demosaiced result…at least until we get hemispherical sensors)! So, if your sensor is not perfectly registered with the screen detail, the image detail will become extinct much more quickly than otherwise. Additionally, if your sensor pixels don’t occur at the same frequency as the screen holes, the image may be quite sharp in some areas where they are registered with the screen’s holes, but not in others. In order to defeat this phenomenon, you must have pixels that occur at a frequency of about twice the rate of the screen holes. This matter is terribly compounded by antialiasing, but if you can grasp the fundamental mechanical side of the issue, you’re almost an expert.

Now, with that explanation of Nyquist you’ll probably need to take a breath. Dwell on Nyquist for a day or two, digesting it, oddly photographing any screens you happen upon, mumbling unintelligibly. Remember, It’s Hip to Be Square.

L