The term apochromatic/apo is used to designate any lens that minimizes color dispersion. In an even more basic description, apo lenses bring all colors of visible light to a single focus point. Before going any further, some basic scientific knowledge is required.
All colors we see are part of the electromagnetic spectrum. In fact, visible light is just a tiny sliver. From radio waves on one end to gamma rays on the other, each type of energy on the spectrum has a different wavelength, measured in nanometers (one billionth of a meter). Although covering just a tiny portion of the spectrum, visible light, red, orange, yellow, green, blue, indigo, and violet (think ROY G. BIV) all have a different wavelength, which complicates the process of bringing them to a single focus point over a short distance, which is exactly what a lens seeks to do. If all light is not focused at a single point, image quality will degrade. Images will appear soft and they will often show chromatic aberration, which appears as false color fringes in high contrast situations. The good news is that there are three main ways around the problem.
1. The first, and oldest, is to have a lens with a long focal ratio that gives the light a longer path to get in focus, thus minimizing focal length differences for different wavelengths of light. In the old days, refracting astronomical telescopes had long focal ratios, often around f15, and showed no false color except on the brightest of objects. In practical terms, a telescope with a four inch lens would be five feet long, which is not very portable. In photographic terms, long focal ratios require slow shutter speeds, which are often undesirable and necessitate use of a tripod in all but the brightest lighting for most people.
2. The second way, the quick fix, is through filter usage. Filters can be threaded onto camera lenses or telescope eyepieces to minimize the false color. Unfortunately, these filters can introduce inaccurate color casts to the image and also reduce the amount of light passing through the lens, which will result in longer shutter speeds on cameras and dimmer views in telescopes.
3. The third, and by far best method, is by utilizing special glass that will bring all colors to a single focus point in the first place.
Unfortunately, APO as an acronym could stand for “another paycheck obliterated.” Special high-grade glass that will bring all the colors to focus at the same point is expensive. Added to this is the fact that these types of glass are hard to work with, which means higher labor costs. The good news is that, for people who demand the highest image quality possible, apo lenses are worth the investment. Fully corrected for chromatic aberration, the false color will disappear. An added bonus is increased correction for spherical aberration, which results in sharper images.
Keeping in mind that pictures are worth a thousand words, I put two telescopes, one achro and the other apo, through a false color optical torture test: an extremely high contrast situation courtesy of tree branches and a bright sky.
Photo notes: All photos (except the last) are heavy crops taken from an original.
Photo 1 was taken with an Orion 120mm f8.3 non-apochromatic refracting telescope ($300 OTA). The false color is very obnoxious.
Photo 2 was taken with the same scope but with an aperture mask that made it into a 60mm f16. False color is greatly reduced, but exposure requires a shutter speed four times longer than photo 1.
Photo 3 was taken with an Orion ED80 f7.5 apo refractor ($500 at release). Absolutely no false color at all, but this 80mm scope costs $200 more than the 120mm model.
Photo 4 is the full, uncropped version of photo 1 to illustrate just how tiny the crops are. Viewed in full, the false color does not appear so extreme. However, the more the image is cropped, the more obvious it becomes. For astronomical uses, which are often far less forgiving than most terrestrial photography, even a full view photo will show ugly violet rings around brighter stars.
As a final note, apo lenses often go by many different names (LD (low dispersion), ED (extra low dispersion), (SAPO) super apochromat, SLD (super low dispersion), fluorite, and FPL-53). The terms all are more similar than different in their final goal: striving for better images, but that doesn't stop people, especially astronomers, from debating what constitutes a “true” apochromat. For me, an apochromat is any lens that shows no false color visually or photographically. Keep it simple.
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