God’s blessings to you on this the seventh day of January in the year of our Lord’s incarnation, 2017.
The world of photography is an interesting one. I’m very much an amateur, but owing to my mother’s much greater interest and budget for pursuing her passion, together with the progress in DSLRs (Digital Single-Lens Reflex cameras) I have a hand-me-down Nikon D300S. It’s an 8 year old camera which has been superseded by a subsequent model, but it was a great camera when it came out and it still takes better pictures that I’m capable of taking. Also, it takes the same lenses that all other Nikons do, and as a very rough rule-of-thumb, the lens is more important than the camera body. Some day I’ll probably invest in a newer body, but especially given the amount of time I have to devote to photography, it will undoubtedly be years before I’ve developed my skill to the point where the camera body is holding me back.
As a Christmas present, I was given a 500mm mirror lens. Whereas normal lenses focus light be refraction inside of glass, mirror lenses work like telescopes and focus light by reflection off of the surface of a pair of concave and convex mirrors. (The larger, concave mirror concentrates light onto the smaller, convex mirror, which then straightens it out and directs it at the camera’s sensor.) Oh, and a 500mm lens is a zoom lens roughly equivalent to 10x magnification from like a telescope or binocular. The curious thing about mirror lenses is that they are wildly cheaper than glass lenses. The cheapest glass-based 500mm lens that Nikon makes is over 12x more expensive than the lens I was given, and why this is the case is, I think, quite interesting.
Refracting light through glass has the problem that different wavelengths of light refract different amounts. This varies with the material, but the problem is that, to oversimplify, red, green, and blue light will actually have different focal points, which results in what is called “chromatic aberration”, or to be less technical, weird, slightly blurry colors. So to combat this, telephoto glass lenses have to be made out of very carefully engineered glasses. I use the plural, because in order to correct the light, telephoto lenses will actually have somewhere between 7 and 14 “elements” (i.e. a telephoto lens is really a system of a bunch of lenses), many of them made of different materials to correct imperfections in what the previous lenses did. As you can imagine, this is expensive, both because of the careful engineering, the precision of assembling that many lenses together, and just making and grinding that many pieces of optically clear glass. It’s sort of a miracle that lenses are as cheap as they are. And the good ones run into the tens of thousands of dollars!
In comparison, reflection works the same for all wavelengths of light, so a mirror lens can be made of just two mirrors as I described above. And mirrors are cheaper to make than polished optically clear glass with no internal distortions. So when you put it all together, mirror lenses are wildly cheaper than glass lenses. (Incidentally, you can also probably see why “mirror lens” is a contradiction in terms, and why the technical term for them is catadioptric optical systems.)
At this point you’re probably wondering why, if mirror lenses are so much cheaper to make at the same quality, they aren’t the standard. To some degree I wonder the same thing, but possibly one of the bigger reasons is that people generally don’t like the bokeh of a mirror lens. (Bokeh is basically how the things which are out of focus blur; glass lenses blur things into circles, which mirror lenses blur them into donuts.) There are shots where bokeh isn’t relevant, but it’s relevant in an awful lot of photography, hence the dominance of glass lenses. There’s also the fact that glass lenses come with auto-focus, and diaphragms to change the amount of light allowed onto the sensor (narrower openings give you greater depth-of-field, but require slower shutter speeds, while bigger openings give you a narrower depth of field; which is better depends on what sort of shot you’re going for). I’m not sure that’s inherent to glass lenses, though; I suspect that mirror lenses have basically found the niche of cheap, and as long as they’re going for that, they’ll sell best if they’re very cheap. At 12x cheaper than a glass lens, a 500mm mirror lens makes sense to play around with; if it was only half the cost of a glass lens, I suspect most people would just pay the extra money for the glass lens. Which brings up, once again, the curious topic that all sorts of things are technologically possible and would even make a lot of sense but aren’t done simply because there’s no market for them. Anyway, as I figure out how to get good results from my mirror lens, I’ll post some pictures on the blog.
(At 500mm, even slight shake in the camera makes the images blurry, so a tripod and a shutter-release remote are necessities, but it turns out that the camera shake caused by mirror-slap is a problem too. If you don’t know, an SLR uses a mirror in front of the sensor to allow you to look through the lens in the viewfinder. This mirror must get out of the way during photographs, and so it does, but you can’t move mass around quickly without it applying force to the body of the camera, and the sensor is mounted to the body of the camera, so it shakes. Normally this doesn’t matter, but for telephoto shots, and especially since a mirror lens is very light and thus doesn’t have enough mass to damp down the vibration, this is a real problem. Fortunately, there’s a mode my camera has where you can press the shutter button once to move the SLR mirror out of the way, and a second time to actually take the shot. Unfortunately, there’s nothing you can do about the shake from the shutter moving, at least unless you have a really top-of-the-line DSLR. Come to think of it, this is another reason to prefer glass telephoto lenses. The fact that they weight anywhere from 5-15 pounds (for the really huge ones) damps vibrations, which will give a clearer shot.)
Glory to God in the highest.