Reflex mirror lenses, also known as simply reflex lenses or mirror lenses, use a different technique than refractive lenses to focus the light on the image sensor of a camera. Refractive lenses, or the standard design of lens you would think of if I were to just say “camera lens,” use glass lens elements to augment the path of the light that enters the end of the lens and focus it down on the image sensor of the camera. The only elements involved in the refractive lens design are shaped pieces of glass. However, a reflex mirror lens uses both glass elements and mirrors to bounce and augment the light path.
Technically speaking there are three major parts to a reflex lens: a front lens element, a collector element and a corrector element. If you were really picky you could group the front lens and the corrector as a single element as I’ll describe in just a moment, but for the purposes of this discussion I’ll address them as three. The front lens element gathers the light into the lens body similar to how the front lens of a refractive lens does. The collector, usually a concave mirror with a hole in the center, does exactly what its name applies. It collects the light gathered into the lens and then reflects it back toward the center of the front element, where the corrector element is located. The corrector element, often a mirror, but sometimes a mirror and lens combination then bounces the light path down through the hole in the center of the collector, through the aperture, and onto the image sensor or film. This is illustrated here:
Although I did not illustrate it, there can often be additional glass elements to further focus and correct the light located between the hole in the collector and the image sensor.
This design and process of using mirrors to focus the light was originally developed for use in telescopes, and is still used in most telescopes today from the small personal telescope to the large room sized telescopes used in observatories around the world. Although there are many variations, there are three design types that stand out as the most popular, particular for use in the lens for cameras. They are the Schmidt-Cassegrain design, Maksutov-Cassegrain design, and the Argunov Cassegrain design. These designs are all named after the people who developed the part of the lens, with the first name referring to the person who developed the design for the corrector and the second name for the design of the collector. As you can see, all three use the Cassegrain collector and as such I am only going to differentiate them by discussion the corrector element. If you are interested in the actual people behind these designs, I encourage you to look them up on the web, but I am not going to focus on them here.
The Schmidt corrector uses a very slightly convex front lens element and flat corrector mirror mounted on the back of the front element. This is a very common design found in the lenses made for cameras and in small amateur telescopes. The problem with the Schmidt design for camera lenses is that as the flat corrector has limited power to flatten the image and can cause aberration and distortion in the image. The Maksutov design uses a slightly concave front element and as such a slightly convex corrector mirror. The shape of the mirror helps to flatten the image and correct for aberration and distortion of the image. The last design, the Argunov, which is similar to the Schmidt, but uses additional glass elements in front of the corrector (in terms of the direction of the light path) to correct and better focus the light. Although this can produce better images, it is often only used in larger telescopes and fairly infrequently in camera lenses, although sometime additional elements are incorporated into some of the Maksutov designed lenses. All of these designs adjust the focus of the light be moving the mirrors closer together or farther apart.
So I don’t know about you, but I’ve had enough of me talking about the actual designs and the names of the people developed them. This blog is about cameras, lenses and taking photos, so I’m moving away from the technical and now talking about the advantages and disadvantages of this type of lens for photography.
On the surface, the advantage of reflex lenses appear to be very strong, and would make you wonder why there are not more of them used by photographers today. That may be more clear when I touch on the disadvantages, but let’s focus on the positives for a moment. The biggest advantage of reflex mirror lenses is simply stated as “more bang for your buck.” What I mean by this, is that these lenses can pack a much longer focal length into a package which is not only smaller and lighter, but also cheaper than the equivalent focal length in refractive lens. How is this so? Well if you look at the design above you’ll notice that the light enters the lens, travels the majority of the length of the lens to the collector, where it is reflected the majority of the length back to the corrector mirror before traveling the length of the lens again to the image sensor. This “folded” light path means you can have a lens body that is a fraction of the length of a standard refractive lens. The collector mirror and the concave shape also helps to magnify the image even more so the length of lens is even shorter. Additionally as there are few heavy glass elements, and only a couple of mirrors, the weight of the lens is much lighter. In a refractive design, a 500mm lens can measure nearly 16 inches in length and way upwards of 9 lbs. I don’t know about you, but that’s not a lens you can carry around and shoot without the assistance of a tripod of a monopod. Those refractive lenses are the ones you see on the side of a baseball game or a on the sideline of a football field. That’s a big lens to move around. Additionally, not just because of their weight, but because of their length, they have major issues with shake caused by the photographers hands, the wind or the action of the camera which is another reason form amble support. The same focal length of a reflex lens can measure only 4 inches in length and weigh just a little over a pound. That is a huge difference, and can mean being tied to a tripod/monopod or potentially being able to handhold a lens with major magnification power. As a major bonus, the reflex lens may only cost a few hundred dollars where the refractive lens could easily cost you upwards of $8,000.
Another advantage of these lenses is that they do not have issue with chromatic aberration, or color shifting, that often plagues long refractive lenses. Because the light is not passing through a series of spherical glass elements, but is rather being bounced between mirrors, the colors tend to stay true across the whole image, even at the edges where CA is usually an issue.
So why then would anybody spend $8,000 on a refractive lens that weights a ton and is tough to maneuver when they could have lens they could walk around with that only weight around and costs just a few hundred bucks? Here’s why. Quality. Reflex lenses have major drawback when it comes to image quality, and I’m going to touch on a few of them here.
As I mentioned above, reflex lens correctors, in addition to reflecting the light to the focal plane, are designed to help eliminate some of the aberration, in the form of distortion, that is inherent in this lens design. With that being said, there are no perfect correctors and there is still some distortion which is different than the typical “pincushion” distortion seen in longer focal length refractive lenses. This distortion for many photographers is unacceptable and there is little you can do in post processing to hide it. However, that is all in the eye of the beholder. A strong image will often make slight issues with aberration appear non-existent and so I personally do not hold this as such an issue as a professional looking for perfection would. I’m just looking to capture good and interesting photos as I work my way up to seeking perfection (at which point I too would probably not use a reflex lens). Also and I don’t have $8,000-$10,000 do drop on a refractive lens.
The second major drawback is a lack of resolution and contrast. The reflex design, the fact that the center of the light bath is blocked by the corrector mirror and the reflected light has a hole due to the collector mirror, can actually lower the overall resolution and particularly the contrast of the lens. This is fairly evident when you use one. Your photos do come out a little flat looking. Contrast can always be boosted in post-processing, but you don’t want to rely on post-processing to get a good image. This is also dependent on what and where you are shooting. Due to their focal length, reflex lenses work great for capturing close shots of the moon and other celestial bodies. The dark sky and bright bodies are not as affected by the lower contrast of the lens, although, it is still noticeable (just not as much). Overall, I find this to be the biggest drawback of this lens design, but I’ll touch on whether or not I think this is enough to completely avoid these lenses a little later on in this post.
Another big drawback is the fixed aperture design of reflect lenses. Due to the design and having the center of the light path blocked, a variable aperture design is not possible in these reflex lenses. As such, each lens has a fixed aperture, and it is usually fairly slow. For example, it is possible to to get a 500mm lens with an f/6.3 aperture or and f/8 aperture, but that aperture is the only option. For longer lenses, the aperture gets smaller. Most 1000mm lenses have a fixed aperture of f/11. With a long focal length, these small apertures can make shooting pretty difficult and could potentially ruin the benefit of potentially handholding the lens. Nikon used to make a 500mm reflex lens with an f/5.6 aperture, but this was the fastest lens I’ve seen and it is still a full stop slower than typical 500mm refractive lenses. Practically speaking, this is the feature of these lenses that bugs me the post, because when I typically shoot, the aperture is typically the setting I focus on first, and then I check my shutter speeds. I love have control over the depth of field so this is my biggest dislike about these lenses.
The last major disadvantage to these lenses is the bokeh, or the out of focus area in a photograph, can have a funny donut shape to it. This donut bokeh can be distracting to the image, and many find it displeasing in general. I personally don’t mind it and think it can add some interesting effects to some photos, but I would really like to be able to turn it on and off (I know this is not an option, but I’m just saying if it was an option, I would want it) depending on what scene I was shooting. The best thing to do is to look up images online which were shot with reflex lenses and decide for yourself if it really is personal preference. I should also highlight, that due to the narrower fixed apertures in reflex lenses, in many situations, particularly those that are evenly lit, the bokeh is not all the pronounces. However if you have a spot of bright light, for instance the sun reflecting off the water, or a street light in a scene at dusk, the bokeh will be visible.
So as you can see, the advantages of these reflex lenses are pretty great, but the disadvantages may outweigh them (depending on the photographer). Reflex lenses, although have been made for shorter focal length is the past, really are not made any more for focal lengths less than 400mm. Without the benefit of the small package for a large focal length, the more those disadvantages are weighted and most companies understand this. They have also fallen out of favor with the big lens manufacturers who would rather focus on tweaking their higher quality refractive lenses than spending time making reflex lenses. Both Canon and Nikon used to make reflex lenses and you can still find them used for fairly good prices. Nikon made a 500mm, 1000mm and even a 2000mm lens under its Reflex-Nikkor lineup. The 1000mm and particularly the 2000mm lenses are very rare (the 2000mm is also a beast of a lens) and often still fetch prices in the thousands. The 500mm came in a few varieties and can still be found for a few hundred dollars (although those prices can be affected by collectors). 3rd party manufacturers such as Vivitar, Samyang and Opteka still make manual focus reflex lenses in 500mm and 800mm lengths. The only first party manufacturer of reflex lenses is Sony, which they inherited from their acquisition of Minolta. Their 500mm is probably one of the better lenses on the market today and it is the only reflex lens with autofocus. This lens works on their Alpha lineup of DSLRS.
If you venture outside of the U.S. market, you can find reflex lens under the Rubinar brand. These lenses are Russian-made and are some of the best reflex lenses out there (but still not rivaling high-end refractive lenses). They are made using the same design as used by the company MTO which was started by Dmitri Dimitriev Maksutov, of the same fame as the Maksutov-Cassegrain lens design. The downside of the MTOs as well as the Rubinar is the weight. These lenses use a lot of metal and as such tend to weight more than those made by other manufacturers. I personally own two MTOs: a 550mm f/8 MTO-500 and a beastly 1100mm f/10.5 MTO-1000A. The 1000A weighs about 8 lbs which definitely requires a tripod (but still shy of the 9 lbs for a 500mm refractive lens). They both use an M42 mount (see this post for more on adapting lenses). I picked up both of these lenses on EBay from sellers in Russia. They are about 40 years old, but still are in great working order.
So you now know that I own two of these reflex style lenses, so you must think I am going to tell you to go out and get one. Well, not exactly. As I have said a few times in this post, a lot of the features, advantages, and disadvantages of this style of lens are really personal choices by the photographer. These lenses are certainly not for everyone. I would say that if you have a desire to get out and shoot with some long glass and are not in the financial position or have the desire to drop $8,000-$10,000 on some long refractive glass, then give these a try (they are also much better than some of the junk refractive glass that is often sold under off-brands for cheap on EBay…don’t buy those). If you are looking for top of the line image quality, you are not going to get that out of a reflex lens, but that does not mean you can’t take great, interesting photos. Remember there’s currently a whole slew of photographers making a name for themselves today with exhibits filled with photos taken with toy cameras, so technical image quality is not always everything. So as you can tell, I’m not going to tell you to go buy one, or recommend them whole-heartedly, but I am also not going to completely trash them either. If you have potential need for a long lens, or a desire to try one, then pick one up and see how it goes. It certainly is one of the less expensive options to try out in the world of photography and may open your eyes to something you enjoy. Whatever you decide to do, go out and capture the world around you.
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