Today’s Question: I am sorry, but what is a mirror lens? It is a term and an item I have never come across, in film photography or in digital. Are there any such lenses currently available?
Tim’s Quick Answer: A mirror lens (also called a reflex lens or catadioptric lens) is a lens that employs a convex mirror in place of one or more lens elements. These lenses were smaller and lighter than the more common models, but they don’t offer an adjustable lens aperture, and out of focus areas could create circular artifacts that many photographers found objectionable. Mirror lenses are still available from some manufacturers, such as a 500mm model from Opteka you can find here: https://amzn.to/2QNoNqE
More Detail: When I mentioned mirror lenses in a previous Ask Tim Grey eNewsletter about bokeh, the thought crossed my mind that some readers may not be familiar with these lenses. It also made me realize I might be perceived as a “dinosaur” for making the reference. After all, these lenses were most popular in the 1970’s and 1980’s.
Mirror lenses are commonly used in telescopes, and have also been available as photographic lenses. Many mirror lenses for photography were telephoto, because that is where the key advantage of a mirror lens has the greatest impact. Because a convex mirror is used in place of one or more lens elements that would generally be made of glass, a mirror lens can be significantly lighter (and smaller) than a “conventional” photographic lens.
However, mirror lenses also have some issues that contributed to their lack of popularity today. As noted above, there is no adjustable aperture on a mirror lens. That means you can’t adjust the depth of field with a mirror lens. In addition, if the shutter speed doesn’t provide enough control over the exposure, you may need to add a neutral density filter.
In the context of out of focus areas (as noted in my previous answer about bokeh), mirror lenses can produce ring-shaped artifacts that many photographers found distracting and undesirable. In other words, while a conventional lens would result in bright highlight artifacts in the shape of the lens aperture, a mirror lens would create “donut” shaped highlight artifacts.
So, mirror lenses do have some advantages, but I would say the disadvantages are more significant, which is why they are not in common use by photographers today.