A telephoto lens, is a component that attaches to a camera in order to look up close at faraway objects. This type of lens sometimes relies on complex optics to function. In contrast, a mirror telephoto lens duplicates such capability in a smaller component, with the aid of a special mirrored design. This type of lens is typically cheaper and heavier than standard telephoto types. To choose the best mirror telephoto lens, it's vital to understand how common fixed specifications like aperture size and focal distance measure up to the type of shooting you do.
Generally, a mirror telephoto lens can be characterized by its focal length, in millimeters, and a mirror aperture size, measured in f-stops. This type is relatively small compared with typical telephoto lens designs. Essentially, light enters the lens and bounces off a center mirror. The light reflects back to the tube's front interior, where corrective back-facing mirrors cast the light into the camera. Adding this zigzagging path gives a mirror telephoto lens similar performance to a much larger and more expensive lens.
Certain disadvantages come with this type of lens, however. Most products come with a fixed aperture, meaning it's not possible to adjust depth of field, though a few exceptions do exist. Products may perform with varying degrees of sharpness, from poor to very sharp.
Using a secondary mirror also sometimes introduces doughnut-shaped light artifacts that might interfere or enhance the type of photography you do. This type of lens may also be slower and heavier. Still, it may have a reach comparable to more expensive telephoto lenses.
When choosing a mirror telephoto lens, review other specifications as well. Note how many elements are involved in its lens construction. Determine its minimum focus distance and magnification. Not only will its case dimensions factor in, but also fixed aperture and filter sizes, and which types may accommodate your shooting needs. While many mirror telephoto lens products feature fixed focal length, variable or zoom lenses do exist.
Balancing all these factors, the unique characteristic of this lens is how it can contain the power of a more powerful lens with less space and lower price. This may be worth a look for photographers who desire such performance without the cost of a precision telephoto lens. Typically, this type of lens may function well in shorter distances with less demanding degrees of sharpness; for example, for closeups of wildlife in captivity.
Common mirror telephoto lens products can range in price from hundreds to thousands of dollars. Besides weight and performance, weigh in any additional features that may come with the lens. Products may offer front and rear lens caps, hoods, and different warranties. Choosing the right lens can pack powerful shots in a small package.