Why are telephoto lens designs usually used for long focal lengths, not short ones?
Asked 11/23/2023
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A telephoto lens is defined by its optical design: its physical length is shorter than its focal length. If that’s the case, why are telephoto designs usually associated with long lenses like 200mm or 300mm? Why don’t we commonly see telephoto lenses with short focal lengths such as 20mm or 10mm?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
2y ago
2 Answers
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Let's put it the other way round:
If you were to construct a lens for a desired focal length, under which circumstances would you use a telephoto lens construction?
To make it concrete, let's talk about the standard 35mm format ("full-size" 24mm x 36mm sensor).
For a 28mm focal length, why would you try to make the lens shorter than 28mm? I don't see a significant benefit, but that length constraint would make the construction more difficult.
But for a 300mm focal length, customers will highly appreciate a lens that is shorter than that, so it makes sense to use the "telephoto tricks" when constructing the optics.
Originally by user79539. Source · Licensed CC BY-SA 4.0
user79539
2y ago
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Because a telephoto design mainly solves a problem that matters at long focal lengths: physical size.
For a simple lens focused at distant subjects, the sensor-to-lens distance is roughly the focal length. So a 300mm lens would otherwise need to be about 300mm long, which is bulky; making it physically shorter is a real benefit. That’s why telephoto constructions are useful for long lenses.
At short focal lengths, there’s much less to gain. A 28mm lens is already compact, so making it shorter than 28mm brings little practical advantage while making the optics more difficult.
At very short focal lengths such as 10mm, it can become impractical on many cameras. On SLRs, the mount and mirror box already place the rear of the lens much farther from the sensor than 10mm, so a true short telephoto layout would run into mechanical limits. The lens mount can also obstruct such a wide field of view.
In short: telephoto design is mostly used when shortening a long lens is worthwhile. For short focal lengths, other wide-angle optical designs are usually more practical.
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