Are long prime lenses for full-frame cameras usually true telephoto designs?
Asked 4/23/2014
4 views
2 answers
0
I understand that, technically, a telephoto lens is a long-focus lens whose physical length is shorter than its focal length because it uses a telephoto optical group. But many long prime lenses still look physically very long, especially at 300mm, 400mm, 600mm, and beyond.
For fixed-focal-length lenses made for full-frame and smaller formats, are most lenses around 85mm and longer actually telephoto in the technical sense, rather than just being called "telephoto" informally? Is there a practical limit to how much shorter than the focal length a telephoto design can be, and does that explain why very long lenses are still physically large?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
2
Pretty much every commercially available prime lens longer than 70mm for 35mm size or smaller cameras is telephoto in both senses of the wikipedia definition you quote: They are longer in focal length than the diagonal of the sensor they are designed for and they are shorter in physical length than their focal length.
For instance, the Canon 800mm f/5.6 L IS only about 460mm long from the rear to the front optical elements. Add the registration distance from the flange to the sensor and you are at just over 500mm for an 800mm focal length. Include the supplied hood and the tip of the lens housing is still only 650mm in front of the sensor.
The EF 85mm f/1.4 is only 80mm or so long. So is the EF 85mm f/1.8. That makes them both just shorter than their 85mm focal lengths. The EF 100mm f/2 L is about 81mm long, the EF 135mm f/2 is about 120mm long, and the EF 200mm f/2.8 L is roughly 145mm long.
Even though the longer Canon Super Telephoto lenses look totally monstrous in size, they are actually more telephoto in terms of the ratio between their focal length and their physical length (at around 1.33:1) than lenses in the 85-200mm focal length range. Lenses of the same focal length ranges from other manufacturers are similar in size to their Canon counterparts. Canon also makes Diffractive Optics lenses that are even more compact for their focal lengths, but they haven't really caught on because there is a small penalty in terms of image quality compared to the more conventional telephoto lenses. The EF 400mm f/4 DO IS at 244mm long has a focal length to physical length ratio of about 1.66:1.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
12y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. For 35mm/full-frame and smaller formats, most commercially available primes from about 85mm upward are true telephoto designs in the technical sense: their optical length is shorter than their focal length.
Even very long primes that look huge are still usually shorter than their stated focal length once you compare actual lens length plus flange distance to the sensor. So a 300mm, 400mm, or 800mm prime may be physically large, yet still be a telephoto design rather than a simple long-focus lens.
Why are they still so big? Because telephoto design does not let you shrink a lens arbitrarily. As focal length and aperture increase, the front elements must remain large, and correcting aberrations becomes harder. There are practical limits: making a lens much shorter usually brings optical compromises, and designers balance compactness, weight, aperture, and image quality.
There is also a difference between “long focus” and “telephoto”: a lens must first be long-focus for its format, then be physically shorter than that focal length to be telephoto. On full-frame, lenses longer than roughly the format diagonal are long-focus; in practice, most 85mm+ primes are telephoto designs. Non-telephoto long-focus options are more commonly encountered in large-format systems than in modern full-frame or smaller interchangeable-lens cameras.
Recommended products
UniqueBot
AI12y ago
Your Answer
Related Questions
What’s the generic term for a long-focal-length lens if “telephoto” is a specific optical design?
Why are telephoto lens designs usually used for long focal lengths, not short ones?
What’s the difference between a long lens and a telephoto design?
Why can a telescope outperform a long DSLR lens for astrophotography?
How can a 300mm lens be physically much shorter than 300mm?