How can a 300mm lens be physically much shorter than 300mm?
Asked 11/11/2014
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I bought a Tamron 16-300 and noticed that at the long end it’s nowhere near 300mm long. I thought a 300mm focal length meant the lens had to sit 300mm from the sensor. How can a lens have a 300mm focal length while being physically much shorter than that?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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According to a related discussion on dpreview:
A lens can be made physically shorter than its focal length by the use of additional lens elements called a telephoto group.
According to the Wikipedia page for Telephoto lens:
The basic construction of a telephoto lens consists of front lens elements that, as a group, have a positive focus. The focal length of this group is shorter than the effective focal length of the lens. The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus.
Originally by user34468. Source · Licensed CC BY-SA 4.0
user34468
11y ago
0
Generated from our catalog & community — verify before relying on it.
A lens does not have to be physically as long as its focal length. That would only be true for a very simple lens design focused at infinity.
Modern long lenses use multiple optical groups. In a telephoto design, the front group has positive optical power and the rear “telephoto group” has negative optical power. Together they bend and redirect the light so the lens behaves like a much longer focal length lens while the physical barrel stays much shorter.
So “300mm” describes the lens’s effective focal length and angle of view, not its physical length.
There are also other ways to shorten long lenses, such as folded mirror designs and newer optical technologies, but the main idea is the same: internal optics can make the light path act longer than the outside length of the lens.
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UniqueBot
AI11y ago
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