I bought a Tamron 16-300. Question is: how can it have a focal length 0f 300 mm when it is only about 30 mm long? Doesn't the lens have to be 300 mm from the sensing element?

  • Short answer, no. Multi-element lenses can be designed to be much more compact. See: photo.stackexchange.com/questions/38192/… – MikeW Nov 12 '14 at 1:08
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    Note: the distance from the sensor to the front of the camera body is about 30mm. The lens length is quite a bit longer than that. – user13451 Nov 12 '14 at 1:56

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:


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  • Welcome to Photography.SE. Couldn't help but notice the similarity in username. Always a pleasure to have another A Henderson around. – AJ Henderson Nov 12 '14 at 17:30

There are several approaches to getting a lens system with a long focal such that it is closer to the media than its focal length. The most common of these is the 'telephoto' lens

enter image description here

From Wikipedia: https://commons.wikimedia.org/wiki/File:Lens_telephoto_1.svg

What this does is it that the outer lens element (at the front of the lens system) has a positse focus and is bending the light more significantly than a simple lens of some focal length would. Then the second element group (with a negative focus) refocus the light again so that the light appears to have come from a much greater focal length. This allows for a shorter lens system than the focal length would be otherwise.

Another approach is to 'fold' the lens with mirrors.

Nikon 500mm f/8

This is a 500mm lens that is only 109mm long.

The light path itself does have some glass in it, but it is bounced around a bit:

enter image description here

From Wikipiedia: https://en.wikipedia.org/wiki/File:MinoltaRFRokkorX250f56text.svg

You will note that the light comes in, is adjusted a little bit, bounces off the back (and adjusted), bounces off the front (and adjusted) and then goes through a few more pieces of glass. The light path is folded through the lens to get a much longer focal length in a smaller distance.

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  • That Rokkor 250/5.6 was a lovely little lens, by the way. About the same size as a conventional 85mm, and easily hand-holdable down to 1/125 unless you were wired on caffeine (which really cut down on the film-swapping; it would be pretty much right on the money at ASA 100 and 1/1000 at high noon, or ASA 400 and 1/500 in daylight woodland shade if you were shooting little critters). – user32334 Nov 13 '14 at 21:46
  • @user32334 personally, I like the Nikkor 2000 f/11 (only 598mm long). Neat bit on the light path and nature of the doughnut bokeh of reflex lenses at pierretoscani.com/echo_shortpres.html – user13451 Nov 13 '14 at 21:55

Making a lens shorter than a simple lens is exactly what the term telephoto means. Telephoto groupings basically act kind of like a magnifying glass, making the field of view close faster than would happen with a simple lens system.

Traditional telephoto lenses still required a fair bit of space and lots of lenses to operate, but newer things like diffractive optics can condense this down even further to make very short telephoto lenses without sacrificing a whole lot in terms of image quality.

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