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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
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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.

  • 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
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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.

  • 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
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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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