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I accidentally saw Nikon 17-35mm f/2.8 lens and noted that it is rather large for its focal length. Here's the first review of this lens search offered - looks like it's 107 millimeters long. Given its focal length of up to 35 millimeters I would expect it to be more like a pancake lens but 107 millimeters is very far from pancake lens.

I guess there're some design reasons for that.

Why is this lens so long given it's rather short focal lengths?

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The focal length of the camera lens defines its fundamental property. We are talking about such things as angular field of view, relative aperture, depth of field and depth of focus. It is customary to fit lenses to cameras that have a focal length approximately equal to the diagonal measure of the format. Such a lash-up delivers a “normal” angle of view. If the lens mounted is shorter than the diagonal, the angle of view will be wide-angle.

This Nikon 17-35mm is designed to fit on a compact digital (DX) with a format that measures 16mm height by 24mm length. The corner to corner measure of this rectangle is approximately 30mm. In other words, a 30mm on your camera delivers a “normal” view.

The focal length of a lens is a measurement taken when the lens is focused on a far distant subject. We measure from a point called the rear nodal to the focused image which will be the surface of the image sensor. When a wide-angle lens is mounted such as this this one, set to 17mm, the rear nodal must be positioned 17mm from the imaging chip. This is a super short distance, only about ¾ inch.

Such a close lens positioning is virtually impossible, not enough room for lens barrel, glass, mount, aperture and other stuff. What to do? The optician must shift the rear nodal more to the rear. Normally it would fall somewhere within the lens barrel. This shift can be extreme, it could be made to fall in the air behind the lens. This design elongates the back focus distance; now there is room for everything. This is accomplished by inserting several lens elements, some with positive power and some with negative power. The correct combination resembles an inverted telephoto. This is known as a retro-focus lens. Such a lash-up is how you get a 17-35mm zoom to fit on a DX camera.

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    Good post, but just to clarify, this lens covers 35mm film and FX full-frame digital. I've owned one since before I had a DSLR. – Jim MacKenzie May 11 '18 at 15:09
  • @ Jim MacKenzie -- All lenses project a circular image. The center of this circle is the sharpest and brightest part. The image formed away from this central location dims and blurs towards the edges. Thus only the central portion is photography useful. This is called the circle of good definition. It is difficult to make a super wide-angle that will deliver, without noticeable vignette, an image that covers. The longer the back focus the better the coverage. – Alan Marcus May 11 '18 at 16:04
  • I beg to differ with you. Certainly the centre is sharper, but the corners are more than adequately sharp, especially as you stop down. This lens was designed for full-frame. – Jim MacKenzie May 11 '18 at 16:10
  • @ Jim Mac Kenzie -- If you were an insect crawling on the sensor, looking back at the lens from the center (axis) , you would see a circle of light when the shutter opened. If the insect moves to the boundaries, looking back at the lens, an ellipse is seen. The circle has greeter surface area then the ellipse. Thus the center of the image is brighter than at the margins. This is called cosign error. The fact that your lens delivers a satisfactory and uniform image is quite an accompaniment, a tip of the hat to Nikon opticians. – Alan Marcus May 11 '18 at 16:18
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    In contrast to retro-focus (wide angle) lenses which are longer than their focal length (intentionally moved forward, for example, so that the SLR mirror can be raised behind them), telephoto lens bodies are shorter than their focal length (just a convenience). A long lens (like a telescope) doesn't have to be built shorter, but the word telephoto means "shorter than its focal length", i.e., those that are built that way (most longer camera lenses). – WayneF May 11 '18 at 17:48
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There seems to be a fundamental miss understanding that some how the focal length of a lens represents its physical length.

The focal length of a lens refers to the distance it sees or its field of view and is not a reference to its physical size. Although a longer focal length telephoto is physically longer then a shorter focal length lens, the physical length of the lens is not equal to the focal distance number. I.E. a 400 mm lens is not physically 400 mm long. How does one "accidentally" see a lens?

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There are different optical designs. From Wikipedia:

a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length.

What you are seeing is the reverse though which is a retro-focus design, also from Wikipedia:

The retrofocus lens solves this proximity problem through an asymmetrical design that allows the rear element to be farther away from the film plane than its effective focal length would suggest.

This amounts to having a design where a reversed telephoto lens is placed at the back of elements to allow a wide-angle lens to be built for a large sensor.

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