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Short focal lengthcanon

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?

Longfocal length

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.

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.

Short focal lengthphotography

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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.

How to measureshort focal length

What isfocal lengthof lens

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.

There seems to be a fundamental miss understanding that some how the focal length of a lens represents its physical length.

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Short focal lengthvs long

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.

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Focal lengthexamples

Short focal lengthchart

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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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.

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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.

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Screen magnification software allows users with low vision to read what is on the computer screen by enlarging and enhancing the text.