BetterPhoto Q&A
Category: New Answers

Lenses for film cameras and digital

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October 18, 2007

You see, the chip in the D80 is smaller than the size of a 35MM film frame, so the lenses designed for the DSLR are not built to create an image circle as large as what 35MM film lenses do. This is also why comparable DSLR lenses are smaller and lighter than their SLR counterparts.

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October 18, 2007

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October 18, 2007

When it comes to lens interchangeability, there are several factors that must be considered.

Many cameras have unique lens mounts. Some lenses screw-on others bayonet mount, adapter devices are available. Often lens and camera body are mated via mechanical and/or electronic coupling. Maybe a lens will mount but maybe some or all automation features will be lost.

The focal length of a lens, be it fixed or zoom, is chosen based on the size of the film and/or chip (media). In the case of the 35mm film camera the film measurement (frame size) is 24mm height by 36mm length the diagonal measure of this rectangle is 43 ¼ mm.

Lenses are commonly classified by their width of view i.e. their angle of view. Examples are “telephoto” “normal” “wide angle”. These classifications are based exclusively on the diagonal measure of the media (film or chip dimensions). A lens with a focal length that nearly matches the diagonal will be classified as “normal”. For a 35mm film camera this woks out be about 50mm (43 ¼ rounded up the angle of view will be about 53° i.e. the human experience as determined by opticians). Lenses significantly longer, say 100 or more millimeters in length are classified as telephoto’s that magnify and sport an very narrow angle of view whereby shorter focal length lenses are ones like 35mm or shorter with a wide angle of view.

Now digital cameras replace film with a light sensitive chip but it also has dimensions and the same rules apply. Thus classification can get quite complicated. For the most part, most digitals sport a chip that is reduced in size from the film dimension by about 66%. You can check you cameras specifications via its manual. Some digitals are said to be “full frame” thus the chip is 24mm x 36mm as the 35mm film frame has become the “defacto standard”. For most digitals with a chip 66% smaller, “normal” becomes about 28mm (the diagonal measure). Telephoto becomes lenses longer than 66mm and “wide angle” lenses shorter than 23mm. As a side bar; portraiture with a 35mm film camera is best when the lens employed is about 105mm as this combination yields an image with a prospective that about matches the view seen by the subject in his/her make-up / shaving mirror (the most accepted self-image). If the digital indeed shorts a chip 66% smaller, then the ideal portrait focal length becomes 70mm.

Note: The chip size of the Nikon D80 is smaller than the film thus your digital will not see a vignette (darker edges. A 35mm film lens is designed for the larger film dimentions the digital chip is smaller thus no vignette will be experenced.

Alan Marcus (marginal technical mumbo jumbo)
ammarcus@earthlink.net

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October 18, 2007

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October 18, 2007