dont get this lens stuff.. i want a 35mm like my 18-200mm vr II at 35m

Started Nov 6, 2012 | Discussions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 13,233
Re: Down to basics

Focal length is only accurate at or close to infinity with most lenses.

Once upon a time most lenses were symmetrical optics - with a "technically accurate" angle of view when focused close.

In the discussion so far only the 50mm f1.8 has a "correct" angle of view based on a symmetrical design optics.

As a guide, vary aperture AF zooms and 1:1 AF macros widen their angle of view focused close compared to a symmetrical 50mm f1.8 - which is why in close focus the 18-200 at 50mm has a different viewfinder crop to the 50mm f1.8 D. At infinity the angle of view should be the same.

Most wide angle primes have a narrower angle of view (become more telephoto) than a symmetrical design the closer you focus.

To check if a lens is likely to change its angle of view in close focus relative to a symmetrical optic take it off the camera body, use the lens depth of field preview button and see if the aperture size looks the same apparent size viewed from the front or the back of the lens. If the apparent size is different (it usually is) angle of view will be different to a symmetrical design of the same focal length in close focus. Note sometimes a zoom is symmetrical at one zoom setting.

Why do most lenses do this?

With telephoto primes, not being symmetrical means shorter, lighter, cheaper to make longer focal length lenses with better close up performance.

Without retrofocus wide angle designs to give space for a DSLR mirror to lift the widest angle of view would be about 24mm.

In vary-aperture zooms it is a way to get decent optical quality at much lower cost.

In 1:1 macros it gives best close up performance (the number one requirement of a macro) with less weight and faster AF.

The trade off with all these modern advantages (a separate topic but I regard them as advantages) is for the same infinity focal length the viewfinder crop can be different at close focus distances.

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.

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