The current system

The focal length of camera lenses is often referred to by its “35mm equivalent” length. The actual focal length is meaningless when quoted on its own, as the “effect” of the lens depends on the ratio of the focal length to the film or CCD/CMOS sensor size. For example, a 50mm lens is a standard lens (gives a view that looks like a natural human-eye view) in the 35mm system, but is wide-angle in medium and large format cameras, and a short to medium telephoto in APS and most digital cameras.

Digital sensor sizes are expressed using an archaic system relating to CRT sizes. Read this explanation of sensor sizes at the Digital Photography Review web site.

In the not-too-distant future, film will become obsolete, and the old 35mm notations will seem archaic and arbitrary.

My proposal

I don’t know if this is an original thought of mine. I suspect not: if it is, then the rest of the world are less imaginative than I thought. If it is original, I hereby place the idea into the public domain.

Lenses should have their focal length denoted by a dimensionless number: the right-hand side of the ratio of the focal length to the diagonal of the intended format, followed by a “d” for diagonal (not to be confused with D for dioptre). For example, in the 35mm system (36×24mm frame size with 43.3mm diagonal), a 100mm lens would be a 2.31d lens (100/43.3mm). Numbers significantly less than one refer to a wideangle; those much greater than one indicates telephoto. The “standard” 50mm lens would be 1.15d.

As another example, my old Nikon Coolpix 2000 has a real 5.8–17.4mm zoom lens and a 1/2.7″ sensor. This is equivalent to 38–114mm in the 35mm system. My system would have it listed as 0.88–2.63d, which is easily understood as slight wideangle to medium-short telephoto, and retains the ease of calculating zoom ratios (3× here).

My Minolta DiMAGE A1 has a 7× zoom range, from 7.2mm to 50.8mm on a 2/3″ sensor, which is 28–200mm equivalent. This would be expressed as 0.65–4.62d: a medium wide-angle to medium telephoto.

The full range of numbers likely to be seen in all but the most esoteric use would be from about 0.18d (8mm on 35mm) to 28d (1200mm on 35mm).

Limitations

Some lenses may end up being used in different formats. Most current digital SLRs that take carry-over 35mm lenses have this issue, and use a conversion factor due to the CCD being smaller than 35mm film size. This makes all lenses slightly “longer” than when used with 35mm.

A simple solution would be to also list the intended format size or the real focal length on lenses where this may be an issue. On non-interchangeable lenses (excluding cameras with removable backs…) this is not a problem.

The use of the image diagonal means that comparisons between squarer formats and panoramic formats would be difficult: but then they’re not exactly easy today.