Quote:
Originally Posted by greenwing
Most DX users don't know, and don't need to know, what any particular focal length looks like on 35mm. Why insist that they know that 18mm is 27mm, other than to fill up column inches? It only really matters to owners of FX cameras, and then I don't think they'll be particularly concerned about it.
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I agree. The only value in knowing the focal length is to get a feel for the degree of magnification that the lens will provide. In the days or yore when we used to cram stuff called, "Film" into out 35mm cameras the rule was (more or less) 50mm is 'standard', i.e. 1:1 and anything bigger than that was heading towards the telephoto (magnified) end of the spectrum and any smaller number indicated wide-angle. Thus, 100mm was roughly 2:1
This remains true in FX formats, but when we think in terms of DX we have to say that 35mm is about standard and a 70mm will be equivalent to the 35mm (or FX) 100mm
If you want to get techie about it, the 'standard' focal length is established by measuring the diagonal of the equivalent square image. In other words; the 35mm, FX format is 24mm x 36mm, so the square format would be 36mm x 36mm and the diagonal of that works out to 50.9mm. Therefore 50mm is the standard or normal focal length. The DX format is 18mm x 24mm which gives us 33.9mm on the diagonal.
Not so important these days, but 40 years ago we had little 16mm cameras, Olympus had a ½ frame, 35mm was common, as was 60mm x 60mm. 50mm x 70mm was creeping in and 4" x 5" was still in use. Losing favour, but still around was Quarter Plate, Half Plate, Full Plate (6½" x 8½") and 8" x 10". It was important to know your way round focal lengths.