Martin Evans
New member
I am reading a book by a well respected author, who has written many books about the technicalities and art of photography. Early on I find in a photo caption: "... the shot was composed with a 20 mm lens ..."
What a meaningless number this now is! For most of our lifetimes, it would probably have meant a lens with a 20mm focal length fitted to a 35mm film camera. What might it mean nowadays? Probably not a lens on a quarter-plate camera, and maybe not even on a medium-format camera. But is it on a 35mm or 'full frame' camera, an APS-C sensor camera, or one of the innumerable smaller digital sensor cameras? On a 35 mm camera, a 20 mm lens would definitely be a 'wide-angle'. On the Canon compact that I carry with me when I don't have my dSLR, 20mm is medium-telephoto, equivalent in view to a 100 mm lens on a 35mm camera.
What can we do about this? The immediate answer would be to specify angle of view rather than focal length. But even angle of view is not precise. If we specify angle of view across the diagonal of film or sensor, we have to remember that not all sensors keep the old 24 x 36 mm ratio of 135 film. Furthermore, virtually all optical maths revolves around a focal length measurement.
I thought that Doug might touch on this in his detailed essays about the "reach" of a lens, or perspective and focal length, but we have become too used to talking about focal length simply in terms of millimetres.
It will be an uphill battle to change this nomenclature for the 100 million or thereabouts photographers in this world - let alone the powerful manufacturers. But has anyone any useful suggestions about proceeding in a more rational and precise direction?
Idle thoughts on a Friday night...
Martin
What a meaningless number this now is! For most of our lifetimes, it would probably have meant a lens with a 20mm focal length fitted to a 35mm film camera. What might it mean nowadays? Probably not a lens on a quarter-plate camera, and maybe not even on a medium-format camera. But is it on a 35mm or 'full frame' camera, an APS-C sensor camera, or one of the innumerable smaller digital sensor cameras? On a 35 mm camera, a 20 mm lens would definitely be a 'wide-angle'. On the Canon compact that I carry with me when I don't have my dSLR, 20mm is medium-telephoto, equivalent in view to a 100 mm lens on a 35mm camera.
What can we do about this? The immediate answer would be to specify angle of view rather than focal length. But even angle of view is not precise. If we specify angle of view across the diagonal of film or sensor, we have to remember that not all sensors keep the old 24 x 36 mm ratio of 135 film. Furthermore, virtually all optical maths revolves around a focal length measurement.
I thought that Doug might touch on this in his detailed essays about the "reach" of a lens, or perspective and focal length, but we have become too used to talking about focal length simply in terms of millimetres.
It will be an uphill battle to change this nomenclature for the 100 million or thereabouts photographers in this world - let alone the powerful manufacturers. But has anyone any useful suggestions about proceeding in a more rational and precise direction?
Idle thoughts on a Friday night...
Martin