So.. simply put.. not all cameras have the same size sensor ("film"). This means if you put the same lens on them, they might not all capture the same frame. A lens with a certain focal length will project a certain image. But the angle of view of the actual
recorded photo can be bigger or smaller than that image projection. So a certain lens can
appear to have a different angle of view when you put it on, for example, Pentax Q camera, Pentax K-5, and Pentax 645Z. Because these cameras have a different sized sensor, which can capture less of the projected image (like the Q, which is very small) or a bigger sensor which captures more of the projected image (like 645Z, very big sensor).
Equivalent focal length was merely something that manufacturers told people who already know what a 50mm lens or a 200mm lens looks like on the old 24x36mm film SLR cameras. These people could be surprised if they put the same 50mm lens from their old Pentax K-1000 onto a new K-5, and the
recorded image would be different.
The problem is that all of this "equivalent" stuff can fool new photographers into thinking that buying a 200mm lens from another film or sensor format will give them a different angle from the 200mm they have on their camera. But it won't. On your camera, all 200mm lenses will appear the same. Or 24mm or 50mm or whatever. Doesn't matter if the lens was made in 1950s or if it was made for 67 format. Just note that if you use a lens made for a smaller format (like for the Q), then the recorded image will have black edges; as the lens' projected image will not cover the whole sensor. Whole thing is a big silly: equivalent means that the lens on a certain camera will have an angle of view similar to a different lens mounted on a different camera.
Anyway, if you are only taking photos with one camera just ignore all of the "equivalent" stuff. You will only have to learn about this if you use cameras of different sensor size (or film size). For now, just look at the actual focal length of the lens and remember what it looks like on your camera. All lenses with a given focal length will give (approximately) the same angle of view on your camera.
Originally posted by fstop18 So in the above example the manufacturer should designate the lens as 35-270mm not 20-200mm.
No, because the lens only
appears to be equivalent to that second number when mounted on
certain cameras. Just look at the true focal length, which is a property specific to the lens and has nothing to do with the camera it is mounted on, and remember its look on your camera. The only time you will look at equivalences is when you will mount those lenses on cameras with different sensor size. Then you might notice that 35mm is wide angle on one camera, but not wide angle on another; even though the lens itself doesn't change, its just the camera recording more or less of its projected image.