>The bottom line is that contrary to a lot of internet >chatter, focusing does not change the focal length of the >lens.
I have only just read this comment.
Technically focal length is only measured at infinity focus.
In decades long gone all lenses were "symmetrical" and increased their physical length in proportion to their ininity focal length as you focused closer.
Regardless of infinity focal length a symmetrical lens at 1:1 doubles it physical length and the distance from the subject to the film plane is 4 times the infinity focal length.
This increasing of the physical length does narrow the angle of view with less than 100% of the light going through the front of the lens reaching the film. The unused light is absorbed within the mirror box and usually results in an increase in lens flare.
This physical increase in length in theory reduces depth of field. However the physical size of the aperture is unchanged so it is effectively a smaller aperture which gives more dof. The net effect of the 2 changes is dof is unaltered.
If you want wide angles on SLR's, compact telephotos, closer focus macros, mirror lenses and zooms you usually cannot have a symmetrical lens.
The result is that as you focus close the image size is not the same as with a symmetrical lens. Generally wide angles increase effective focal length and telephotos loose focal length. In extreme cases like the 500 mirror or the 28-105 at half life size the reduction is equivalent to 50% of the angle of view with a symmetrical lens.
All you need to know is that the lens design has an effect on the angle of view in close up, and that whilst this can effect screen brightness and dof what you see through the viewfinder is what you get.
The detail is of importance if you want to know exact dof or exact image size at a particular focus distance without looking through the viewfinder or using dof preview. It can be of importance with hand held meters where in close up correct exposure may vary by a full f stop between different lenses - but TTL metering sorts out any differences.
But for most shooting situations what you see is what you get and modern SLR's get the exposure right.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.