I recently picked up a superzoom lens, the Nikon 28-300mm. Although I got it primarily for its versatility, my intuition was that a lens that can do 300mm focal length at 50 cm, as indeed it can, would also offer reasonable magnification for macro shots.
I was shocked to discover that, within a distance of about 5 metres, my 105mm macro lens with 2x teleconverter offers a considerably narrower field of view at 210 mm than my 28-300mm lens does at 300 mm! I found a forum thread on this lens which explains:
Anybody expecting to be able to use it as a macro should carefully check the maximum magnification: 0.32x. Being a IF lens, the Nikkor dramatically increases the angle of view upon closer focus. [...] 0.32x at 50cm roughly calculates to a focal length of 92mm at [minimum focusing distance]... so "dramatically" could have been written even in capital letters.
I'd like to understand better what principles of lens construction and/or physics lead to this counterintuitive behavior. On a pragmatic level: it's clear that I can derive the effective field of view at minimum focusing distance from the maximum magnification listed in the specifications, but how do I go about determining the effective field of view at other distances? For example, how would I determine the field of view of my 28-300mm lens at 300mm and 3 metres? Can these be calculated or must they be determined empirically? If they must be determined empirically, are there people who publicly document this sort of thing?