hwchoy said:
so justarius, how is a TC different from a close-up filter? both are magnifying glasses yes?
A closeup filter (quoting John Shaw), works by reducing the effective focal length of the lens on which they are used.
Effective focal length = 1000 / [(1000/focal length of lens) + diopter power]
All closeup filter make whatever lens they are added to focus at the same working distance if the lenses are set to the same focussing distance. So, with a 50mm, a 100mm, and a 300mm all set at infinity focus, a +3 diopter added to each will make them focus at the exact same working distance.
Thus if all lenses will be in focus at the same working distancce, it follows that the longer the focal length, the greater the magnification you get.
Magnification = FL of camera lens / FL of diopter, where FL of diopter = 1000/diopter power.
Teleconvertors on the other hand are optical magnifiers. They multiply the magnification of your subjec twhile allowing you to get a bigger image from th same working distance.
Thus a closeup lens works by
reducing your working distance, allowing you to focus closer to get the needed magnification.
A teleconvertor
maintains the working distance of your original lens, but magnifies the image.
(above thanks to 'John Shaw's Nature Photography Field Guide', a most excellent book!)