**macroimage** wrote in post #7578745According to the leaflet that came with the 500D filter, using a EF 200mm f/2.8 L II USM and the 500D will give a magnification of 0.57-0.39x. The 0.4x above assumes that you are set to infinity focus. You can get closer. At infinity on the ring, focus will be 500mm from the sensor plane with a 500D filter.

200mm is 5 diopters. A 500D filter is 2 diopters so you get 7 diopters total when you combine the lenses. This gives a 143mm effective focal length with 200mm of extension at infinity. The 57mm of extra extension (200-143) divided into 143mm gives 0.398.

Thanks a lot for your detailed response! Unfortunately, I am unfamiliar with all the terminology. For example, I've no idea what a "diopter" is (though I can, of course, google it).

So while I'd love a tutorial on what all that means, the immediate concern is what is the closest I can frame with the 200 / 2.8L + 500D (I'm assuming it's 500mm) and what's the furthest I can frame.

Using the expression m ~ FL / (d - FL), where FL represents focal length and d represents subject-sensor distance, I get m ~ 0.67 for d = 500mm, so I'm assuming that the 0.57 you wrote above was a typo?

In addition, solving for d gives d ~ [(1+m) / m] · FL. So, for m=0.67, I get d ~ 500mm and for m = 0.398, I get d ~ 700mm (but I don't know where the m ~ 0.398 figure came from). So, would I be correct in assuming that I can focus only between 500mm and 700mm with a 500D filter on a 200 / 2.8L?

Anyway, thanks for the explanation, even if I don't get it right now. I'm off to bed, so maybe I'll lie awake trying to figure it out rather than sleep. : )