"Diopter" is just a measure of how strong a close-up lens is.
The focal length of a close-up lens with x diopters is 1/x meter.
So a 2 diopter lens has a focal length of 1/2m=50cm
That's what the Canon 500D close-up lens is.
If you put a 2 diopter close-up on your primary lens you cannot focus further away than 50cm (from the front-lens) but you can get much closer, when you focus your primary lens to it's closest distance.
Now, let's get practical (from the Excel-Labs of Dr. T
On a 50mm
prime lens which goes as close as 45cm normally (with a magnification of 1:7 approx.) the 2 diopter close-up lens should get you down to 28cm with an approx. magnification of 1:4
On a tele-lens (or zoom) at 200mm
focal length that normally focusses down to 1.44m (magnification of 1:5) the 2 diopter close-up lens will let you get down to 1:1.5
at 60cm distance from focal plane!
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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