Originally Posted by
Inkanyezi
In general, a close-up lens added to a macro lens should be avoided, as it might introduce imaging errors.
When the close-up lens is achromatic, as the 2 diopters Canon 500D, it still can produce sharp results, particularly in the centre of the image, as there are no longitudinal chromatic errors there. However, it does introduce coma, curvature of field and distortion, which is alleviated if the angle of view is narrow.
There are two different, in fact very different, types of macro lenses, where one is focused by extension and the other by changing its focal length (inner focusing). Canon EF 100/2,8L USM Macro is an inner focusing lens, changing its focal lenght from 100 mm at infinity to about 50 mm at 1:1 reproduction.
When a close-up lens is added to a lens, the diopters of the close-up lens are added to those of the original one. A 100 mm lens at infinity, or one of the type that extends by focusing, has a diopter value of 10, and with the 500 mm lens, it becomes 12 diopters.
The inner focusing Canon 100 mm macro thus also becomes 12 diopters when set to infinity, but as it is 20 diopters at 1:1, as focusing is accomplished by altering focal length, Its diopter value changes to about 22 when the close-up lens is attached, increasing reproduction scale about 10 %. Whether it is worth the rather small possible drawbacks of adding coma and field curvature for such a little gain in scale, is up to the user. I would not put a close-up lens on the macro.