Placing an auxiliary close-up lens (or close-up "filter") in front of a macro lens is another option. Inexpensive screw-in or slip-on attachments provide close-focusing at a very low cost. Some two-element versions are qualitatively very good while many inexpensive single element lenses exhibit chromatic aberration and reduced sharpness of the resulting image. When you use macro lenses with different diameters, for each macro lens a close-up lens has to be purchased separately. Most close-up lenses are marked with a +d number in diopter unit, the power of the lens. The diopter (or power) of a lens is defined as 1000 ÷ Fd, where Fd is the focal length of the lens measured in mm. Thus, a lens with a focal length of 50mm has a diopter of +20 = 1000 ÷ 50, and a +4 diopter close-up lens has a focal length of 250mm = 1000 ÷ 4.
The
maximally obtainable magnification can be calculated with the
equation (2F + Fd) ÷ Fd.
For Example: coupling a +20 diopter lens with a 60 mm macro
lens produces maximally a magnification of (2 × 60 + 50) ÷ 50
= 3.4.











