**Theory and praxis of magnification with macro rings**
There is an interesting issue of variable magnification of the lens+macro ring according to focusing distance.

According to macro math, for example:

www.peterforsell.com
the magnification formula is: M= ((mL x F) + E) / F

where

M = magnification

mL = the magnification of the lens alone (0.15 for A50/1.4)

F = the lens focal length in millimeters

E = added extension in millimeters

Magnification in this formula should be constant for given F and E.

I made test with A 50/1.4 and 20mm extension ring.

1) Picture of a ruler, with focusing ring set up on the shortest distance gave 43mm (the sensor wight 23.7mm). So the magnification is 23.7/43=0.55, what is the same as formula

2) Picture of a ruler, but this time with focusing ring set up on infinity gave 60mm, so the magnification is 60/23.7= 0.40

3) I also made test with the DA 50-135+20mm ring. The diference of magnification was much smaller, ca 0.40 vers 0.42. The reason might be internal focusing.

Summary:

Above mentioned formula is not valid for different positions of the lens focusing ring (and focusing distance) . In case of A50/1.4 the formula was valid just for the shortest focusing distance (0.45m).

The reason for that (correct me if I am wrong) could be that lens with macro ring changes its focal length according the position of the focusing ring.