Most zooms are not the focal lenght that you dial in when you are at close focus distance.
Nikon 18-105 becomes a 78mm when focusing at 4 feet for example. Its called varifocal.
Varifocal lens - Wikipedia, the free encyclopedia
There are some formula to calculate the true Focal lenght goes something like this
Focal lenght = d / (2 + m +(1 / m))
d = distance to object in mm
m = magnification (1:1 = 1, 1:2 = 0.5 1:5 = 0.2 etc)
Sigma 28-135 macro zoom gives you. Closest focus gives you distance to objekt, 240mm, magnification is 1:2 = 0.5)
Focal lenght = 240 / (2 + 0.5 + (1 / 0.5)) = 53.3mm
So your zoom is most likely true 50mm only at infinity.
Last edited by melander; 08-28-2009 at 12:07 PM.