Extension tube math


Senior Member
OK, I have been looking at getting some extension tubes for some macro work and trying to figure out the math. On Adorama they have a Kenko extension tube that states "magnification ratio for Tube 25 is about 1/2 with the 50mm standard lens". So when looking at this it would seem that the 50mm lens would act more like a 100mm and would that also equate to a reduction in minimum focus distance by 1/2? According to the specs the 50mm has a minimum focus distance of 1.5ft or 18 inches, would that then change to 9 inches?
I understand the concepts behind the depth of field changes with the extension tubes but not sure on the other affects based on the extension tube numbers.
I am actually looking at getting the Kenko DG auto extension tube set which has the 12, 20, and 36 included. I want to know what each is going to bring to the table.


Happily retired
Staff member
Super Mod
When magnification is quoted in numbers, usually it means that the object you will be photographing will be for 1/2 two times bigger (or is it half the size... not so sure anymore)on the sensor or film than it is in real. 1/1 would be where the object is the same size as in real life.
But it does have to do with size of object relation to size on sensor.

Hope this helps.