Originally Posted by Galah
I wonder if someone out there could help me, please, with the following question.

suppose you have a 100mm, a 200mm and a 300mm lens.

Suppose these lenses have minimum focusing distances of approximately 1metre, 2 metres and 4 metres apiece.

Now you interpose a 50mm extension tube between each lens and the camera.

You should be able to get a maximum magnification of 1:2, 1:4 and 1:6, respectively, OK?
Not quite.

The maximum magnification will be higher.
You only figured in the 50 mm extra extension. Not the extension the lens mount already provides on its own.

For instance, the 100 mm lens having a minimum focussing distance of 1 m would mean that it comes with about 12 mm extension in its mount, which has to be added to the extra 50 mm to figure out the maximum (!) magnification.

"about" 12 mm, because we don't know the internodal distance of that particular lens, which is part of the (minimum) focusing distance (which is measured from film plane to subject).

But supposing infinitely thin lenses (without internodal distance), i.e. assume that the focussing distance is equal to the sum of the object and subject distances, we could use a rearrangement of the classic formula 1/f = 1/u + 1/v.
First, to calculate what the subject and image distances (u and v in the formula) are given their respective sums (1 m, 2 m, 4 m) and the focal lengths (f).
Then, to calculate for each focal length how the subject distance shrinks when the image distance is increased by 50 mm, and what the resulting sum would be.

Well, how does this affect the minimum focusing distance in each case?
I don't like maths. Sorry!