Originally posted by SOldBear The strength of magnification is in terms of diopter, which is the inverse of focal length (in meter). Thus the 28mm is 1/028 = 35 diopters and the 35mm is 1/0.035 = 29 diopters.

For comparison, the Raynox 150 is 4.8 diopters and the Raynox 250 is 8 diopters.

That is correct.

A way to understand what's going on in general is that stacking 2 lenses, focal length f'+focal length f", creates a new lens with a focal length of:

f = f'*f"/(f'+f")

This and the distance of the "new lens" from the image plane is sufficient to estimate magnification, which is

M = (lens_to_image_distance/lens_focal_length) - 1

These two formula are sufficient to estimate magnification etc with complex arrangements of extension tubes, preliminary magnification, multiple combined lenses, etc.

There are many other short cut relationships for special cases like when the primary lens is on the camera and focused at infinity, but the above two equations cover all such special cases.

Dave

The next level of precision is a usually small correction for distance between the stacked lenses, d, and isn't much more complex, simply:

f=f'*f"/(f'+f"- d)

http://en.wikipedia.org/wiki/Lens_(optics)