Reverse mounting question

[dima1109]

I have a Sigma 105mm macro lens, and I've been toying with an idea to take another one of my lenses, an 18-55mm or an 85m, and try reversing it on the end of the macro lens, supposedly to get a little more magnification. My question is - is there a formula/way to calculate the magnification of such tandem with a given macro lens and an arbitrary lens mounted on the end? I know there is a way to calculate the magnification of a lens paired with a set of extension tubes of arbitrary length. Is there something like that for lens?

[LordV]

Magnification = existing magnification + focal length main lens/focal length reversed lens.
So for a 105mm macro lens at min focus with say a reversed 50mm
magnification = 1+ 105/50 =3.1:1 approx.

It's much easier just to photograph the mm scale of a ruler to check the actual magnification.
Magnification = sensor width mm/mm across pic

Brian V.

[dima1109]

I see. So, say, adding a 28mm lens would give me an overall magnification of 4:1?

Another question: how would I calculate the working distance in the reverse mounting case? I'm assuming it would be quite small.

[d.tek]

bump cuz i'd like to know!

[LordV]

I'm sure there is a beautiful complex calculation available somewhere with lots of inputs you won't know the value of :)
Can only guess it will be somewhere between 0 and 1.5" in front of the reversed lens with a 28mm reversed.
Brian V.

[dima1109]

Found some stuff:
http://www.jeffree.co.uk/pages/macro-lens-calcs.html