Converting a lens - figuring out FL and speed?

[Scott --]

Hi, all -

I'm pondering my Darlot Hemispherique Rapide #2, a 9" FL lens, about f/10 or so on its own. It's an RR lens, and so should be convertible (right?). Short of mounting the lens (on a camera for which I haven't made a lensboard, and which doesn't have a bellows), is there a way to calculate the FL and speed of one of the groups alone? I'm curious if this little lens, which will (barely) cover 8x10, will cover 10x12 converted.

Thanks!
Scott

[Richard Rankin]

I just tried my smaller Darlot #1 like that and it doubled in length with one lens, as I was pretty sure it would do, and there is more coverage, but I don't have any way of measuring how much more.
Richard

[Bob Salomon]

This has been outlined many times before. Take a large white board. Darken the room. Point the lens out the window at a distant object. Move the board toward and away from the lens till the object is sharp. Measure the circle at that point. That is how large a circle the lens covers at infinity. If necessary tape a ruler to the board to make it easier to measure.