how to deal with unscrewed lens elements en f-stops?

[stehei]

Hello,

I've got 2 lenses that can be unscrewed in 2 parts,
convertibles, so you like.

How to deal with the F-stops? Should I add 2 stops for
correct lighting? When unscrewed the elements make
the double focal lenght. So my 240mm makes a 500mm
roughly, my 300mm a 600mm.

regards

stefan

[Jim Galli]

You can add 2 stops or you can do like me and take a tape ruler. Measure the aperture and measure the extension then divide. Whichever is easier. If at infinity doing the 2 stop thing is pretty simple. But if you're extended well beyond infinity for a portrait or something it gets easier to just measure the hole and divide into the extension.