Advice on using front cell of G-Claron

[Lightbender]

Hello, I have an early version of a 210mm f9 G-Claron.
I noticed that I can use just the front cell and have a much longer focal length. (300mm?)
Does anyone know what the max aperture would be?
Will there be a focus shift when I stop down the lens?

Thanks for the info!

[Andrew]

9 x 300/210 = 12.9

[anyone, please correct me if I'm wrong]

not sure about focus shift but I guess you'd probably see it on the ground glass if there was enough to be a problem

[swilf]

Andrew is right, of course, but the more convenient way would be to calculate the correction factor.
300^2/210^2 = 2.05, that is, one f-stop.
And I suppose that the amount of aberrations introduced by halving would make focus shift neglible.