I read that we can remove the rear portion of a (for example) plasmat lens, and get something akin to an old-fashioned portrait lens.
I've tried it with a Sironar, and something happens, that's for sure.
To determine the new focal length of this lens, we can focus at infinity and measure the bellows draw.
How do we determine the new f/stop ?
Example: let's say a 150mm f/5.6 lens becomes a 300mm lens. It is now 2x the length.
Does that mean that instead of being f/5.6, it is effectively f/8 (1 stop less) ?