[Peter Zack]

Wiseman, That's correct. the shorter the lens the closer it will focus and the higher the magnification when the bellows is fully extended. The math is fairly simple. If you use a 150mm lens on 150mm bellows (fully extended) the image is 1:1 (or life sized). 100mm lens on a 150mm bellows the image is 1.5 times life sized. If you use a 35mm lens at 150mm of extension the image is 4.28 times life sized. So the same amount of extension = lifesized images.

So you are correct that a 50mm lens at 100mm of extension gives 2:1 or double life sized magnification. Focus distance does not change much (1-2 inches) and is very close to the lens. I could set it up and measure it for you today as I've been meaning to take a couple more pictures of the focus rails etc. A 50mm lens has one issue though with bellows that are not fully extended. The rail(s) that the bellows run on can get in the way with a short lens because the rails stick out front. Since the focus distance is only about 1.5-2 inches you may not be able to get the subject focused unless the bellows is almost fully extended or at least the front element of the lens is out to the edge of the rails.

Back when these were more popular, the common lens to use was a 100mm, 105mm or a 150mm. There were some 75mm lenses but less common. The reason was that the lens could be used with the bellows fully closed to fully extended without issues with the rails. The focus distance from a 50mm to a 100mm lens multiplies by about 10 making it's use much easier.

Since so many of us have old 135mm lenses or can get one from Ebay for $30. I thought this might help some people get started shooting macros when the flowers start appearing.

IMO a bellows (although funny looking and bigger) is better than a set of extension tubes. It's much more adjustable and once you've practiced, understood the exposure settings, focus technique and so on, you can get very good results at even higher magnifications than a macro lens.