Bellows extension vs effective aperture change with accessory lens

[Murray@uptowngallery]

Hello:

Newbie question:

I am prone to lens experiments.

I 'added' a negative 2-element front cell from a Rodenstock-Ysarex Polaroid 110B lens to the back of the 130 mm 3-element lens from a Polaroid 150. I left the shutter/iris mechanism intact.

This resulted in 300-310 mm f.l. as best I can estimate with a tape measure.

It covers the better part of a 10"x13" ground glass, certainly with blurry edges. I'll have to see how much is useable.

I want to estimate, if not determine what the new f and EV numbers will be. At first I thought to apply the bellows extension concept, and I'm a little uncertain because I might be missing one physical measurement.

If I started with 130 mm and now have 310 mm f.l, do I use (310/130)^2 or is it [(310-130)/130]^2?

Then, I imagine the effective aperture is distorted further by the effect of an optical change (analogous to adding an accessory lens), not just the bellows extension.

Am I on the right track, thoughtwise? Can this be measured with a lightmeter and compared to the pre-modified illumination level and have this be added to the bellows factor?

Thank you


Murray

 

[Murray@uptowngallery]

Oops - here is spreadsheet copy

new f # new EV #
actual labeled 130 310
EV(130) f EV(f) T EV(T) f T EV(T) EV(f) EV(310)
9.9 10 8.8 6.3 0.083333333 3.6 20.9 0.083333333 3.6 8.8 12.4
10.9 11 8.8 6.3 0.04 4.6 20.9 0.04 4.6 8.8 13.4
11.9 12 8.8 6.3 0.02 5.6 20.9 0.02 5.6 8.8 14.4
12.9 13 8.8 6.3 0.01 6.6 20.9 0.01 6.6 8.8 15.4
13.9 14 12.5 7.3 0.01 6.6 29.7 0.01 6.6 9.8 16.4
14.9 15 17.5 8.3 0.01 6.6 41.6 0.01 6.6 10.8 17.4
15.9 16 25 9.3 0.01 6.6 59.5 0.01 6.6 11.8 18.4
16.9 17 35 10.3 0.01 6.6 83.2 0.01 6.6 12.8 19.4