Graflex.org

[RichS]

Here's another question I'll probably laugh about in a year...

I have recently aquired a Symmar 240/420 convertible. It is exactly what I wnated and looks good. And yeah, I know about how bad it will be when converted....

But, as always, I played a bit and noticed something I'm hoping someone will know about.

To convert, the front element is removed. The lens is then a 420mm or 16.5 inches. The bellows says 21 inches (roughly) from shutter to GG.
First question. Is this due to a shift in the nodal point because the front element is no longer there? I had hopes of also using this on my GVII, but not with 21 inches of bellows... And a difference of 4.5 inches is actually past the end of the lens. Shutter to end of lens is only 2 inches. Seems odd to me?
Second, I removed the rear element. Had to see what would happen... The front element does work just fine alone. But at 12 inches from shutter to film. I looked the lens up and it's suppose to be a Protar design. Now supposedly you can figure the focal lengths by adding or subtracting the inverse of the indivual FL to find the combined. this works fine for my other convertibles. But not this one. The front element should be 21.7 inches if the rear is 16.5 and combined is 9.45.
Anyone know why the math doesn't work for this lens? Or did I just completely miss something again? Or did they not truly use the Protar design and came up with something odd which is why the front element is not supposed to be used alone?
Mostly a curiosity question, and some info for anyone like me who hoped of using a 16.5 inch lens on a 17 inch rail and then realised that 4 more inches was needed...


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"Ya just can't have too many GVIIs"
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