I was just playing with a new 12 inch Dagor. I searched out his lens because it was supposed to be good, supposed to be convertible, and was made about the same year as my camera. How could I pass it up?

But it brought up the question (for me anyway) about f-stops when converted. Of course the Dagor doesn't have a dual scale like my officially convertible lenses, which I never gave much thought.

After a quick test, I found the lens to be approximately 11 1/2 inches, and just over 23 inches with the front cell removed. That made me think of f-stops since there's no scale...

Here's my 'problem'...

Essentially going through the same lens but with twice the bellows draw is about the same as using a non-converted lens at a 1:1 mag ratio. That's 2 extra stops. Easy and makes sense... But... I'm only using half the lens! My thought would be that there should be more light getting through. So say 1 1/2 stops added when converted (rough guess).

That made sense till I looked at my other officially convertibles. On average, they add 2 1/2 stops when doubling the focal length with a cell removed. So they account for a 1/2 stop _less_ light hitting the film when using half the lens... That doesn't make sense to me...

So, as usual, here's the questions:

Is there any formula that would calc f-stops for a converted lens?

And does anyone know why there's a half stop less light hitting the film when going through half the lens elements?

I hope all that made snse? And yes I can measure the light falling on the GG to make a dual scale, but (again)... After checking the scale I made for a lens I swapped shutters, My scale for the double focal length is only about 1 1/8 stops over the normal focal length... Somthin wrong there... I'll have to do two lenses now

I sure don't know, but I do someone who I bet does. He doesn't post on this site, at least I don't remember seeing him here, but this is a perfect question for Richard Knoppow, whose address is dickburk@ix.netcom.com


<font size=-1>[ This Message was edited by: alecj on 2005-02-22 19:00 ]</font>

Alec, thanks. I've read quite a few post by Richard. He does tend to get technical at times I may pose the question someplace else, but I doubt I'd bother him with a private e-mail. I hate to intrude on people about as much as I enjoy trying to start conversations here

Charles, That's how I do the new scales. I _thought_ it worked at the time but now really doubt what I did was correct? So, since I have to double check that old Gundlach, I may as well do this lens too. Unfortunately, it's in a Compound shutter with the IBT switch right above the f-stop scale, so I can't make a continuouse new scale for the converted range. Looks like I'll miss two stops or so, but I can live with that...


<font size=-1>[ This Message was edited by: RichS on 2005-02-22 20:14 ]</font>