Lens help needed

[Jaq]

I received an 8x10 camera with an old lens - a Goerz Extra-Rapid_lynkeioskop N°6.

There is no diaphragm with the lens, but there is a slit where I suppose you can put a card in. Is this how it works? If so, where can you find these "cards", can one make them?

There is no lens board to attach the lens to the camera. I think the thread on the lens is M67/1. Can one find a lens board or will I have to make one - a quasi impossible task for a clutz like me :confused:

For a shutter, I suppose I will have to use a lens cap as in the old days..

I would appreciate any help with these matters....
Thanks!

Emile

 

[Chazzy]

The slit is for waterhouse stops, and yes, you can make them by making holes in thin metal (or even paper, I suppose). Figure out the focal length of the lens, and then you will be able to calculate the sizes of the holes for the various f stops.

 

[Jaq]

The lens is a 300 I think. I made a lensboard from strong cardboard that is a good temporary fix. At infinity, I measured 300mm to the focal plane, although I am unsure where I must measure from on the lens.
Is there an internet site that helps you make waterhouse stops?
Thanks for the info.
Emile

 

[Ole]

The usual place to measure from is the aperture - in this case the Waterhouse slot. I haven't found your lens in my references, but my first guess is that it's longer than 300mm - perhaps 360mm?

F-stops are rather simple to calculate, at least with longer focal lengths:

Note that f-stops are always given as a ratio - f/16 for example (or f:16, same thing with different notation). The f is actually the focal length, so for f:16 the hole diameter should be 1/16th of the focal length! It's really a little bit more complicated since it should be the diameter of the entrance pupil to be absolutely correct, but in most cases the error with using it as hole diameter is minimal and would lead to less than half a stop overexposure.

 

[Jaq]

I will remeasure the lens and see what I get.
I have subsequently found some sites that explain in some detail how to measure the diameter of the aperture.
All that is left for me is to get working on it!

 

[Jaq]

Ole,
I have remeasured it and it is indeed a 300mm and not longer as you expected. It is also a Series C. Strange though that you don't have any reference to it in your books, from what I understand, there is not much out there that you don't know about.
Anyway, thanks for the help.
Regards

 

[Ole]

Ah - there's the little bit of information I needed to find it: Serie C.

Introduced 1891 as f:6.3, changed to f:5 in 1920. That's presumably also when it became "Extra Rapid", as my references (pre-1920) don't mention the "Extra".

The first version was rated at 70° coverage, that can be assumed to have changed with the increase in speed and changes in the definition of "coverage".

 

[Jaq]

Thanks for the info.
I tried to measure the speed of the lens and came out with f:5.7, so given the margin for error, that's pretty close to f:5
Isn't it strange that there is not an iris diaphragm if it came out after 1920?

What does 70° coverage mean? I am guessing that it has got to do with the maximum format that it covers.