A. Ross Petzval lens: making waterhouse stops

Seele · Joined: 17 Apr 2009 Posts: 742 Location: Sydney Australia

I have this Petzval lens marked A. Ross in rack-and-pinion focussing mount and waterhouse stop slot, which dates it very easily as between 1858 (introduction of waterhouse stops) and 1859 (the death of Andrew Ross), with a serial number of 6392. Its lens diameter is close enough to 42mm and when viewed from the front. the disc structure inside for holding the waterhouse stop is invisible, and according to my measurement its focal length is 7". Without hood its length is 73.5mm, outer barrel diameter 55mm.

Assuming my focal length measurement - by producing a 1:1 magnification ratio and divide distance between target and image by four - is correct, the maximum aperture seems to be around f/4.2, and I think it should be quite a bit faster. It would be good to find its true maximum diameter so that I can make a set of replacement waterhouse stops to put it back into use.

Of course there is the brute force method: using a modern 180mm lens, photograph using different apertures (say everything between 2.8 and 5.6) and then with this lens, with the same lighting and shutter speed. By comparing negative densities it should give a fairly accurate figure.

As my experience with Petzvals is limited (this is only my second) I would appreciate any insight from my fellow correspondents.

arri · Joined: 13 Jul 2014 Posts: 95

You have already calculated the max. speed of the lens in the correct way.
Front lens diameter/focus length.

When you are not sure about the real focus length of a lens you can compare it with another lens.

The most precise way is to focus the lens to infinity, measure the distance between the front and rear of the camera.
After this focus to the lens to a ratio of 1:1 and measure again.
The difference between both distances are the 100% correct focus length.

When you know how long the focus is you have to measure the largest inside diameter of the lens. It is the housing position of the waterhose stops.
F.ex.
The lens has a focus of 210mm and a front lens diameter of 51mm you have a speed of 210/51= 4.1 or better f/4.0
The largest inside diameter is smaller that the 51mm of the front lens element. The reason is that a Petzval lens system large up the aperture diameter with the optical system in the front.
You find it in a lot of other lenses as well.

On my 210mm f/4.0 Petzval lens I have an inside diameter of 42mm at f/4.0
Now divide the front diameter of 51mm/42mm inside diameter and you get a factor of 1.21

When you calculate the diameters of the other aperture values you must use this factor.
First divide the focus length to the aperture value, f.ex. f5.6
210mm/f/5.6 = 37.5mm
Than devide this value to the factor
37.5/1.21= 30.9mm
This is the necessary diameter for the lens with f/5.6

I use black carton for waterhouse stops, very easy and low cost.
Another good material are ABS plastic plates. It is availble in different thicknesses. I have 1mm and 2mm in stock and this are the most used thickness of waterhouse stops.
The 1mm plates can be easy cutted with a knife or a scissors.

First cut a dummy made of carton.
It must have a width of the slot in the lens.
At the front of the W.S. you can cut a radius of the inside barrel diameter.
Insert the dummy into the slot and draw a circle with a pencil on the carton, before you have to removed the front lens element.

Now you have a dummy as a pattern with the correct width, the length, radius and the position of the aperture bore.

With it you can make every other waterhouse stop you need.
I made it for a lot of other lenses, I made the W.S. with carton, ABS plastics, brass and alumnium.
It is a very precise way.