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Pete Watkins
14-Apr-2007, 01:16
At long last I have mounted the lens by The English Lens Co. of London on my Calumet monorail and it looks to be an interesting lens. By focusing the lens to infinity and measuring from the Waterhouse stop slot to the front of the ground glass I have calculated that this lens is 195mm. Is this the correct way to calculate the focal length? I now want to make a few Waterhouse stops and I believe that there is a formula to calculate the hole sizes once you know the focal length, but I can't find any information about this formula. As the lens cost 10 UKP and the camera cost 30 UKP I'm not prepared to send the lens anywhere to have stops made, I want to do it myself! Thanks in anticipation.
Pete.

Ash
14-Apr-2007, 04:02
Pete, how did you get such bargains?!?!!

You'll have to PM me the seller :D

As for focal length, I always go from the rear element. Then again I just estimate pointing it out the window and having an image focus on the wall.

Some of the stops I've seen have simply been numbered. Probably as a fraction. Is it possible that you could use the formulas for pinhole photography to get the correct size dia's?

I think you need the length of the barrel and the diameter of the elements as well for the maximum aperture, I can't quite remember and I'm not as brainy as the others :)

Ash
14-Apr-2007, 04:15
From the Dictionary of Photography 17th Edition, 1950-something... this may take a while to type up.

Diaphragms
.....In early lenses a form of stop devised by Col. Waterhouse was used. Stops, consisting of thin plates of metal, each with a different-sized circular opening, were placed in a slot.... (blah blah blah)
im trying to find the formula, lots of reading!
...For a lens of a single component, cemented or otherwise, the usual position is from 1/4 to 1/7 of the focal length in front of the lens. and a doublet is in the middle

ok it goes on and on and I don't have time to scan the pages. It mentions the 'working' aperture to be f/4 for a 4in lens with a 1in diameter. "the figure 4 denotes the number of times the diameter of the aperture will divide into the focal length" hope that helps. Everything then is a fraction of the 'working' aperture'.

Ernest Purdum
14-Apr-2007, 07:29
If you have enough bellows, you can find the focal length accurately by focusing at infinity, marking the extension, then refocusing on a ruler until the image on the groundglass is the same size. The distance between the two focusing positions is the focal length.

Ernest Purdum
14-Apr-2007, 07:36
"f" numbers are the focal length divided by the apparent aperture of the lens viewed from the front. (The lens is magnifying the actual aperture.) There are several ways of measuring the apparent aperture. One way is to rig up a means of sliding a thread held vertically across in front of the lens. Measure the distance between the thread lined up on one side of the apparent aperture, then the other.

Somebody else will probably chime in with another method, maybe an easier one.

Pete Watkins
15-Apr-2007, 00:04
Many thanks for all the advice up to now. Ash, go to camera fairs and I usually pay the bit extra for the "Earlybird" ticket. I thought that you were planning to go the Photographia 2007 on the 20th of May. I doubt if I'll make it now, my wifes having an operation that week.
Best wishes,
Pete.

Ash
15-Apr-2007, 04:31
Hey Pete,

I was planning to but unfortunately I haven't had any work for a few weeks and my pockets are empty. Poor timing, eh? !!

Good luck with the waterstops. Hope your wife recovers in a speedy fashion :)

Ole Tjugen
15-Apr-2007, 05:16
The "F" in "F-stop" is the focal length of the lens. Notice that on many lenses the maximum aperture is given as a fraction, i.e. F/9. That's your formula: The diameter of the aperture (in that case) is 1/9 of the focal length!

To be absolutely correct it's the diameter of the entrance pupil of the lens, which will in most cases be slightly larger than the diameter of the physical aperture hole size due to the "loupe effect" of the glass in front of the stop. But with most LF lenses this difference can be ignored without causing any problems, as it indeed has been on all the original Waterhouse- and wheel stops I've measured.

So for an F/8 stop make a hole 1/8th of the focal length, for f/11 use 1/11th, and so on.

I measure focal lengths by projection:

I have a long corridor with a window at one end and a blank wall at the other. On this wall I put a piece of cardboard with one vertical line drawn on it. Then i project the image of the window on the cardboard, lining up one edge with the line, and draw a (pencil) line along the other edge of the image of the window.

From the distance between the two lines, the size of the window and the length of the corridor it is possible to calculate the focal length of the lens.

Fortunately I have enough lenses with known focal lengths that I can just look it up in a little table I've made from several such measurements - the window and the corridor are constants, so the only factor determining the size of the image is the focal length of the lens! It may not be 100% precise, but neither are stated focal lengths. It's good enough to determine the difference between a 280mm, a 300mm and a 305mm lens; and that is better precision than I need.

Doug Kerr
15-Apr-2007, 05:31
Hi, Pete,


At long last I have mounted the lens by The English Lens Co. of London on my Calumet monorail and it looks to be an interesting lens. By focusing the lens to infinity and measuring from the Waterhouse stop slot to the front of the ground glass I have calculated that this lens is 195mm. Is this the correct way to calculate the focal length?

The f/number (N) is the focal length (f) divided by the diameter of the entrance pupil (D), which is the apparent diameter of the stop as seen from in front of the lens:

N=f/D

Thus the needed apparent diameter of the stop, D, is given by:

D=f/N

To deal with that, you will need to know the ratio of the apparent diameter of the stop to the actual diameter. That is best detemined by inserting a test stop whose actual diameter (d') is known (any practical diameter will do) and then observing its apparent diameter (D') from in front of the lens.

Ideally, this should be observed from a substantial distance to avoid parallax.

My technique is to fasten a calibrated "ruler" (I use a 6" machinist's scale) across the front of the lens so that the marked edge passes through the center of the lens. Then, with the lens situated so that there is a light surface behind it (so the apparent stop will appear bright - on the camera with the ground glass illuminated from behind works fine), I photograph the lens from in front with a digital camera and a long focal length lens from a distance of 5 feet or so. I then examine the image to determine where the edges of the apparent stop fall on the scale, and thus detemine the diameter of the apparent stop (D').

Once we know the ratio of D' to d' for a lens (it will be constant for all apertures), we can determine the needed actual diameter of the stop, d, to produce a desired f/number, N, with this formula:

d = (f/N)(d'/D')

where N is the desired f/number, f is the focal length of the lens, d is the needed diameter of the stop, and d' and D' are the actual and apparent diameters of the test stop.