calculate aperture for waterhouse

[mylek]

Hello,

i'll like to make some waterhouse stops for a unknown Petzval 300mm (12") but i can't figure out how to calculate it properly its aperture.
Front glass has a diameter of 80mm so f/3.8 and the diameter of the waterhouse slot is 60mm for f/5. Rear lenses diameter is 76mm.
Which one should i used to make it a 1 and 2 stops that will be related to its original aperture.

Regards!

[Ray Heath]

G'day mylek
Simple, you know the focal length, you know where the waterhouse slot is;
focal length divided by the f stop number equals the diameter of the aperture (the actual hole in the waterhouse stop).
e.g. 300mm divided by f22 = 13.64mm.

Much harder when the focal length is unknown and/or where to measure from is difficult to determine.

For my homemade simple lens constructions I use the black plastic covers from notebooks and visual art journals.

133000

133001

Ray

[Jim Jones]

First make a temporary Waterhouse stop with a fairly large aperture out of cardboard. Insert it into the lens and measure its apparent diameter while looking through the front of the lens. When measuring, move your eye from side to side by the apparent diameter of the stop to eliminate any parallax. This will give you the f/number of the temporary stop. You can scale the diameter of any future Waterhouse stop up or down and the f/number will scale in inverse proportion. I can't tell you what the f/number of the original Waterhouse stops was. Others with more experience with Petzval lenses may help you with that.

[Emmanuel BIGLER]

Hello from France!

Just a few words to add to what Jim's said.
As a general rule for all compound lenses, the f-number should be computed with respect to the entrance pupil diameter, not the physical iris diameter, as explained by Jim. The entrance pupil is the image of the iris as seen through the front lens group looking backward from the front of the lens.
In a Petzval design made of two thin groups, when no waterhouse stop is inserted, the diameter of the entrance pupil is simply the diameter of the lens mount for the front group, as directly measured.
Hence the calculation at full aperture N = 300 mm / 80 mm for a Petzval lens at full aperture is perfectly correct.
This situation is not specific to the Petzval design, but common to all optical instruments made of two thin groups only, when the rear group is not too small in diameter, this is the case for the Petzval lens, the entrance pupil is simply the mount of the first thin group.

The actual location of the f-stop has an important influence on aberrations and distorsion, so insert your waterhouse stop where the manufacturer has designed it, and follow Jim's procedure to evaluate your actual f-number by evaluating the actual diameter of the entrance pupil i.e. the diameter of the image of the iris as seen through the front lens group.
However it is not forbidden to add a home-made iris just in front of the first lens group and experiment! In this case the diameter of the entrance pupil will be the actual physical diameter of the iris.
I would anticipate a very different behavior of the Petzval lens, whether it is stopped down at the front lens group or at the "official" waterhouse iris slot in the middle of the lens. Amateurs of uncommon bokeh should try this !

The evaluation of the entrance pupil diameter does not have to be very precise, if you tolerate an error of 1/3 f-stop on your actual f-number, this error corresponds to an error of about 20% on the actual area of the pupil and this corresponds to an error of 10% on the measurement of the diameter.
I'm fairly confident that estimating the diameter of the entrance pupil can be easily done, by any means, with a precision of only 10%

[Gundlach]

I measure the fixed aperture in the lens body - ( that opening / fixture that holds the waterhouse stops). I do a little math and calculate the area of that opening. I calculate what half of that area is, that is a the calculation for a one stop decrease in exposure. I do this for the remainder of the stops. You can calculate 1/3 stops 1/2 stops what ever you think you need.

[mylek]

Thanks for all the answer.