Hello all,
Recently I came into possession of a not particularly unique brass lens and shutter from the early 1900's. For fun I thought I'd give it a go on my 4x5, but there was one problem, the shutter was missing its aperture scale. This is likely because the lens had mismatched front and rear elements, and whichever photog used it all that time ago knew from practice about where to put the dial.
To set this right I decided to recreate, in brass, an appropriate aperture scale. Seeing as I was unable to find a guide when setting out on this mission, I decided to write down what I did and what I learned - in the hope that it may prove useful to another.
Necessary Tools
Calipers
Compass (for drawing arcs and circles)
Thick, Smooth Paper
Computer
Scanner
Laser Printer or Photo Copier (you CANNOT print inkjet onto transfer paper)
Heat Transfer Paper for making printed PCB’s (NOT for iron on shirts)
Iron or Heated Press
.015 in Brass Sheet
Cue Tips
Salty Water
Car Battery Charger
Tin Snips or Heavy-Duty Scissors
Flat and Round Files
Drill
Small Screws
Small Screw Driver (to match screws)
Optional: Thread Tap
The first step I will rather rapidly brush over, as those who know the focal length of their lens can skip this step, and there are many good guides here and elsewhere on how to determine the focal length if it is unknown.
1. To determine the focal length of your lens simply measure from the center of the lens to the inside surface of your ground glass when a subject at infinity is in focus. You could also do this with a piece of paper and a ruler. That’s it, pretty simple right??
Not so simple, thanks to Dan for clearing this matter up (I guess a film degree doesn't qualify me for optical engineering!)
The real way to do this
Originally Posted by Dan Fromm
The second step involves actually finding what dial position corresponds to what f-stop.
2. First determine the maximum aperture of your lens. To do this open the diaphragm all the way and measure the apparent diameter of the opening as viewed through the front element with a set of calipers. It is important to measure the apparent diameter and not the actual diameter as the formula for determining f-stop is focal length / entrance pupil diameter. entrance pupil diameter is affected by the magnification of the front element. Mark the f-stop and its position on the shutter with a fine sharpie.
Now you can work the opposite way to find specific incremental stops. Say you want to mark the location of f/4. Simply divide the focal length of your lens by 4 to get the diameter of the entrance pupil. Set you calipers to that and mark on the shutter dial where f/4 is. Continue this until you have marked all the desired f-stops.
The third step involves creating a 2d drawing of the shutter and the stop locations so that a design for the aperture scale can be drawn up.
3. Using a pair of calipers set a compass to the radius the shutter housing, so as to draw out a circle of equal circumference. Now choose any point on this circumference to mark as the location of the widest (or any really) f-stop. This point, combined with the arc of the drawn circle will serve to locate all the other f-stops. Now set the compass to the distance between the original mark on the shutter and the next mark over, being sure to measure on the edge of the shutter. Replace the pointed end of the compass on the first marked f-stop on your drawing and draw an arc. Where that arc intersects the circumference of the circle is the location of that f-stop on the drawing. Continue this, always measuring from the first mark to avoid adding up errors, until all f-stops are marked. Locating the screw holes is slightly different as they do not intersect with the arc of the shutters circumference. However, marking their location is just as easy. Simply measure and draw an arc from two each of the already marked f-stops instead of one. Where those arcs intersect is the location of the screws. When you are done your drawing should look like this.
The fourth step is to draw up the actual scale on the computer and print it out.
4. Scan in the image you have drawn up and use whatever software you are most comfortable with to draw up the final design. I used Illustrator, and my final design looked like this.
Next print out the design on your heat toner transfer paper (remembering to mirror it) and follow the manufactures instructions when transferring it to the brass sheet.
Now comes the fun part, the actual etching!
5. Using a car battery charger, attach the positive lead to the brass sheet and the negative end to a cue tip that is soaked in very salty water. Carefully dab the cue tip over the design keeping contact for a count of 3 before letting off. Once the cue tip becomes discolored it should be exchanged for a new one, and the solution on the cue tip should be refreshed between changes. I wore heavy rubber gloves as accidental shorting causes large sparks and could be painful......... and electricity is scary. Once the scale has been etched to a sufficient depth rinse it off and use tin snips to roughly cut it out of the sheet and then a round file and a flat file to remove remaining excess material.
Finishing up.
6. Evenly coat the entire front of the scale with paint. Once dry sand the paint off so that it remains only in the etched areas. Drill out the marked holes, screw it on, and you’re done!
A final Look
And a test image I made in my back yard
A couple of notes.
Obviously, battery chargers can be dangerous, so be careful.
To preserve the life of the leads, it is a good idea to connect them to the brass and the cue tip using sacrificial alligator leads, this protects them from the corrosive process of etching.
For the life of me I could not find screws that fit the threaded holes already in the shutter for attaching the scale (even from McMaster-Carr). However, I found I was able to rethread the existing holes to No.1-64.
I used stainless steel screws, which should be fine according to galvanic reaction charts, but brass would be ideal to avoid corrosion.
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