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Any idea about making your own waterhouse stops (never having used any actual ones before...)
Simple materials, measurements for aperture size, aperture shape etc?
(BTW - these are for the lens you sent me Jim G - was waiting when I got back from vacation - neato looking old thing!)
McMaster Carr has a thin black plastic like material which machines like wood, it comes in different thicknesses, I've made Waterhouse stops from these, I first used a circular saw to machine this material into 'strip's about the width of the diameter of the lens I was using this for. I'd then machine off the two opposing corners of the end of a strip, machining off more or less, but always at 45 degrees, until the end of the strip fit 'flush' on the inside of the barrel opposite the insertion slot for the Waterhouse stop.
I'd then measure out two intersecting lines to find the exact center of the strip as it sits in the barrel, after finding that, I'd secure/clamp the strip to a 1x4 section of wood, I'd do this so that when using of my forstner bits, I don't get 'tear out' on the hole I drilled. A forstner bit will then give you a nice neat hole, and since forstner bits come in several diameters, you can make the wholes incremental.
I made 2 of these, go to the McMaster Carr website you can get plenty of this material for $20.00, it's cheap, you'll need a circular saw, a drill, somebodies forstner bits you can borrow.
I can think of this stuff, but it's on the MC website, w/description, they'll state that it machines like wook, but is hard as rock, and comes in very thin sheets.
You asked. I think it's for a Wollensak Verito.
As a second thought, you can make a full diameter stop and photograph any shape or size onto TechPan, Kodalith, Aero duplicating film, ect, and attach it to the stop. Would be easier than cutting out something like this.
Hrmmm...That's interesting.
So, when making an aperture with a non-conventional shape, how do you figure out the f/number ? Do you measure all of the removed surface area, or just the circular hole in the center ? If its just the center circle, then what formula did you use to calculate the amount of area to remove for the 'rays' ?
Thanks !
Steve
Tim, we made a set some months ago from thin sheet metal that had been discarded in a skip. (Tip: skips in industrial areas are great sources for DIY material). Cut down to size with good scissors, drilled different sets of imagon-look-a-like holes based on a trial and error workflow. Once we have a waterhouse stop that throws a pleasing image on the GG (check in the highlights and shadows), we measure the f-stop for that particular waterhouse stop as follows. Mount the lens, measure the light from the GG wide open for reference. Mount the stop, measure a second time and read off the difference in stops from the meter. Another way would be to use a Sinar-type light meter that you stick behind the GG, don't know the tech-word for that device offhand. It's really not that difficult. Making the right shape for the waterhouse stops is harder when you do it on eyesight rather than calculus. I'm sure our way of doing this might not accord with other people's standards, but it's all I can offer by way of advice.Quote:
Originally Posted by tim atherton
All this is a lot simpler if you just need regular round stops. Do a google for f-stop calculus for lenses that need new scales and work from that. Have the formula somewhere else than in my memory....
HTH, Norm
Start by making a stop the size of the full aperture. Keep that, and the disk you cut out of it. Weigh both, and note the weights. Call them D and S for Disk and Stop.
When you've made a pleasing stop, weigh it. Lets'call that F.
Then F-S=D-H, where H is the weight of the material you've cut away. So H=D-F+S.
The light transmission in % of the full aperture is H/D*100. 50% is one stop down, 25% two stops down, and so on.
Ole,
I see. So all in all, the surface area of the stop is to remain the same, regardless of shape.
Exactly! Since the light transmission is proportional to the area of the opening, two openings with different shapes but the same area will have the same effective aperture.
Most interesting and helpful! In the past I have made them with brass shim and a lot of calculations. Ole's way sounds much easier! Thanks to all!
C Webb
Hi there,
No one mentioned an 8X10 darkslide for material, works perfectly and is large enough for several. Jonathan's idea with the forstner bits should cut clean enough not to need beveling. You can also fold-over 'card stock' like a filter-slip to try out different designs.
Try http://www.whizkidtech.redprince.net/zoneplate/ for ideas for shapes and sizes. On another thread people agreed that the soft-focus effect stops at f/8 so I guess most if not all soft-focus lenses use spherical aberation from the outer diameter of the glass for the effect.
A 'concentric circles' aperture should reduce the f/stop for dof and still use the outer diameter for the soft-focus effect.
Just another thought.