Thickness limits of pinhole appatures

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eli griggs

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I'd like to know others thoughts on making appatures out of brass, copper, gold alloy and silver, American coin of 90%, some 40%, sterling and some fine, 9999.

I have plenty of copper and brass on hand, in different thicknesses and a small piece of dental gold, a filling of about 12k.

The silver types listed above are also nice too work with

I also have jewelers tools, etc for working these metals and I want advice on which metal is an ideal ductile material that will allow a smooth, round hole, by whatever method folks have to hand.

I can blacken all but the gold, with Liver of Sulfer but my biggest concern is how round a hole will each make and how thick the metal should be.

I have a good micrometer, so thickness can be measured.

This pinhole is for a full frame 5"x7" pinhole camera and I want it to as sharp as possible this no burrs or oblong openings
 

ic-racer

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As the pinhole gets smaller sharpness increases to a maximum, after which, the image just gets darker, but not any sharper. With a view camera, you can use some trial and error to get the brightest, sharpest hole.
 

AgX

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You can place a very thin metal sheet under pressure between thicker sheets and drill it then. This yields a better hole than drilling the thin sheet directly.
 

awty

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You can place a very thin metal sheet under pressure between thicker sheets and drill it then. This yields a better hole than drilling the thin sheet directly.

That is clever, must remember for next time.

pinhole metal should be rigid, perfectly flat and very thin. Most use brass or stainless steel.
 

MattKing

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I have a friend who has had good success using the material from beer cans, blackened suitably.
He also has the resources to be able to mount very thin brass (IIRC) shim stock in an appropriate holder, and has had excellent results with that.
He has a neat trick where he mounts the result in an enlarger and projects the (very dim) image of the pinhole on a wall. That way he catches pinholes that are significantly off from being a circle, or exhibit large edge burrs. He discards those.
He also uses another enlarger set to a high magnification with a short lens to calculate the actual pinhole size.
 

AgX

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That is clever, must remember for next time.

pinhole metal should be rigid, perfectly flat and very thin. Most use brass or stainless steel.

One could use stainless gauge or shim strips.
 

Donald Qualls

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Pinhole metal does not need to be rigid, as long as it's supported both when you drill the hole and when mounted in the camera.

I've made many pinholes from .001" (= .025 mm) brass shim stock; this stuff is less stiff than regular printer paper. The pinholes I've made in this material have ranged from 0.2 to 0.7 mm with good results.

Ideally, the thinner the layer with the actual pinhole, the better; if you have a thickness comparable to the diameter of the hole, you lose sharpness due to the doubled diffraction effect (from the front edge of the hole, and from the rear). The true optimum thickness is zero at the edge of the hole; this can be approached by "dimple and sand" method of drilling the hole in a somewhat thicker material like beverage can aluminum; the edge of the hole can be pretty much a knife edge (but it's hard to control the hole size with this method, hence why I haven't used it myself).

Also, smaller isn't better beyond the point where diffraction effects outweigh the simple geometric projection of the hole. There's an optimum for any given projection distance -- the best compromise between geometry and diffraction -- and there's a calculator for it online at Mr. Pinhole web site. This provides a guideline, however, not an exact solution; if you expect to make a lot of close-up shots, for instance, you will gain by making your hole a few percent smaller than the calculated optimum.

Another factor is that the longer the projection distance ("focal length"), the larger the optimum hole, which overall detracts from final sharpness. Couple that with the well-known effects of large vs. small film, and that 5x7 will likely be best with around 100 mm projection (pretty wide, a bit less than half a "normal" focal length) as the best tradeoff between "too wide to compose for" and "optimum hole gives too much fuzz". Even 4x5 with 135-150 mm projection looks good in a contact print, however, and any large film at any reasonable projection distance will tend to look better, just because you normally view from further way or with less magnification the larger the film.
 

ic-racer

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Bottom line is that for large format you can’t make the hole too small, because no matter what, you are going to have long exposure times. Some foil, a pin and two seconds of work are all that is needed.
 
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