size pinhole for large negative

I would like to make a large pinhole camera to be able to make 16x20

paper negatives. I would like the negative to be as sharp as

possible. I then can choose the softness of the finished image later.

What size pinhole would I need and at what distance would I want the

paper to be for the sharpest image possible. I know little about the

mechanics of pinhole photography so please bear with me. I shoot

large format almost exclusively. But I have a project that I think

would look nice as a pinhole work. Very distorted. Thanks for any

help.

James, turn your question around - you have a 16x20 camera, what focal length would you use if you were using a lens? Once you have chosen a focal length, you can look up (or calculate) the optimum pinhole size.

<p>Wide angle and close-up gives the sharpest images apparently, so if you used a 360mm focal length on 16x20 (roughly equivalent to 90mm on a 4x5) then you'd need the pinhole to be 0.71mm ... with a working aperture of F493.

<p>

Other ways of calculating would differ in the result. Absolute accuracy of the hole size is not critical, but you do need a thin profile material (you want a hole, not a 'tunnel') and it should be a crisp circle. My experience of paper negs is that glossy (OK) or matt (better) doesn't make a huge difference, but stay away from pearl finish.

<p>

Have fun :)

Actually, close-up loses sharpness -- it's easy to see, geometrically, that significantly diverging rays will increase the size of the image spot cast by any point in the subject. If a camera is to be used mostly for extreme close-up or macro images, it's advantageous to reduce the hole size slightly from the "optimum" commonly found by calculation using the usual formula, dividing the square root of focal length by 25 (all in millimeters).

 

Generally, the longer the focal length (while keeping optimum hole size) the greater the relative sharpness (that is, the larger features in the image are relative to the spot cast by a point), but for a given film size, wider angles give better absolute sharpness (because you're using a small hole, and nothing smaller than the hole will be resolved in the image).

I didn't mean to imply macro, Donald, only near distance. And I'm guessing that James' concern is absolute, not relative, sharpness. <p>James, I'd be interested to hear what kind of 'distorted' look you're after - pin 'holes' are not the only option.

The freeware program Pinhole Designer provides a useful tool for sizing pinholes, calculating exposure time (with compensation for some films), angle of view and Zone Plates. it is available at:

 

http://www.pinhole.cz/en/pinholedesigner/

 

The rest of the site is pretty goo too.