Need help with Zoneplate exposure

[JeffD]

I have an interesting pinhole turret built into a 4x5 lensboard, made by Abelson Scopeworks. Unfortunately the company seems defunct, so I can't inquire with them.

Included in the turret are two zone plates, one for 75mm and one designed for 150mm.

I'm wondering if anyone can help me guestimate what an fstop might be for these two zone plates, assuming i'm using them at the 75mm and 150mm focal lengths.

 

[Donald Qualls]

Unlike pinholes, zone plates actually have a genuine focus, so you need to use them at the indicated focal length. There's a formula somewhere for calculating the effective focal ratio of a zone plate, but it's probably simpler, if you have a low-light meter, to compare the ground glass brightness from a lens of the same focal length at the smallest aperture against that with the zone plate. That will give a correction factor -- for instance, if your lens was at f/45 and the zone plate gives two stops less on the ground glass, you'd be at f/90 equivalent with the zone plate (and that's a likely range to be in -- the whole point of zone plates is that they're MUCH faster than a pinhole, but still aren't a lens).

 

[JeffD]

I had read that zoneplates may be around 3 stops faster, so I’m guessing it could be 2 or 3 stops.

I have a Pentax spotmeter. I hadn’t thought about taking a reading off the Back of the ground glass, but that might work...

Unlike pinholes, zone plates actually have a genuine focus, so you need to use them at the indicated focal length. There's a formula somewhere for calculating the effective focal ratio of a zone plate, but it's probably simpler, if you have a low-light meter, to compare the ground glass brightness from a lens of the same focal length at the smallest aperture against that with the zone plate. That will give a correction factor -- for instance, if your lens was at f/45 and the zone plate gives two stops less on the ground glass, you'd be at f/90 equivalent with the zone plate (and that's a likely range to be in -- the whole point of zone plates is that they're MUCH faster than a pinhole, but still aren't a lens).

 

[Donald Qualls]

Obviously, you'll want to take all reasonable steps to limit or exclude reflections and ambient light falling on the ground glass, but if you can set up hot lights in an interior studio, and work under a dark cloth, you should be able to get readings that are close enough for film tests, at least.

 

[Jim Jones]

Since adjacent zone plate rings are nearly the same width, a zone plate passes about half as much light as a round diaphram with the same outside diameter. Thus, try about one stop longer exposure (allowing for reciprocity failure) than you would for a lens photograph with the aperture diameter set to the zone plate outermost ring.

 

[Jim Jones]

Since adjacent zone plate rings are nearly the same width, a zone plate passes about half as much light as a round diaphragm with the same outside diameter. For calculating zone plate exposure, think of your zone plate as a lens with a diaphragm diameter 0.7 times the outside diameter of your zone plate. I haven't actually tried this, so one of you good people might prove or disprove it, which is much better than my mere theory.

 

[Donald Qualls]

For calculating zone plate exposure, think of your zone plate as a lens with a diaphragm diameter 0.7 times the outside diameter of your zone plate.

That's slick -- alternatively, you could just measure the diameter of the outer ring and meter for one stop smaller. At the least, it ought to get you close enough for negative film, and a few tests would let you get within a tenth stop if you need that level of accuracy (though why you'd use Velvia in a pinhole or zone plate camera, I'm not sure).