It is the one thing that you can do with a pinhole that you can’t do with a lens. The normal attraction of curved film planes is to achieve extremely wide angles without the overwhelming vignetting inherent in very short distances to the pinhole with a flat image plane. To accomplish this, curved cameras are usually designed with an arc of uniform radius so that the pinhole remains exactly the same distance to the image plane, hence the f ratio is uniform across the field.
With the Pinhole Lab Camera, it’s possible to use a piece of paper 10 inches long that, when attached to back of the camera, creates a curve that almost extends to the front of the camera with the distance from the pinhole varying from 120mm to 60mm. (I know it’s not exactly a parabola but I couldn’t resist the alliteration.)
In the center, with a .55mm pinhole, it’s f231. By the time you’re at the front, it’s f109, a four stop difference. Something that modifies this is that in the center, the pinhole is circular from the point of view of the film, but by the time you get to the edge, it’s rather foreshortened into a narrow ellipse. Since what determines exposure is the area of the aperture, it should be passing less light to the film. Would those things cancel out?
I started out with a scene that I thought would have pretty uniform exposure, the little alley in the back of the Frontenac Flats. The levels of this scan have been adjusted to achieve the full range but otherwise there’s no burning or dodging. It does look like the exposure is pretty even across the frame.