Atelier Bonryu(E)
zone plate photography
Laboratory: Zone Plate Photography
Taking Zone Plate Photographs
- Not Only Soft But Also Unblurred -
From descriptions up to now it may be said that the zone plate photography is a generalization of the pinhole photography. Though the diffraction phenomenon is used in a positive manner for imaging by a zone plate, usually it is not necessary to be considered in the case of pinhole photography. In order to take an unblurred photograph, however, the diffraction phenomenon should be considered even in the case of pinhole photography. Following after this consideration and by carrying out a thorough investigation of pinhole photography we finally arrive at zone plate photography. Actually if we optimize the diameter of a pinhole with respect to the resolution of the projected image the diameter of the pinhole coincides with the diameter of a positive zone plate with N=1. Therefore, an optimized pinhole can be considered as a special case of zone plates.
By the way as a zone plate utilizes the diffraction phenomenon it has several features which cannot be seen in pinhole photography. As these features are well described by keywords of “resolving power and contrast” and “chromatic aberration”, we consider these issues in some detail.
Resolving power and contrast of a zone plate photograph: The common knowledge of photographers who have an experience to have taken a zone plate photograph is that a zone plate photograph looks softer than a pinhole photograph. In scientific or engineering applications, however, one of the advantages of a zone plate photograph is that a sufficiently high resolution is realized by a zone plate among lensless optical systems. Aren’t these contradicting each other? In reality these are not contradicting each other.
The softness of a zone plate photograph is attributed to (1) a background light and (2) a chromatic aberration. In the following we describe these features briefly. Detailed explanations are given in the pages of remarks *5 and *6. As a zone plate has a definite focal length unlike a pinhole, careful focusing is necessary for taking a photograph by a zone plate, especially, with a long focal length or a large number of zones. Careless defocusing is, of course, a reason of unclearness but a blur due to this reason can be avoided, and we do not consider it here.
Background light (*5): First we search for whereabouts of the light incident to the zone plate. It should be noted that a half of the incident light is reflected at the front of the opaque zones and, in the following, we consider a light passing through the transparent zones of the plate. A light on the optical axis goes forward along the axis. A part of light incident to an outer transparent zone is diffracted at the zone and goes straight to the focal point but most part of the light goes straight in the direction of the incident light. As in the case of a Fresnel zone plate there are sub-focal points as well as the main focal point, parts of the light go to sub-focal points in the image screen side or diverge as if they are emitted from sub-focal points in the object side. Anyway the quantity of the light converging the main focal point is only one tenth of the total light. Therefore, about 4/10 of the total light is not converged and distributed around the focal point. This is the background light which causes the low contrast of a zone plate photograph. This is a phenomenon specific in a zone plate photograph which is not seen in a pinhole photograph.
