Atelier Bonryu(E)
zone plate photography
Laboratory: Zone Plate Photography
Taking Zone Plate Photographs
- Finishing with Unblurred Photographs -
Finishing with a unblurred zone plate photograph: As described at the page “Not only soft but also unblurred”, truthfully a zone plate photograph is unblurred. The cause of why a zone plate photograph looks very soft in comparison with a usual pinhole photograph is the “background light” inevitable in a zone plate photograph. Though the chromatic aberration also contributes to the softness the background light seems no less important for this phenomenon in our experience. Results of these causes certainly correlate with the difference in the conditions to take a photograph such as a photogenic object, a spectrum distribution of a light, and so on. Here, at first, we try to make an unblurred zone plate photograph by removing the effect of a background light due to a large “object” such as an ambient light. Detailed descriptions of the background light will be given afterward.
The phenomenon that a photograph looks soft is often seen when we see an image projected on a screen in a bright room. In such a situation even a unblurred drawings or an unblurred photograph look very soft and with a low contrast. By curtaining off windows and turning off electric lamps we can see an original unblurred drawing or photograph. On the basis of the principle it is easy to take the background light due to the ambient light off from the photograph and to make it unblurred. The intensity of the ambient light is, of course, non-uniform, but the non-uniformity is weak in comparison with the change of the light intensity due to the light from the main object. Therefore, we subtract a quantity of brightness uniformly from all over the photograph. This is approximately carried out by moving the graph in the histogram to the left without changing the shape. For this purpose image data processing codes, Photoshop, GIMP, and so on or image data analysis codes such as imageJ can be used. As a halo, a characteristic blur of a zone plate photograph around an image of the “main” object, is a localized phenomenon, it is not removed by this operation and the photograph remains as it was keeping the appearance of a zone plate photograph. Because a zone plate photograph originally carries information on detailed structure of the image, it is possible by this operation to make a photograph with higher resolving power than a pinhole photograph. The following pictures show examples of the effects of the operation. For example, leaf veins are found to become clearer by this processing.
An example of level adjustment for a zone plate photograph of comfrey
A histogram is a graph which expresses number of pixels versus the brightness level (256 levels) of a photograph. The left upper figure shows the histogram of a photograph before adjustment, where there are no pixels in the left side (no dark pixels) because of the background light due to the ambient light. Then we move the tone curve to the left side with keeping its gradient (the left center figure), which means that a same quantity of brightness is subtracted from all the pixels. Then the original histogram is moved to the left side with keeping the shape as shown in the left lower figure and the uniform ambient light was removed.
It should be noted that this process is not always applicable. When the ambient light is very strong this process reduces the dynamic range effectually and a photograph with high resolving power cannot be obtained.
(Slide show)
