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A Subjektiv is fun. But it also helps to understand the world of optics better. One understands even more if one knows more. We are therefore gathering articles about the physical backgrounds here.
Nowadays a zone plate with 65mm focal distance can easily be inserted into the Subjektiv. But what is a zone plate? Strictly speaking, it is a piece of film that contains a curly pattern. But of course not just any curly pattern. A zone plate consists of alternate black and white rings. The areas of these zones always have the same size, ie the white circle in the middle lets as much light pass as a white swirl on the edge. Let’s honour Fresnel once more by remembering school physics. Constructive interference combines light waves in one focal point, while the parts of the wave that would lead to destructive interference are being blocked by the black curls of the zone plate. On one hand this is genial, on the other it’s quite complicated. Even this bit of information delivers interesting photographic effects. Especially when a zone plate is compared to a pinhole lens.
Contrast properties
The first point is the film. Film can be very transparent, but it will never be as transparent as the high-quality glass of a lens or the air of a pinhole lens. On the other hand, the black of the zone plate isn’t a metal socket, aperture or pinhole metallic foil, but the exposed silver of film and therefore a lot less light permeable for light rays than we would like. But do we really want this anyway? Maybe not, because this piece of film has contrast properties which are ideal for high contrasts in a picture. The zone plate can only smile about the eight to nine apertures of a digital SLR or the 13 apertures that some B+W films achieve. Especially digital photographers can only admire the contrast capabilities, which their cameras can suddenly handle.
A zone plate is often as unfocussed as the common assumption of a pinhole camera. It achieves its maximum sharpness only in its focal point. To express it differently: While a pinhole theoretically offers unlimited depth of field (and practically still focuses sharply from the double distance hole-sensor/film to unlimited distances), a zone pate should be focussed. This means that a zone plate is suited more to SLR cameras with the Subjektiv than to fixed wood and DIY cameras.
This can of course be easily ignored in the wide-angle area, with a depth of field at aperture 32. But those that are brave enough to focus can sometimes literally feel like nailing a pudding to a wall. To call it one focal point can sound much too euphemistic, but if you keep turning the scale, you will quickly realize that it can get a lot more unfocussed. Viewing the images, nobody will guess anyway that they could have been a lot more focussed.
Light capacity and light rings
Because zone plates bundle much more light to create a picture than a tiny hole, they offer a lot more light. Using apertures of 30 to 50 make it sound exaggerated to talk about light capacity. But compared to the apertures of 138 to 250 of pinhole cameras they are strong. Which brings us back to reasons why one should use a SLR. Apart from all these criteria the zone plate image has one other typical characteristic. In light reflexes the shape of the aperture won’t be visible, as it is with ordinary lenses, but on the same way a zone plate will be in the image, somehow in the aperture shape. It will become even more mystical with regular aureoles that can form around the lighter exposed areas. These light rings would probably have helped the dramatic August Strindberg more than 100 years ago with his ghost photography. What a pity that he never had the opportunity.
Praxis
In practise, zone plate photography might have some difficulties, but the impressionistic charm of this type of image is making is hard to resist. Because its effect, just like any other dominant change in photography can be in danger of overuse, we would discourage the use of sole zone plate cameras. But where the zone plate is an additional option, like the optic modules of the Subjektiv, we would really like to encourage the thoughtful use of a zone plate. Because anybody can take focussed photos these days.
The background of the Subjektiv
Taking photos with the zone plate.
The zone plate is old. Older than photography. But both have more than just the fact that we can take photos with it in common. The zone plate was invented by Augustin Jean Fresnel. The same man, by the way, who all large format photographers can thank for his pleasantly bright adjustment discs. When Nicéphore Nièpce took the first and still existing photo in 1826, Fresnel only had a few months to live. Dominique Francois Arago was a study partner and important friend of Fresnel. He demonstrated on 19.8.1839 Daguerres exposé to the scientific and artistic academies in Paris, which made him the master of ceremonies of the official hour of birth of photography. Then nobody had the idea, despite this timely and personal proximity, to photograph with the zone plate. Its advantages were discovered and used only much later thorough holography and radio microscopy. But just like everything else concerning photography, there are always photographers who are interested in different ways of image making.Nowadays a zone plate with 65mm focal distance can easily be inserted into the Subjektiv. But what is a zone plate? Strictly speaking, it is a piece of film that contains a curly pattern. But of course not just any curly pattern. A zone plate consists of alternate black and white rings. The areas of these zones always have the same size, ie the white circle in the middle lets as much light pass as a white swirl on the edge. Let’s honour Fresnel once more by remembering school physics. Constructive interference combines light waves in one focal point, while the parts of the wave that would lead to destructive interference are being blocked by the black curls of the zone plate. On one hand this is genial, on the other it’s quite complicated. Even this bit of information delivers interesting photographic effects. Especially when a zone plate is compared to a pinhole lens.
Contrast properties
The first point is the film. Film can be very transparent, but it will never be as transparent as the high-quality glass of a lens or the air of a pinhole lens. On the other hand, the black of the zone plate isn’t a metal socket, aperture or pinhole metallic foil, but the exposed silver of film and therefore a lot less light permeable for light rays than we would like. But do we really want this anyway? Maybe not, because this piece of film has contrast properties which are ideal for high contrasts in a picture. The zone plate can only smile about the eight to nine apertures of a digital SLR or the 13 apertures that some B+W films achieve. Especially digital photographers can only admire the contrast capabilities, which their cameras can suddenly handle.
Focus
If the image concept is based on constructive interference, then this means on the other hand, that the wavelength is important for the calculation of the focal point. A zone plate is therefore only ever exactly focussed for one wavelength. In a physics experiment this works really well with laser light, but fails in the image photography due to the significantly wider wavelength bands. Chromatic aberrations really hit their target here.A zone plate is often as unfocussed as the common assumption of a pinhole camera. It achieves its maximum sharpness only in its focal point. To express it differently: While a pinhole theoretically offers unlimited depth of field (and practically still focuses sharply from the double distance hole-sensor/film to unlimited distances), a zone pate should be focussed. This means that a zone plate is suited more to SLR cameras with the Subjektiv than to fixed wood and DIY cameras.
This can of course be easily ignored in the wide-angle area, with a depth of field at aperture 32. But those that are brave enough to focus can sometimes literally feel like nailing a pudding to a wall. To call it one focal point can sound much too euphemistic, but if you keep turning the scale, you will quickly realize that it can get a lot more unfocussed. Viewing the images, nobody will guess anyway that they could have been a lot more focussed.
Light capacity and light rings
Because zone plates bundle much more light to create a picture than a tiny hole, they offer a lot more light. Using apertures of 30 to 50 make it sound exaggerated to talk about light capacity. But compared to the apertures of 138 to 250 of pinhole cameras they are strong. Which brings us back to reasons why one should use a SLR. Apart from all these criteria the zone plate image has one other typical characteristic. In light reflexes the shape of the aperture won’t be visible, as it is with ordinary lenses, but on the same way a zone plate will be in the image, somehow in the aperture shape. It will become even more mystical with regular aureoles that can form around the lighter exposed areas. These light rings would probably have helped the dramatic August Strindberg more than 100 years ago with his ghost photography. What a pity that he never had the opportunity.
Praxis
In practise, zone plate photography might have some difficulties, but the impressionistic charm of this type of image is making is hard to resist. Because its effect, just like any other dominant change in photography can be in danger of overuse, we would discourage the use of sole zone plate cameras. But where the zone plate is an additional option, like the optic modules of the Subjektiv, we would really like to encourage the thoughtful use of a zone plate. Because anybody can take focussed photos these days.