Rodenstock Imagon lenses for the Pentacon Six
the components of a lens for a Single-Lens Reflex
Ask any person familiar with film-based SLR cameras to describe the general characteristics of lenses, and they will probably come up with something along the lines of:
The two-element Imagon lenses do not have a telephoto design (nor could they, with just two elements). Therefore, a 200mm Imagon, for example, needs to be 200mm from the film. The distance from the film to the lens flange on the front of the Pentacon Six is 74.1mm, and the helical mount is not likely to be 125.9mm long. Therefore, it will probably be necessary to add a prolongation tube either in front of or behind the focussing helical, with a camera mount on the back of the helical or on the back of the tube. The length of the tube will depend on the focal length of the lens (i.e., the distance that the lens needs to be from the film in order to bring an object at "infinity" into focus).
Fortunately, some committed retailers did make up kits of all the right parts, so it was possible to buy an Imagon that you could attach to your camera and start taking pictures straight away. The principal retailer and promoter of the Imagon for the Pentacon Six was Schmachtenberg of Solingen, (West) Germany. Schmachtenberg also manufactured focussing tubes for the Imagon, and his tubes are shorter than the Zörkendörfer tubes. Unfortunately, his business no longer exists, although you may find equipment or packaging that bears his name.
What is the right focal length for the Pentacon Six?
The Imagon was originally designed as a portrait lens, and over many decades it was manufactured in a wide range of focal lengths. The "right" focal length for a given camera depended on the format of the camera: the larger the film format, the longer the length that was required.
At the time of writing, Wikipedia lists the following Rodenstock Imagon lenses, along with the format for which they were intended:
Over many years Rodenstock marketed both the 120mm and the 200mm Imagons for 6×6 cameras, although for most purposes (and definitely for portraiture) the 200mm Imagons actually have too narrow an angle of view for 6×6. With the 200mm Imagon it would therefore be necessary to increase substantially the distance between the camera and the subject in order to get the right framing for a good portrait.
Schmachtenberg is credited with having convinced Rodenstock in the 1980s that there was a market for a 150mm Imagon, after which a 150mm Imagon was specially calculated and manufactured. It then appeared on the market for a short time. I here report on both a 120mm Imagon and a 150mm Imagon for the Pentacon Six. The general features of these lenses and their use apply also to Imagons of other focal lengths.
The helical in the above image was supplied to the first owner by Schmachtenberg of Solingen. The prolongation tube behind it is necessary in order to put the Imagon at the right distance from some 35mm cameras. This extension tube and the Pentacon Six lens mount were manufactured as a custom order for this lens and camera by the photo machinist Glenn Evans of Glenn View in Chicago, USA. (See the Glennview website here.) The Pentacon Six lens mount screws directly onto the back of the focussing helical and the lens can then be focussed from "beyond" infinity to quite close up.
Mounting accessories on the front of the lens
The lens does have a screw thread at the back, to enable it to be screwed into a shutter, a lens board or a focussing helical, but there is no screw thread at the front, nor even a bayonet mounting. The components that go in front of the lens – the aperture discs, the filter and the lens shade – clip together, thanks to a partially-hidden spring at the front of the lens and at the front of each aperture disc and the filter.
The aperture discs
The lens hood (shade), ND4 filter and three aperture discs for the 150mm Imagon.
The straight pieces of clear metal at 90° intervals recessed within the front of the filter and each of the discs are springs to hold anything clipped onto the front.
Note that each of the aperture discs has a large hole in the middle and that the diameter of this hole is different for each of the discs. It is this hole that forms the aperture, and like all apertures, it has three functions:
These discs click in place onto the front of the lens. Only one disc is ever used at a time. They cannot be "doubled up". Rodenstock advises focussing the image before adding the chosen disc. The lens hood should be clicked onto the front of the chosen disc.
The diameter of the aperture discs is identical for the 120mm and 150mm Imagons, but the aperture values are different, because of the different focal lengths.
To avoid confusion when working with both lenses, I have therefore labelled the discs with the focal length. Some (later?) discs from Rodenstock come with the focal length already marked on the discs.
Readers will observe that there is a series of holes arranged on two circles round the main aperture hole in each disc. It is these holes that control the variable diffusion effect of the disc. The purpose is that, while the core image is sharp, the peripheral holes superimpose onto the captured image diffusion of highlights in the subject area. The size of these peripheral holes can be easily adjusted by the user. When they are fully open, maximum diffusion for that disc occurs. When they are fully closed, minimum diffusion for that disc occurs. Naturally, when they are open, they also let through more image-forming light, and when they are closed, they let through less light, which is why each disc has two numbers printed on it. Intermediate "H-stops" can be guesstimated, and this should be well within the exposure lattitude of modern negative film, both colour and black and white. If desired, through-the-lens metering can be used to obtain a more precise reading, which may be especially useful with colour reversal ("slide") film.
How does one open and close the peripheral holes?
Each "disc" is in fact made of two concentric discs, one in front of the other, with identical holes in exactly the same places. These two discs can be rotated in relation to each other. When the holes are all exactly aligned, as in the first image here on the left, all the holes are fully open. When the front disc is rotated to the right a few millimetres, the peripheral holes are fully closed, as in the last image on the right here. At intermediate positions, as in the middle image here, the peripheral holes can be more or less open or closed.
Here are all three "H-discs" for the 150mm Imagon, each one with the peripheral holes approximately half closed.
I have lightened the discs in this image and increased the contrast, in order to make it easier to see the detail of how the holes close.
If you have an opportunity to buy Imagon aperture discs on their own, make sure that they are right for the lens with which you wish to use them, as the lenses of different focal lengths and their corresponding aperture discs are of widely differing diameters. In any case, the given "H-number" is only right for a lens of the focal length for which it was calculated. Some aperture discs have printed on them the focal length of the lens for which they are designed, but that is not the case with the set of discs shown here.
The Neutral Density filter
As the greatest diffusion effect is obtained at the largest aperture and with the peripheral diffusion holes open, in some situations even the fastest shutter speed may not prevent over exposure. For this reason, a neutral density filter may become essential in order to obtain the correct exposure within the shutter speed range that is available to the photographer.
One photographer even told me that for portraits he prefers to use his Imagon lens with no diffusion discs at all. This is probably not recommended, since even the disc with the largest aperture improves the resolution of the lens. However, the results will depend on the lighting that he uses and on the subjects of his pictures.
It is important to add the supplied lens hood in front of the filter, to prevent stray light falling on its surface and degrading the image.
The complete 150mm Imagon set (top row), with the 120mm Imagon below it.
(Of course, a focussing helical and a lens mount are also required!)
Close-up view of the 120mm and 150mm Imagons, with their lens hoods (labelled by me)
The photographer needs to learn how to use the Imagon lens, which means making tests and recording the settings chosen for each picture, since it is not possible to see fully in the viewfinder the effects of the diffusion discs with this unusual lens. I have already taken my first pictures with this lens on my Pentacon Six, and hope to be able to report on the results soon.
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© TRA May 2016 Revised: August 2016