Rodenstock IMAGON questions

[Samuel Hotton]

First question: I have a Rodenstock Imagon 250mm lens in a Copal #3 shutter that I purchased NEW in 1981. The three aperture discs that came with mine are all marked f=200. I JUST noticed this! Should it not be f=250 as I have a 250mm lens, NOT a 200mm lens. Did the factory put the wrong discs in my box, OR do they use the same aperture discs for both 200mm and 250mm lenses?
IF someone out there has a Imagon 250mm lens, would you please look on your aperture discs and see if it reads f= 200 or f=250.

Second question: Why are the apertures marked as H= 5.8, 7.7, 9.5, 11.5 Even the lens is marked H=5.8?? These H numbers are NOT equal to f numbers! One will over expose your film IF you use the H number on your light meter. Does ANYONE know the history or meaning of H numbers??

With thanks,
Sam H.

[leicam5]

I had a Imagon 250 mm for 15 years till it got stolen about 5 years ago, what made me very, very, sad indeed.
I can not recall whether ore not the f number was engraved on the discs, but if the H number of the 'clearest' disc is the same as the H number on the lens, then I think that the set of discs might be the same for a 200 mm and a 250 mm lens, who knows,
The 'H' is coming from the German 'HELLIGHEIT', meaning 'clearness — brightness' and, in this case, the amount of light that is available to expose the film when the disc is mounted and opened ore closed (and all the steps between that). These discs are no real apertures, the centre hole, which is not changing when turning the disc's ring, is perhaps somewhat like an aperture, but the little holes around it are regulating the amount of optic (spheric-) aberrations and also the transmission of light. One might not forget that a so called 'soft' image is more brighter than a rather 'normal-to-hard' image. A soft image 'contains' more light and, by this, is exposing the film whit that typical amount of light.
The Imagon whit out a disc is far more softer, thus more brighter (hell), than an other lens of the same focal length and diameter (opening).
So, each disc has a fixed pseudo aperture and a very own amount of light transmission via the perepheric holes. This is why each disc, and the lens not having an aperture only a diameter and typical (aberrated) light coming through (!), is considered to have a HELLIGHET (brightness) and not a F number.
You will see that, when the disc with the highest H value is mounted and closed, there is far less light 'coming through' than compared to a normal working lens with a normal aperture system and closed to the same amount, if you understand what I mean.
Helligheit has to do with the (artistic?) effect of the amount of light, and aperture with the mathematic amount of light (energy), reaching the film.

I hope, dear Samuel, that my pidgin English was clear (bright) enough, so that you could understand all what was to do about 'H'.

Good luck whit that marvellous lens,

Philippe

[AgX]

But as both lenses have got the same nominal aperture, the 250mm lens will have a larger front lens diameter. And, especially as the the circumference of the the aperture discs is essential for controlling off-center rays, by making use of an spheric aberration, how can both sets of discs have the same diameter?

[leicam5]

Dear Agx,

Good question!
As an answer I did a quick scan of the Rodenstock brochure of 1989. As you can see in the attachment, the 200 mm and the 250 mm have the same HELLIGHEIT.
Do not ask me for an explication...

Philippe

[AgX]

As a wild guess:
200 to 250 is not that much of a difference. As to be expected it was done concerning the image circle.
I thus would take those maximum aperture statements for the unobtruded lens with a grain of salt. So I guess both versions got the same mount for the disc and the same discs.

But perhaps someone has got both versions and the accompanying discs and can tell us more.


But nevertheless those H values on the discs should give the same effect concerning exposure as a regular iris-diaphragm with the same value.

[raucousimages]

You have the wrong discs. I have an Imagon 250 made in the mid 90s and it has discs marked H-250. With the correct discs the H value is very close to the corosponding F stop in exposure, there should be little or no over/under exposure.

My favorite way to use the lens is to get the proper F-stop tape from Copal for a 250mm lens, install it on the shutter (you will get 2 tapes and 4 screws) and just use it without the discs. Use the f-stops instead. Exposure is dead on and you can vary softness and DOF with ease.

[Chazzy]

You have the wrong discs. I have an Imagon 250 made in the mid 90s and it has discs marked H-250. With the correct discs the H value is very close to the corosponding F stop in exposure, there should be little or no over/under exposure.

Are they equivalent with the little holes open or closed?

[leicam5]

You have the wrong discs. I have an Imagon 250 made in the mid 90s and it has discs marked H-250.

I think it duos not matter that much, but it matters.
The peripheral holes are used to control the aberrations rather than the amount of light. And aberrations is all that matters with this lens.
This is, perhaps, why there was a ND 4 filter added to the set.
The centre hole of the three discs is more to control, step by step, the aberration and the little holes to control this variously. The aberrations are a little more present in the border than the centre of the image circle.
The higher the 'H' value the higher the so called relative sharpness (= lesser aberrations), and till a certain amount the depth of field. When the disc with the smallest hole is mounted and closed, the Imagon can produce almost 'normal' images.
The discs are marked from H 5,8 to H 11,5, and so on, not just one number. This is why the discs are numbered in H values rather than in F values. And if the H values happen to correspond to F values, then this is only when full open ore - closed.
Originally, the Imagon was mounted in a shutter whit out an Iris.

Do not consider the Imagon, a (soft-focus-) achromat, as a common lens. It was not designed as such by Rodenstock, as a matter of fact they called it a 'Weichzeigner'.

Philippe

[raucousimages]

I wonder if the differences in the H200 discs and the H250 discs is the size of the holes. That would efect the exposure also. I have the 250mm and the H250 discs. I use the following H to F conversion with good results.

H11.5=f11.5
H9.5=f8 3/4
H7.7=f5.6 2/3
H5.8=f5.6 1/6

But as I said before my favorite way to use it is by adding the f-stop tapes to the shutter and just use the f-stops.

[Dave_B]

I have a 250mm Imagon and the disks with it are labeled f=250 and then the range of H numbers.
Cheers,
Dave b.