Imagon again

[Mark Sawyer]

measure the iris opening and divide......

easiest to use a metric measuring device to measure the opening. 250mm lens. 31.25mm opening. = f8.

I've wondered if this is why American and British lenses were often marked in metric rather than or along with inches, even before metric started taking over: to get rid of those pesky fractions-of-an-inch when dividing...

[cowanw]

I'm going to have to disagree with Bob here. You have focused the main sharp image under the same condition it will be used at with the holes closed. Adding focus shift afterwards from the little holes being opened is what you want, because they're supposed to be focused off the sharp focal plane due to spherical aberration. That's what makes a soft focus lens work.

There is a range of choices with regards to best focus and we are looking at the opposite of the usual problem with smaller formats of focussing wide open and then stopping down. Mark suggests we focus stopped down and then open up the aperture (by opening the disc holes) to a larger f stop. Sort of the equivalent of focusing a Verito at f8 and then opening to f5.6 to shoot.
It is helpful to me to try to understand the ray diagram on this site.
http://toothwalker.org/optics/spherical.html
If I have it right, Mark is suggesting we focus at point C and Bob is suggesting point B.
It is also important to realize that focus shift moves closer with a larger f stop. As applied to a portrait when we open the peripheral circles and keep the focus of the centre disc, the nose may become sharper but the eyes will still be in the attractive (behind the focus point) part of the DOF. ( the Graf Variable and the Cooke RVP are the opposite of this.)

[Armin Seeholzer]

The Imagon was in the early days also called "Tiefenbildner" in german because of the enormous DOF which it creates even at H 5,8 it gives a very long DOF which is usefully sharp!

Cheers Armin

[Mark Sawyer]

...Mark suggests we focus stopped down and then open up the aperture (by opening the disc holes) to a larger f stop. Sort of the equivalent of focusing a Verito at f8 and then opening to f5.6 to shoot.

Not the equivalent, because in the Verito, changing the aperture changes the depth of field significantly, which may cause you to miss the point of critical focus. Also, with more of the periphery of the lens making the Verito's image, it is a significant part of the overall focusing. I always focus the Verito at the taking aperture.

The little holes in the Imagon aperture are a small part of the overall exposure, meant as a soft overlay over the sharp image from the main aperture hole. Focus using that sharp image, which is the taking aperture as far as the sharp focus is concerned, then open the outer holes to overlay the soft image, (actually a sharp image, but on other focal planes).

But in practice, it doesn't make much difference...

[Bob Salomon - HP Marketing]

This is what the Imagon instruction book, latest versions, says:

"OPTICAL FEATURES OF THE IMAGON
The Imagon is a single-component cemented doublet with adequate corrections of all aberrations other then spherical. A special feature of this aberration is that it does not adequately define the focal length of the lens, for marginal rays meet in a different plane from central rays. The degree of this spherical aberration depends on the aperture. At large apertures it almost completely blurs the image but progressively disappears on stopping down. At the same time the plane of sharpest focus shifts. The object on which a lens with spherical aberration is focused at full aperture is no longer in the plane of sharpest focus of the stopped-down lens. HENCE THE RODENSTOCK IMAGON MUST ALWAYS BE FOCUSED AT THE APERTURE AT WHICH IT IS TO BE USED.

TAKING TECHNIQUES
....Due to its special characteristics THE IMAGON HAS TO BE FOCUSED AT THE APERTURE AT WHICH YOU INTEND TO EXPOSE THE PICTURE.

Always determine the zone of relative sharpness with the marginal diaphragm apertures fully closed. With the perforated daiphragm open, the ground glass screen image is too vague and gives you little idea of the detail rendering. Although opening the marginal apertures slightly alters the focal length, this is negligible. Forget about the complicated instructions for soft focus lenses found in older textbooks."

[Mark Sawyer]

And this is what the designer of the Imagon originally said:

"The following may be helpful to simplify correct focussing on the ground glass: with the small holes of the rotary diagram totally closed, thereby using only the central rays, not the slightest difficulty in focussing should be experienced..."

Closed makes more sense for precise focusing, open for seeing the soft effect. Doesn't matter too much either way, though...

[Armin Seeholzer]

Bob even if you put it in ultra large and red letters it get not more true! Mark and I also using it at the main aperture but we do then sometimes open up the small holes and this gives no shift in the center at all.
But anyway I use it most of the times without the discs anyway, which may kill the Imagon look but gives a normal SF look. And I know I do not have the permission from Rodenstock, but it does'nt matter anyway;--))))
And yes for all which like it complicated do it like Bob is pushing!

Cheers Armin

[cowanw]

Always determine the zone of relative sharpness with the marginal diaphragm apertures fully closed. With the perforated daiphragm open, the ground glass screen image is too vague and gives you little idea of the detail rendering. Although opening the marginal apertures slightly alters the focal length, this is negligible. Forget about the complicated instructions for soft focus lenses found in older textbooks."

"The following may be helpful to simplify correct focussing on the ground glass: with the small holes of the rotary diagram totally closed, thereby using only the central rays, not the slightest difficulty in focussing should be experienced..."

I think we are all saying the same thing.

"Bob even if you put it in ultra large and red letters it get not more true! Mark and I also using it at the main aperture but we do then sometimes open up the small holes and this gives no shift in the center at all.
But anyway I use it most of the times without the discs anyway, which may kill the Imagon look but gives a normal SF look. And I know I do not have the permission from Rodenstock, but it does'nt matter anyway;--))))
And yes for all which like it complicated do it like Bob is pushing!"

You make it sound combative. I for one am delighted when an industry rep participates in the forum.
All in all, someone needs to shoot a ruler open and shut and settle the issue.
A lens align
http://michaeltapesdesign.com/
would be perfect for this.

[Bob Salomon - HP Marketing]

"The following may be helpful to simplify correct focussing on the ground glass: with the small holes of the rotary diagram totally closed, thereby using only the central rays, not the slightest difficulty in focussing should be experienced..."

I think we are all saying the same thing.

No, we are not. What I stated from the passages in the instruction book was that there IS a focus shift when you change the setting of the small openings. So if you focus with them closed and shoot with them open you have created a focus shift.

[Mark Sawyer]

I for one respect Bob's position, and am very glad he's here! He knows what he's talking about and has corrected me in the past, for which I'm grateful. I learned something!

Either way of focusing is fine, and both have been recommended by higher authorities than either of us, (Rodenstock, the manufacturer, and Kuhn, the designer).

To Bob's latest point, I agree, when you open the little holes, you allow a focus shift effect to be added over the sharper image. That focus shift from the outer portions of the lens is the "Imagon Effect", and if you don't want that to throw off your focusing on the main, sharp image from the central opening, you should close the holes. And if you could correct for all focus shift in focusing, you'd lose the "Imagon Effect". Soft focus depends on there being a zonal focus shift (spherical aberration) across the lens area.

An advantage to Bob's "open hole" method is that you can see the softness as you focus.

But we're just arguing theory, and Yogi Berra was right: "In theory, practice and theory are the same thing. In practice, they're not." In practice, I've focused the lens both ways, and it's fine either way. And the Imagon is a lovely lens. Cheers, Bob!