In Over My Head Rodenstock Imagon Question

[Ed Vatza]

I kind of asked about this in another thread regarding the Rodenstock Imagon 200mm f/5.8 lens. I would like to delve deeper into this question here. The basic issue is this... I am missing the 5.8/7.7 aperture disk for the Imagon 200mm. It is the disk that many Imagon users use as would I like to do. I have not been able to find a 5.8/7.7 disk for the 200 BUT I did purchase a 5.8/7.7 disk for the Imagon 250mm lens. This disk is on its way from Japan as I write. I plan to use the 250 disk on the 200 lens. Since all three Imagons (200, 250 and 300) consist of a rear lens and Copal 3 shutter, there should be no issue with the disks fitting the different lenses. A Copal 3 is a Copal 3 whether it is mounted on a 200, 250 or 300. So the 250 disk I purchased should fit on my 200mm lens.

But this begs the question in my mind of what is the difference. Here is where I think I have gotten in over my head. Digging around I found the Project Nayuki (https://www.nayuki.io/page/absolute-...lens-apertures) which addresses absolute and relative lens apertures. Pushing some numbers around, I came up with the following. I know the 200mm has a 200mm focal length and an absolute aperture of 5.8. I don't know the aperture diameter but I can compute it to be 34.5mm. Likewise I can compute the aperture diameter of the 250mm f/5.8 disk as 43.1mm. So there would be the difference. The 250mm 5.8 disk would have a 43.1mm aperture diameter compared to the 34.5mm aperture diameter of the 200mm 5.8 disk.

Remember, I think I am in over my head. So if I put the 250 5.8 disk on the 200 5.8 lens what happens? The way I read it, I would have a 200mm lens with an aperture diameter to 43.1mm. That computes to an aperture of 4.6 given the aperture diameter for the 250mm 5.8 disk. Now that's a 4.6 aperture based on the disk opening but the lens itself has a 5.8 aperture. So even though the disk would be wider (4.6) the lens still couldn't go any wider the 5.8. Right? SO the 250 5.8 disk should work on the 200 5.8 lens. But the reverse is not true. A 200 5.8 disk would end up stopped down to around 7.2 on a 250mm lens. Correct?

Please help me pull my head out of the water. Thanks.

Ed

[Bob Salomon]

The disks are different as the center hole size is different.

The size of the hole in the center determines how much of the center of the lens affects the image, the peripheral holes determines how much of the periphery of the lens affects the image, the periphery holes can be opened or closed to more precisely control the peripheral rays effect.

Each Imagon actually, by design, has two different focal lengths, the main one in the center of the lens and a different one surrounding it.

Since a 200 is shorter then a 250mm lens the size of the center hole on each of the 3 disks is different for each lens. If you use the first disk for the 250 on the 200 with the same lighting at the same distance with the same subject you will not get the same effect. Regardless of the opening or partial or full closing of the peripheral holes.

Who knows, you might be happy with the result, but it won’t be the expected result and, since the center hole size is different so will be the T stop!

[Tin Can]

Bob is correct, I was just about to write something similar but less compelling and he is the voice of experience.

I own a 360 Imagon with all discs. Fun but not the best lens for my usages.

I do like your link, https://www.nayuki.io/page/absolute-...lens-apertures But it doesn't address variable periferal multiple holes.

You need the matching discs to lens to be perfect.

[cowanw]

While I fully appreciate what Bob says about the function of a Imagon, I wonder if he would be so kind as to answer this. Is there a difference with
1. using a disc with the peripheral holes totally closed and
2. using an shutter aperture of he same size as the centre hole on the disc?

[Peter De Smidt]

Yes!

[cowanw]

The centre hole for the 200 is about 21 mm which I presume is f 7.7. The peripheral holes go out to 41mm and equals f 5.8.
The centre hole for the 250 is about 26 mm which is f 7.7. the peripheral holes go out to 48 mm and equals 5.8
Obviously the peripheral holes of the 250 are larger in diameter; very imprecisely 2 and 3 mm for the 200 versus 3 and 4 mm for the 250. the maximum diameter of the ring small hole to small hole on the 200 is 4.1 mm, very close to the same for the 250.
Math should work to figure a 26 mm aperture for the 200 mm lens but I expect you would have to do comparison film tests, perhaps with a densitometer or Stouffer tester, to figure what open holes from a 250 lens mean by way of aperture.
As Bob says the look will be different. I would predict more than expected flare and blur, especially when enlarged, perhaps more like a Verito in the degree of diffusion.

[Bob Salomon]

While I fully appreciate what Bob says about the function of a Imagon, I wonder if he would be so kind as to answer this. Is there a difference with
1. using a disc with the peripheral holes totally closed and
2. using an shutter aperture of he same size as the centre hole on the disc?

Yes, try it. The disk and the shutter’s diaphragm are in physically different locations and the effect will be different.

[Bob Salomon]

The centre hole for the 200 is about 21 mm which I presume is f 7.7. The peripheral holes go out to 41mm and equals f 5.8.
The centre hole for the 250 is about 26 mm which is f 7.7. the peripheral holes go out to 48 mm and equals 5.8
Obviously the peripheral holes of the 250 are larger in diameter; very imprecisely 2 and 3 mm for the 200 versus 3 and 4 mm for the 250. the maximum diameter of the ring small hole to small hole on the 200 is 4.1 mm, very close to the same for the 250.
Math should work to figure a 26 mm aperture for the 200 mm lens but I expect you would have to do comparison film tests, perhaps with a densitometer or Stouffer tester, to figure what open holes from a 250 lens mean by way of aperture.
As Bob says the look will be different. I would predict more than expected flare and blur, especially when enlarged, perhaps more like a Verito in the degree of diffusion.

The Imagon does not use F stops, it uses TStops. Not the same, but close.

[cowanw]

My bad!

[Ed Vatza]

Thanks for the info. My calculations may not be right but the concept was. The 250mm 5.8/7.7 disk is scheduled to arrive today. Then we move into the try it on the 200mm Imagon and see what happens phase. And in the meantime I am still in search of a 200mm 5.8/7.7 disk.

Ed