Imagon: h/stop vs. f/stop

[Mark Sawyer]

Just playing with a 300mm Imagon on an 8x10, trying to figure out the differences between the h/stops and f/stops...

With the factory h/9.5 disc, "soup strainer" holes open:



With conventional f/stops at f/9:



And because I just couldn't help myself, wide open (f/5.8):


I went with a bit harsher light than I usually do, as that's where the Imagon is said to work best, but I don't see that it needs that situation any more or less than other soft lenses.

At h/9.5, the aperture has a wider "overall" diameter than the f/9 aperture, but the outer area is lessened in its effect as it is a series of small holes. Most of the h/9.5 aperture area is the smaller central area, and you can see it has a deeper depth of field than the f/9 The f/9 has a bit more "glow" around the highlights too, (see the back edge of Audree's hand), as the shallower depth of field allows the aberrations to show more, especially around the highlights.

Sadly, the h/9.5-11.5 disc is the only h/disc I have. I'd like to play with the others someday.

Strange lenses, these Imagons, but I like them...

[Robert Hughes]

Wow! Is that one of those new iFish?

[Bob Salomon]

Then you are in luck! We have one 7.7 to 9.5 disk left in stock and three of the 9.5 to 11.8 disks for the 300mm. These were the only disks supplied with the 300mm in Copal shutter as the physical opening in the shutter was too small to allow the 5.8 to 7.7 disks to be used.

What is the difference between using the holes in the edges of the disks and the aperture control in the shutter? Simple. An Imagon is a lens with two different types of corrections. One central and one on the edges of the lens. When you use the aperture control in the lens or shutter you mask off the effect of the outside of the lens and lose the Imagon effect. The disks preserve the effect of the periphery of the lens casting its effect over the central image. The disks have different size center holes. The smaller the center hole the sharper the image. The more of the surrounding holes are open the softer the image. Thus the softest image is no disk, next is the 7.7 disk with all holes open, next that disk with all outside holes closed and so forth. Sharpest image is with the 9.5 to 11.8 disk with all outside holes fully closed.

The above pertains to the 300mm only as the shorter 120, 150, 200 and 250mm versions of the Imagon were supplied with 3 disks each.

If you would like to buy your missing disk you can call our order department at 800 735-4373 and ask for Edwin. All disk are new stock left over from when the lenses were still available new.

[Mark Sawyer]

Wow! Is that one of those new iFish?

Nope, it's the iWhale! (Now with balleen technology...)

Then you are in luck! We have one 7.7 to 9.5 disk left in stock and three of the 9.5 to 11.8 disks for the 300mm. These were the only disks supplied with the 300mm in Copal shutter as the physical opening in the shutter was too small to allow the 5.8 to 7.7 disks to be used.

Thanks, Bob! But I'm afraid mine is an older version in the Compound shutter, so I'm guessing those won't fit... Great information on using the lens, though! The Imagon has always been one of my favorites, and for my style, I need to find that h/5.8-7.7 disc. Which leads me to a question I've never thought of before:

If the Imagon is a 5.8 aperture lens wide open, how can one block so much aperture area with the h/5.8 disc and still have an aperture of 5.8?

[Jason Greenberg Motamedi]

It has been a while since I sold my 300mm Imagon in Compound 4 (I sold it to the Chinese Envoy to the UN in 2001, and I always wondered what sort of images he made with it), but as I recall, they are actually faster than 5.8 with no disk. I don't remember how much though.

Anyhow, I just bought a 300mm in a Copal missing that disk, so I jumped on the opportunity. Bob mentioned that there are actually a few more available.

[Bob Salomon]

Nope, it's the iWhale! (Now with balleen technology...)



Thanks, Bob! But I'm afraid mine is an older version in the Compound shutter, so I'm guessing those won't fit... Great information on using the lens, though! The Imagon has always been one of my favorites, and for my style, I need to find that h/5.8-7.7 disc. Which leads me to a question I've never thought of before:

If the Imagon is a 5.8 aperture lens wide open, how can one block so much aperture area with the h/5.8 disc and still have an aperture of 5.8?

They should still fit there just isn't a 5.8 disk available any longer.

[Peter K]

If the Imagon is a 5.8 aperture lens wide open, how can one block so much aperture area with the h/5.8 disc and still have an aperture of 5.8?

The optical aperture is f/5.8, f/6.7 if the 300mm is mounted in a Copal #3s with it's 45mm opening and the nominal focal-length is the effective focal-lenght, but the sensitometrical aperture is larger because of the stray-light such a sphericaly undercorrected lens produces. (Don't hit me too hard, I'm using the Imagon too )

So the "H"-value - from german "Helligkeit" = brightness - corresponds with the "T"-value used with movie-taking-lenses.

Peter

[Mark Sawyer]

The optical aperture is f/5.8, f/6.7 if the 300mm is mounted in a Copal #3s with it's 45mm opening and the nominal focal-length is the effective focal-lenght, but the sensitometrical aperture is larger because of the stray-light such a sphericaly undercorrected lens produces. (Don't hit me too hard, I'm using the Imagon too )

So the "H"-value - from german "Helligkeit" = brightness - corresponds with the "T"-value used with movie-taking-lenses.

Peter

It seems like the light that strays in from outside the frame area should be nearly (but not quite) offset by the light that strays out from inside the picture area. But I get what you're saying.

The barrel indicates it as "h/5.8", seemingly without a disc, and the disc offers the option of "h/5.8-7.7" depending on whether the holes are open or closed. Do you know the [i]effective[i/] f/stop of a wide open Imagon, and the i]effective[i/] f/stop using the h/5.8 disc?

Or do I need to figure in soil loss?

T value

The United States Department of Agriculture defines the T Value as the maximum average soil loss (in tons per year) that will still allow economical maintenance of the current level of production into the future.

[Armin Seeholzer]

Hi Mark

The 250mm wich I have is without the disc at around f 4,5!

Cheers Armin

[Peter K]

The barrel indicates it as "h/5.8", seemingly without a disc, and the disc offers the option of "h/5.8-7.7" depending on whether the holes are open or closed. Do you know the [i]effective[i/] f/stop of a wide open Imagon, and the [i]effective[i/] f/stop using the h/5.8 disc?

The barrel, also the old version with three rows of holes, always indicates "H/5.8" because Staeble and later Rodenstock don't recommends the use of the Imagon without a disc.

The aperture of my 360mm wide open is f/5.8, 62mm measured with Steinheil's method. But measured a graycard with a Profi-select TTL in the image plane the brightness, measured with 5.8-disc, is the same as with as with a Symmar 360mm.

And as Armin mentioned before, without disc the brightness increases about 0.6 f-stop.

Or do I need to figure in soil loss?

Sorry, it's "t-stop" not "t-value"

Peter