Peach Blossom Bokeh! Another of my infamous blogs

[Arne Croell]

"It's a cemented doublet , or more precisely: a spherically under-corrected achromat. Any single meniscus will be chromatic."

"Achromatic meniscus" was a common term used for a cemented achromatic doublet in meniscus shape, combining the adavantage of the single meniscus wrt spherical aberration etc., as determined by Wollaston, but with additional achromatization. This was the lens type Chevalier made for Daguerre for his first Daguerreotypes. Check this URL from George Eastman House and the lens type name:

<url>http://www.eastman.org/fm/lol/htmlsrc/mB515400003_ful.html</url>

[Huw Evans]

Thanks to the original poster for an interesting comparison. I think a couple of points need to be made about the Imagon, though.

Firstly, I know this is quite close up (looks like it's about 1:1) but I suspect it's still pushing the limits of design coverage for the 250mm - IIRC it's meant for 5x4 at infinity, so maybe I'm splitting hairs there.

But more importantly, I don't think they were intended to be used without a disk (though obviously they can be). With no disk in at all you do get that really OTT flarey effect, but even the largest diameter disk reduces that dramatically. In fact the effect is very controllable, and if you go as far as to use the smallest disk with the holes closed off you get a very clean, sharp image. I did once compared it directly with my 360mm Sironar-N and although the Imagon shot appears just as sharp (well, on a contact print anyway) it shows what I can only describe as a lovely smooth 'creaminess', to the highlights especially. I did try to capture that quality in a scan some years ago, but it doesn't really come across on the web, so you'll have to take my word for it.

[Bob Salomon]

"Are they out of production now?"

yes. And, as someone else pointed out the 250 is designed for 4x5 and the 300 for 5x7.

The Imagon can be used with or without a disk and the exposure with no disk is the same as the exposure with the first disk used fully opened opened. Same exposure different degree of softness. the smaller the center hole in the disk the sharper the image. The fewer of the surrounding holes open on the disk the sharper the iage. The sharpest image is with the smallest disk with the surrounding holes closed. The softest is with no disk.

Never adjust the exposure with aperture blades as that masks the outside of the Imagon's lens and prevents the edge rays from creating the Imagon "effect".

[Huw Evans]

I've just noticed I missed a bit out of my post above - the comparison I was talking about with the 360mm Sironar was a 1950s 360mm Imagon, which, IIRC, is only intended for whole plate, but seems to me to be adequte on 10x8, and would be more directly comparable with the two other lenses used in the original poster's test.

[Paul Metcalf]

Jim-
This has nothing to do with the subject, and I'm not a digital scanner at all (that I'll admit), but it looks like all of your frames have the same line running down the left side. If this isn't a scanning issue, I'd say it's a film issue (if all from same package), or a downright miracle that all these lenses throw the same aberation in the same location (of course, in Tonapah, anything can happen, there are some weird happenin's for sure out in the desert - but that's Top Secret, I'm sure).

[Jim Galli]

"Jim- This has nothing to do with the subject, and I'm not a digital scanner at all (that I'll admit), but it looks like all of your frames have the same line running down the left side. If this isn't a scanning issue...."

The Epson 1640 is getting tired. If these were color that line would be green. Lint inside? The little green men are too big to get inside the Epson.

[Michael Alpert]

"The Imagon can be used with or without a disk and the exposure with no disk is the same as the exposure with the first disk used fully opened opened."

Bob,

I would think that the aperture with no disc would be larger than with the first disc fully open. After all, there must be some blockage of light from the disc. Perhaps you can explain what I am not understanding. Thanks.

Jim,

I think your photographs are fine. I also am attracted most to the Imagon photograph.

[Bob Salomon]

Michael,

If you close the aperture blades in the shutter you will mask the rays from the edges of the lens. These are the rays that cast the soft image on the image.

[Michael Alpert]

Bob,

Thank you for your response, but I think I was not articulate enough. I know that one does not use the aperture blades with an Imagon. Still, when you place one of the discs on the lens, you must be diminishing the amount of light that is going through the lens, even with holes fully open. So the setting for exposure with the disc must be diminished by an f-stop or so. Thus my question: When using a 200mm or a 250mm Imagon, what is the difference in exposure between (1) using the disc with the largest openings (holes fully opened) and (2) using no disc?

[Bob Salomon]

Michael,

The Imagon does not use F stops. It uses H Stops. They would be similar to an F stop as they are the exposure for the shadows on negative film.

The H Stop is controlled by the disks. There re 3 disks with the 120 to 250mm Imagon and 2 for the 300 in shutter. each disk has a different sized center hole. The smaller the hole the smaller the H-stop and the sharper the image.

Each disk also has a ring of holes around the edge of the disk that controls the amount of light being cast on the center rays by the edge rays. These holes can also be opened and closed to control the H Stop as well as the sharpness. The more holes open the softer the image. Maximum softness is no disk. Largest disk with all holes open is the same exposure but sharper. That disk with all holes closed is, with the 200 & 250mm) H 7.7 and is sharper then with all edge holes open (5.8).

Second disk with all holes open is sharper then the first disk at 7.7 and it is also 7.7. That disk with all edge holes closed is sharper again and becomes 9.5. The smallest center hole disk is 9.5 with all holes open and is pretty sharp. With the edge holes closed it is 11.5 and becomes very sharp.

The Imagon creates a very pleasent halo where the highlights disperse into the shadows. To obtain the correct amount of haloing for a given print size it is very important that the proper focal length lens is used. Otherwise the expected Imagon effect will be either exagerated or degraded. The amount of hoaloing at any given time is controlled by the disk used, the position of the outer holes and the lighting ratio used as well as the type of lighting used. It does not perform properly with soft lights. It needs strong spectrals to create the halo. It does not work well with flat to normal contrast ratios. It does not want a 3:1 ratio between the key and fill. It wants 6:1 or even more.

The Imagon is not a lens that would be mastered in one day. There are no set directions. It is a lens that must be experimented with to master.