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renes
18-Jan-2012, 16:03
Can someone explain what combined focal length and the fastest f/stop will give 120mm f/4 cell + 200mm f/8 cell? Any sugestion about its coverage?

I can not find any data of Hugo Meyer Plasmats.

Ole Tjugen
18-Jan-2012, 16:06
Do you mean using a single cell from each of those two complete lenses? If so, we need the focal lengths of each cell. Or make an educated guess.

I didn't think a single Plasmat cell could be as fast as f:4, that's why I ask..

renes
18-Jan-2012, 16:35
Do you mean using a single cell from each of those two complete lenses? If so, we need the focal lengths of each cell. Or make an educated guess.

I didn't think a single Plasmat cell could be as fast as f:4, that's why I ask..

I meant using combined lenses: 120/4 and 200/8. It seems it's a pair of cells used in one Doppel Plasmat lens. I do not have the lens, it's description of one guy who wants to sell it. I will ask him to confirm both FL and f-stops.

Edit.
I thing it's 120mm f/4 but one cell only (probably rear) is 200mm f/8.

Dan Fromm
18-Jan-2012, 16:54
Piotr, the VM says quite a lot about Plasmats. Mentions that a pair of f/8 cells combined made an f/4 lens.

Spend the money, buy a copy of the VM.

Mark Sawyer
18-Jan-2012, 17:29
There's a pretty simple formula for finding out the combined focal length of two positive cells: add the two focal lengths together and divide by four:

120mm + 200mm = 320mm
320mm / 4 = 80mm

You have an 80mm lens.

A 200mm f/8 has a smaller aperture than a 120mm f/4, (25mm vs. 30mm measured through the lens), so it will choke down the aperture. If you ignore the difference in magnification between the two elements, a 25mm aperture on an 80mm lens gives you f/3.2, which is probably pretty close. Allow a little more magnification from the stronger lens and a little more distance, you have about an f/3 or f/3.1 lens. BTW, the smaller focal length usually goes on the front.

Ole Tjugen
18-Jan-2012, 17:31
A front cell of approximately 300mm f/12 and a 200mm f/8 rear cell would indeed make a combined 120mm lens. That is neglecting the spacing correction, which is wrong but not very far off.

Coverage - could possibly just cover 9x12cm at small apertures.

Mark, you're disregarding that only a COMPLETE Plasmat will be as fast as f:4, so you can't get one 120mm f:4 cell.

And the smaller focal length always goes on the rear of a Plasmat lens.

Mark Sawyer
18-Jan-2012, 17:59
Mark, you're disregarding that only a COMPLETE Plasmat will be as fast as f:4, so you can't get one 120mm f:4 cell.

And the smaller focal length always goes on the rear of a Plasmat lens.

Quite right on both counts, I took Renes too literally when he spoke of a "120mm f/4 lens" in his first post, and I was thinking of other triple convertibles like the Turner Reich and Cooke Series XV configurations that have the longer length at the rear. I ran my convertible 8.5"/14" Acuton (a plasmat) through my mind, and Ole's right. I'd never thought about it before!

renes
19-Jan-2012, 07:56
Ole, Mark, I am a bit confused than becouse the owner confirmed: front cell says D.R.P. Dr.Rudolph Mayer & Co. Gorlitz. Doppel-Plasmat 1:4.F=12cm, rear cell says Plasmatlinse 1:8 F=20 cm. Both cells have the same No.292472! so they make rather a right pair.

How to explain it?

Dan Fromm
19-Jan-2012, 09:36
That two equal 200/8 Plasmat cells make a 120/4 lens when combined is plausible.

Remember Zeiss' nomenclature. A single Ser. VIIa Protar cell is badged Protarlinse, the front cell of a Ser. VII Doppel-Protar (two Ser. VIIa cells) is badged Doppel-Protar.

Ole Tjugen
19-Jan-2012, 12:29
As Dan said:

Plasmatlinse = Single cell, data given or use of single cell.
Doppel-Plasmat = Complete lens with two cells, data given for complete configuration.

Just as with Protarlinse / Doppel-Protar; Anastigmatlinse / Doppelanastigmat.

That both cells have the same serial number is good; then it's an original pair and you can be sure that the combined focal length really is 120mm.

renes
19-Jan-2012, 12:44
Thanks.

Mark Sawyer
19-Jan-2012, 13:04
That two equal 200/8 Plasmat cells make a 120/4 lens when combined is plausible.

I think all the Meyer plasmats were triple convertibles, so the two cells should have different focal lengths.

Dan Fromm
19-Jan-2012, 13:53
Not according to the VM, Mark. Buy a copy. Our own Dan Collucci (posts as ccharrison) sells it.

FWIW, I have four convertible (according to the manufacturers, not according to users who've discovered that their lenses' individual cells form images) plasmat types. All have equal cells. Focal lengths are 105/178, 135/235, 150/250, and 210/350.

I've inadvertently -- too many cells off shutters at the same time -- mixed 105 front with 150 rear and vice versa. Both mismatched pairs formed images. Big deal.

Cheers,

Dan