What, exactly, is a plasmat?

[johndc]

Can someone explain to me what the term plasmat refers to?

 

I know it's a type of lens, different than a Tessar or triplet,but what criteria

does a lens have to satisfy to be called a Plasmat-type?

 

Is it (or was it) a specific formula that has now given its name to any lens

that is similar to the original design (like the Tessar), or is it more general

than that?

 

Thanks.

[michael_briggs2]

A Plasmat-type lens is a 6 element, 4 group quasi-symmetric lens that has wide coverage,

around 70 degrees. In this era, this is the most common choice for middle focal lengths for

a LF format: the Symmars, Sironars, Nikkor-W, Fuji-Ws. In his book, "A History of the

Photographic Lens", Rudolf Kingslake describes the Plasmat design as an air-spaced

derivative

of the Dagor design, which is 6 elements in 2 groups. He briefly mentions E. Arbeit in 1903,

but mostly attributes the design to Paul Rudolph in 1911, who is also the designer of the

Tessar.

[johndc]

Hmmm. It's funny because I've long been aware of the 6/4 symmetrical design, but I never knew it was referred to as a Plasmat. If I recall correctly, both the front and back sections of the lens are positive -- so does that mean that most convertible lenses are also plasmats?

 

Is my B&L Protar a plasmat?

[dan_fromm2]

6/4 plasmat: ()) air gap (( diaphragm )) air gap (()

 

6/4 double gauss: (( air gap ()( diaphragm )() air gap ))

[pico_digoliardi]

<i>()) air gap (( diaphragm )) air gap (() </i><p>

Neat notation. Is the diaphram really between two elements? How about.<p>

())_(|)_(() <p>

[dan_fromm2]

Pico,

 

6/4 plasmat: outer groups, cemented doublets; inner groups singlet menisci. Well-known types include many, not all, f/5.6 enlarging lenses; Schneider Symmar, Componon, late G-Clarons; Rodenstock Sironar, ApoGerogon, Rodagon; Boyer Saphir B, Saphir BX, Zircon. There are, I'm sure, many others.

 

6/4 double gauss: outer groups, singlet menisci; inner groups, cemented doublets. Well-known types include Zeiss Planar; Schneider Xenon; Rodenstock Heligon; Agfa Solagon; Dallmeyer Super Six, Rareac; some TTH Anastigmats. There are, I'm sure, many others. Note that some Planars are 6/4, others 5/4, 7/4, ... Xenotars are 5/4. So is the Wray Unilite.

[johndc]

Based on Dan's description, I drew this up in OSLO.

[dan_fromm2]

Yep, that's one.

[dave_s]

<i>

so does that mean that most convertible lenses are also plasmats?

</i>

<P>

Many are plasmats (e.g. convertible Symmars), but some are not (e.g. Dagors and Angulons).

<p>

<i>

Is my B&L Protar a plasmat?

</i>

<P>

No.