Little secret to share

[Pancolart]

A while ago i was asking which full frame SLR lenses cover wider image circle (ideal to adapt to mirrorless medium format digital). Finally i stumbled upon: CZJ Triotar 4/135 (fat) for Praktina. I checked image circle since large rear element seemed strange. 6x6!

[D1N0]

Yes the triotar is an adapted mf design.

[Slalom]

If you could post any images of the lense it would be helpfull

[D1N0]

This is mine (not the fat version, which is just a different body type) in m42
Carl Zeiss Jena Triotar F=13,5cm 1:4 by The lens profile, on Flickr

[jamaeolus]

I have CZJ tessar 80mm I'm pretty certain was designed for mf. It is a giant thing. Came unmentioned in a lot of exakta camera lenses and widgets.it has a bell shaped base with set screws which I have never messed with. I understood from forum reading it's actually a P6 EXAKTA adapter.

[iangreenhalgh1]

A secret? Not at all, just simple physics.

The Triotar 135 isn't a telephoto design, it's focal length is the same as the distance it's optical centre must be from the image plane to focus to infinity. The image circle will be about 135mm as non-telephoto and non-retrofocal objectives usually have image circles the same diameter as their focal length.

The Sonnar 4/135 is a telephoto design, hence it is a physically shorter lens than the Triotar and the image circle is smaller too - not big enough to cover 6x9 at infinity, whereas the Triotar covers 6x9 with room to spare, as do all the other triplets and Tessars of the same focal length.

[dan_]

Some of the Leica Visoflex lenses are known to cover at least medium format. Hektor 135mm covers even the 4x5 inches format! For the 200mm f/4 Telyt there are adapters to use it on Pentax 645 (I had one and tested it, together with my Hektor, on Pentax 645).

I read somewhere that Leica had, at one point in time, the intention of making some LF lenses and they designed them but never finalised their intention. When they started the Visoflex system they reused the design for their first Visoflex lenses.

See here:
https://www.pentaxforums.com/forums/48-pentax-medium-format/136207-leica-lenses-645d.html

[e6filmuser]

[Pancolart]

[iangreenhalgh1]

This talk of some lenses are 'medium format' or not is silly.

Each lens design has an angle of coverage, standard tessar types like the Tessar, Xenar, Industar and half a million other brand names, are between 45 and 70 degrees. For f4.5 large format Tessars, Zeiss quoted 55 degrees wide open, 60 stopped down. Older f6.3 Tessars can cover upto 70 in many cases.

Anyways, for a 135mm tessar type with a 55 degree angle of coverage, the image circle is around 140mm in diameter. How much of that illuminated circle is of usably high definition depends on the particular lens.

So you see why I say a good rule of thumb for non-telephoto and non-retrofocal lenses is that the image circle is roughly the same diameter as the focal length. You can see this in practice if you look at what lenses were put on old MF folders.

6x6 (56x56mm actual size) requires a 79mm image circle, this is why these cameras have 75 or 80mm lenses.

6x9 (56x82mm actual size) requires a 99mm image circle, this is why these cameras have 100 or 105mm lenses.

Triplets like the Triotar are generally about equal in coverage to tessars - 55 or so degrees wide open, which is why MF folders came with either tiplets or tessars, the designs have approximately the same flange focal distance and angle of coverage so they could be used interchangeably on the same camera body design, allowing manufacturers to produce both cheaper (triplet) and more expensive (tessar) models that differed only in what lens was fitted.

The Sonnar 135mm is a different beast, it is a telephoto lens and has a smaller angle of coverage around 40 degrees - the 135 Sonnar won't cover 6x9, the illuminated image circle is around 90mm, but the outer portion is mush, so the usable image circle is only about 75-80mm, meaning a 135 sonnar can be used on a 6x6 format camera but not on 6x9. I have a Jupiter-11 Sonnar clone mounted in shutter and have tried it on my 6x9 century Graphic, hence I know what the coverage is.In

The other very common lens design of this type is the Planar/double gauss, these also have a similar angle of coverage to Tessars and triplets, but more of the illuminated image circle is of usably high definition. Some Zeiss figures for MF Planars:

80mm has a 64 degree angle of coverage so has a 100mm image circle
100mm has a 62 degree angle of coverage so has a 120mm image circle
135mm has a 64 degree angle of coverage (67 for the later T* coated version) so has a 170mm image circle (T* version is 180mm)

So to know what the image circle of any given lens is, you need to know the angle of coverage and the focal length. For most triplets, tessars and planar types, you can assume it is around 60 degrees, other types, you will have to do some research.

One big caveat though is that there is a difference between how big an image circle a lens will illuminate and what portion of that image circle is sufficiently well defined to be usable. I've seen relatively modern lenses like Schneider Symmars (double gauss) that had relatively high definition right to the edge of the image circle, but older designs where only the middle half or even third is well defined, a good example of this are early Petzvals, I tried a 5inch one and it illuminated a huge image circle but only a quite small portion in the centre was usable, the rest was just mush.

[Pancolart]

[iangreenhalgh1]