Use half of the Sironar 150mm

[LuigiGallerani]

Dear all,
I am an Italian, newcomers in LF.
My first question is very simple. I have noticed that the Sironar 150mm could be unmounted from Copal0, and, using only the front part is like having a longer lens.
Does it make sense to use half of the lens to get a longer lens? I know that this for sure will compromise the quality, but, has this other implications?

[GeorgesGiralt]

Hello Luigi,
The older Sironar, like the Symmar from Schneider are convertible lenses. More modern designs are not useable in this way.
You normally unscrew the front element to get a little less than double focal length of the complete lens.
doing so you transform a six lenses design in a three lenses longer focal length. This has a big quality drawback, and oblige you to focus at real aperture (because the diaphragm is in front of the lens).
Last but not least,(and I don't now what) you will be obliged, for focusing at infinity, to give more length on the bellow than you should think. (for example, on a 265 mm focal length the bellow extend more than the 265 mm !)
Frankly, you'd better buy a second lens of longer focal length if you need it than using half of the "normal" lens.

[Mark Sampson]

Luigi, give it a try. For example, Schneider made the line of 'convertible' Symmar lenses for about 20 years, 1952-72, and suggested that the converted (longer) focal length was useful for portraits. Meaning that the corners might not be sharp. I tested a 180/315 Symmar once @ 315mm and it was better than I thought it would be. Perhaps the Sironar will work well converted- a few sheets of film will tell you.

[Bob Salomon]

"You normally unscrew the front element to get a little less than double focal length of the complete lens."
Yes, with the Symar. However Rodenstock's instructions for the Sironar were to unscrew the rear element to convert it. That way the aperture and shutter blades are not so exposed to the elements.

[LuigiGallerani]

Thanks you all! So I discover my lens is a convertible one.

What could be the difference of unscrewing the front instead of the rear element from the optical point of view? (it there is one?). The lens design looks symmetric. 6elemnts 4 groups.

[Bob Salomon]

Thanks you all! So I discover my lens is a convertible one.

What could be the difference of unscrewing the front instead of the rear element from the optical point of view? (it there is one?). The lens design looks symmetric. 6elemnts 4 groups.

Unscrew the front and the shutter blades and aperture blades are exposed. Unscrew the rear and they are protected.
The 1975 Rodenstock Precision Optics Inc. price list states:

"The Sironar series of view camera lenses are NEARLY symmetrical, six element "standard" taking lens for medium and large format cameras. The new design has combined maximum sharpness with optimum contrast. It is possible to use only the front component alone as a telephoto lens. The focal change is about three times the original focal length."

In case any one is curious, Rodenstock Precision Optic, Inc. was a wholly owned subsidiary of the Rodenstock factory in Germany.

The 8/75 Rodenstock literature page for the Sironar lenses states the following:

"Another essential advantage of Sironar-type lenses is that their special optical design allows the front component alone to be used as a "telephoto lens". In this case, it is only necessary to unscrew the rear component. At maximum aperture the Sironar front component may be used as a portrait lens (if a Rodenstock Imagon should not be available). This "incompletely corrected" system will add an artistic touch to the pictures. It should, however, be remembered that in this case the speed of the lens decreases in proportion to the increased focal length. The corresponding scales are engraved on the mount in yellow. With the front component alone, optimum sharpness will be obtained between f/45 and f/64. At smaller apertures, resolution will be degraded by diffraction."

So, while Schneider may have recommended removing the front, Rodenstock was very specific about using the front components and removing the rear component.

[Eric Rose]

Wow, very cool. I will have to give this a try.

[GeorgesGiralt]

Hello Luigi,
I've digged up the documentation of the old Schneider Angulon (the 6.8 version, with very small lenses).
They state that there are 3 focal length available with this lens : one with the whole lens, one with only the rear element, and one with the front element.
They say that "they are only useable for portraiture or landscape at infinity and with the lens stopped down 2/3)" .....
So three lenses for the price of one !
I've to try it !