15" Tele-Raptar front group FL question

[Dugan]

Hi,
I recently acquired a 15" f5.6 Tele-Raptar in barrel. Black, with a hood...(aerial camera version?)
I would like to use just the front group on the front of an Alphax #4 shutter ( It threads in perfectly).
My question is:
How does one determine focal length and aperture on just part of a tele-design lens?
My normal procedure for determining FL of a non-tele design lens is to put a lens on the camera, focus on a distant object, and measure from aperture to film plane...
That doesn't work with a tele design... is the "front group only" no longer considered a tele design?
So...the "FL divided by the aperture size as measured through the front element of the lens" measurement would be incorrect as a result, if the FL was measured incorrectly.
If anybody who has done this conversion could please help me out, I'd appreciate it...because I will need to make an aperture scale for the shutter.
I have searched the forum for info about this conversion, but haven't seen these questions addressed.
Thank you!

[reddesert]

The usual design for a telephoto lens is front group(s) with positive power (converging) followed by rear group(s) with negative power. The rear groups make the bundle of rays converge in a longer cone, as if converging from a plane in front of the lens. Like this illustration: https://en.wikipedia.org/wiki/Telephoto_lens#/media/File:Lens_telephoto_1.svg

The front group by itself should act like a normal positive lens, so you will get a rough measure of focal length by measuring from the group to the focal plane.

FWIW, there is a somewhat more accurate way of measuring focal length of any lens. That is to measure the extension required for some magnification ratio. For example, you focus the lens at infinity, and mark the infinity position. Then you set up a situation where you have a close subject and set it up for magnification of say 1/5 (5 inches on the subject is 1 inch on the ground glass). Focus and recheck the magnification, then measure e = extension, how far the bellows extended past infinity.

Now you can use the magnification equation, for ex https://en.wikipedia.org/wiki/Magnification

M = (d_image - f) / f = e / f.

So if you set up for a magnification of 1/5 and the extension was 30mm, then you have a 150mm focal length.

A useful thing about this method is that it should work for any unit-focusing lens, including telephoto and retrofocus. This is the same formula that goes into all those bellows factor calculators, we're just using it for a different purpose.

[Dugan]

Thank you for that information, I will try it out.

[mdarnton]

With the front in front it is about f/2,8, in back, about f/5.,6 The focal length is around 180mm--I have never attempted to measure it precisely. The effect is different in front or in back: in back it acts like a normal soft focus lens, and in front it is much softer! Used alone it has a small amount of barrel or pincushion distortion, depending on whether in front or back. A couple examples in both modes are scattered throughout my LF Flickr. This is one of my favorite lenses in all three modes and I have four or five of them in various boards. I use it on 4x5 but it easily covers 5x7. When in SF mode, it will appear to be uncertain where to focus, but I get the best results by focusing as closely as possible (racking out the lens) within the apparent focus range.

Be aware that some models of this lens in barrel have the components locked into the barrel with a small set screw down near the base of the component.

[Dugan]

Thank you for the info!
Mine has a huge hood on the front that doesn't want to come off, so I will be trying it out "front only" for now...not sure if it will fit through the front standard of my camera.

[Dugan]

I did the "focus on a distant object & measure from aperture to film plane" procedure and came up with 5.5" / 140mm. I'm kind of surprised that it measured as that short...but I can tell it's going to be a fun lens to use.

[mdarnton]

The center of the elements, basically where you should be measuring, in that lens is quite a bit forward.The position of the aperture is irrelevant.

[Dugan]

Excellent, thank you!