Rangefinder focusing different lenses of the same focal length

[drew.saunders]

Ebony has a nice list of the flange focal distance (front of lensboard to film plane at infinity) for a large list of lenses at http://www.ebonycamera.com/articles/lenses.html

150mm lenses vary from about 142 to 168mm.

Drew

[Emmanuel BIGLER]

Hello from France !

So, the question is : should I have a perfectly matched cam for my "150mm" lens ?
This has been also raised on the French LF forum and I have tried to find a magic formula that will help you to decide whether you'll be wrong or not using an un-matched cam labeled "150mm".

In fact as soon as your infinity stops are well calibrated for your lens with un-matched cam, the problem, technically speaking, is equivalent to the problem of a twin-lens reflex with un-matched twin lenses (viewing and taking lenses with different focal lengths, but both are properly set for infinity).
Namely, you are OK for large-distance objects but the rangefinder suggest wrong focusing distances for close-up, the closer you are, the more your images will be out of focus.
So without any computation we already know that the lens-to-subject distance, in fact the image magnification ratio, should appear in the formula ! For those who only shoot at infinity, no cam is needed ever after proper adjustement of the infinity stop ! ;-)

The details of the calculations are here (in French)

http://www.galerie-photo.info/forum/...99#reply_68623

... but hopefully the useful formula is exceedingly simple and very close to a good ol' Depth of Field formula.

Let F1 and F2 be the two focal lengths in question : F1 is the focal length for which the cam has been precisely adjusted by our beloved craftsmen in Munich, and let F2 be the actual focal length of the other, un-matched lens, mounted with the wrong cam but properly set for infinity.

The question is : how much can I tolerate on the focal length difference F1-F2 ?
The answer is, so simply :

abs(F2-F1) < c.N/(2M)

where
"abs" means : absolute value of the difference i.e. the difference without the minus sign;
"c" is the circle of confusion that you consider as an acceptable sharpness in your mis-focused image;
"N" is the f-stop number;
"M" is the magnification ratio M = (image size)/(object size).

"M" is used in the universal formula that yields the additional bellows extension "X" required to focus at a certain magnification "M" : X = M . F

As you can see, there is no need to precisely know where the principal (or nodal) points are located, provided that your lens is properly focused at infinity : the trick is that the universal formula deals with the additional extension with respect to the true focus, the formula deos not ask anything more than the focal length F. !!! Whatever the lens might be !!

So let's put some practical figures inside the tolerance formula
abs(F2-F1) < c.N/(2M)

Start with M=1/20 i.e. for a 150mm lens (6 inches), the object will be located at about 3 metres (10 feet) from the camera. Let's take c=90 microns, this is the value extracted form the other magic formula c=f/1720 (2 minutes of arc) ; this is just a rough guide, you can be more or less stringent,, it's your own style, you decide !
Lets stop down to f/11, N=11, we get, in millimetres
abs(F2-F1) < 0.09 x 11 x 20 /2 = 9.9 mm

I doubt that you'll ever find such a variation in focal length between two lenses of same manufacturers and same model ! However, if you consider different lenses of same nominal focal length "150" of various makes & manufacturers... nobody knows.
So if you have a 150mm lens of same make & manufacturer as the one for which the cam was made, my feeling is that you can go for it : the problem is that on a used technika, if you find a cam without its dedicated lens, you do not know which lens was actually in use with the cam !

However, imagine that you've found a nice Linhof-selected Zeiss 3.5-150mm Planar, and that you insist on using it hand-held wide open at the same magnification ratio M=1/20.
Then, the allowed tolerance on the focal length differences F1-F2 drops to only 3 mm. So on practice, if you want to use the full potential of a Technika or any other precision rangefinder camera with lenses wide open, you should have the proper calibrated cam.
(for rangefinder cameras of poor precision this whole thread is irrelevant )

Or imagine that you only insist on using the lens stopped doww to f/11 but demand c=30 microns as a sharpness criterion (c=30 microns is what was standard for lenses on 35mm cameras !), then you are in the same situation as above and will need to have a dedicated cam machined for you. Or be les stringent, or eventually focus on the ground glass.. wrong answer, since you demand a precisely-coupled rangefinder camera !

So the answer is : well, depend if you are a demanding photographer or not ;-)

To Dan Fromm :
..Bed or infinity stops..

Dan, as "they" often say in my e-mail input box (this is English, so I am not sure that I understand what "they" say ) :
Never stop to be infinitely better in bed-stops !

[David A. Goldfarb]

Thanks for the detailed explanation, Emmanuel. This completely fits my experience. When I need to shoot wide open with rangefinder focus on my Technikas, I only use lenses with matched cams. When I have enough light to stop down one or two stops, then I might use the two lenses that I have with unmatched cams, if they are of more convenient focal length.

[ljsegil]

Thanks Emmanuel, your explanation does a lot to put a real world perspective on the issue. It's nice to see a problem given a magnitude, rather than just a black/white answer.
Larry