Rangefinder focusing different lenses of the same focal length

[ljsegil]

I have a question about rangefinder focusing with LF lenses. I am getting my Technika cammed and calibrated for a 150mm lens. I have been told by one of our commercial authorities that I will only be able to use that particular cam to focus that particular lens and that it will not work to focus other lenses of 150mm focal length. The more I think (actually thinking is a poor word for the processes that occur in my mind) about this, the more confused I become. It seems to me that any 150mm lens should have the same focusing distance from the rear nodal point to the ground glass (one focal length or 150mm) when focused at infinity as any other lens of that focal length. Similarly, if I shoot at 1:1 reproduction ratio, all 150mm lenses ought to focus at twice that distance, or 300mm from the rear nodal point to the ground glass. If these two points are the same, it seems to me that most likely the relationship between the object distance and the image distance for any 150mm lens ought to be linear and superimposed, otherwise the focusing object/image distance relationships for different 150mm lenses would have to be some kind of awfully funky curves with a pair of surprisingly coincident points. Thus, it seems to me that a cam made for one 150mm lens ought to work with other lenses of the same focal length. The only difficulty that I can think (again that probably inaccurate verb) of is that the rear nodal point may differ between lenses, although within a single focal length I wouldn't think that it would differ by much, and the effect of differing rear nodal points would cause the most misfocusing at infinity when the image distance is least, and the effect would decrease as object distance decreased and image distance increased making the difference due to differing rear nodal points a smaller percentage of the image distance.
So, does any of this make any sense? Can multiple lenses of the same focal length be used with the single cam, or would the error be too large at any distance? What other problems are there that I have not thought of (undoubtedly many)? I look forward to once more benefiting from the collective knowledge, expertise, and experience of the group.
Thanks ahead of time,
Larry

[Oren Grad]

The problem is that just because a lens is marked with a focal length of 150mm, doesn't mean that its focal length is actually 150mm. Actual focal lengths vary across different brands/models because of design differences, and within a given model because of sample variation.

Flange-to-film specifications can vary as well for a given focal length, but that can be accommodated by adjusting the infinity stops.

[Ole Tjugen]

The position of the rear nodal point does differ between lenses.

Go HERE (http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/index.htm) and look up the data for Xenar, Symmar and Symmar-S, especially the flange focal distance and principal point separation.

[Dan Fromm]

Focal lengths are nominal. Actual focal length is, typically, within 5% of nominal. That's why everyone insists that RF cams are lens-specific. You might luck out and find two lenses of the same nominal and actual focal lengths, but the odds are against.

I've had 20 38/4.5 Biogons, all extracted from aerial cameras. These lenses were collimated to the camera by shims between the rear of the shutter and the camera body. The shims came in 0.01 mm steps. The lenses' focal lengths were measured to 0.1 mm; the range I saw was 38.3 - 38.8 mm. Their flange to film distances were measured to 0.01 mm. All measurements marked on the barrel, of course.

These lenses were made for by a first-class lens maker for an exacting military application. Even so, actual focal lengths differed from nominal (38.5 mm) by as much as 1%. And this brings up another point. The 38/4.5 Biogon's nominal, per Zeiss, focal length is actually 38.5 mm. Focal length as reported for marketing purposes isn't always nominal focal length, let alone actual.

So, LJ, if you want to change lenses, by all means test whether the old cam is right for the new lens but don't blindly assume that it will be.

Cheers,

Dan

[Dan Fromm]

The position of the rear nodal point does differ between lenses.

Go HERE (http://www.schneideroptics.com/info/vintage_lens_data/large_format_lenses/index.htm) and look up the data for Xenar, Symmar and Symmar-S, especially the flange focal distance and principal point separation.Ole, I hope you didn't mean to say that with the lens focused at infinity the rear nodal point is not always exactly one focal length from the film.

But yes, of course the distance from the rear node to any other fixed point in/on the lens can vary from lens to lens of the same nominal focal length.

Cheers,

Dan

[Ole Tjugen]

Ole, I hope you didn't mean to say that with the lens focused at infinity the rear nodal point is not always exactly one focal length from the film.

No, only that the position of the rear nodal point relative to the rear flange of the shutter varies. Which again means that the lens board position at infinity focus is different, unless you put shims between the board and the shutter, or have some kind of tunable lensboard positioning mechanism.

[David A. Goldfarb]

Sometimes you get lucky, but in general, as Ole and Dan have stated, you can't count on two lenses being the same, even if they are the same lens, because the refractive index of the glass may vary from sample to sample.

That said, if you have an unmatched cam for a lens of a certain focal length, it might not be perfect wide open, but it might be good enough stopped down one or two stops. I have a 65/8 Super-Angulon and a 135/5.6 Caltar II-N that I've tested with an unmatched cam on my 2x3" Tech V, and I wouldn't trust them at maximum aperture, but down one stop, they're right on.

[ljsegil]

Oh well, I guess there is still no such thing as a free lunch (lens?).
But thanks all for the great info and explanations. I guess I'll go stew in my naivate and ignorance a while longer. Or maybe I should quit trying this thinking stuff and make a photo instead.
LJS

[Bob Salomon]

Larry,

It would be much easier if you simply called me at 800 735-4373 and get all of your questions answered or Marflex at 252-652-4401.

As I told you the other day on another site each cam is cut for a specific focal length. If you try to use that cut cam with another lens of the same focal length it will not focus correctly over the entire length of the cam. When people try to do this they then try to re-position the infinity stops to compensate but that still won't correct for the complete focusing range so they then try to fine tune the infinity stop position. The screws that hold the infinity stops in place are pointed to dig in and stay put on the chrome finished rails that they are mounted on. Moving them leaves pit marks that can not be removed.

Secondly the cam will not allow you to rangefinder focus at 1:2 or 1:1 magnification. It only works over a regular range from infinity to the minimum focus distance. The minimum focus distance is different for each focal length but no lens will be rangefinder coupled at close ranges. One reason for this, of course, is that lenses corrected for normal ranges of 1:10 or 1:20 are not the best performers at 1:1. That is the job of macro lenses. Macro lenses like the 120 or 180 Apo Macro Sironar could be rangefinder coupled as well but since these lenses are not good performers at Infinity or 1:10 or 1:20 they usually are not coupled.

Please call one of us if you still need further clarification. We will be open again on Tuesday.

[Dan Fromm]

No, only that the position of the rear nodal point relative to the rear flange of the shutter varies. Which again means that the lens board position at infinity focus is different, unless you put shims between the board and the shutter, or have some kind of tunable lensboard positioning mechanism.Bed or infinity stops, Ole. But on many (all?) of the RF cameras that have them, they have to be adjusted for each lens that's to be used on the camera. Think, e.g., Speed Graphic.

David, its not only variation in the glass' properties from batch to batch of glass, there are also variations in elements' thickness, radii of curvature, and spacing from lens to lens within a batch of lenses. My 38 Biogons were from two batches (serial numbers seen were 49305xx, 4997891 - 4998087) and I had some whose serial numbers were one (1) apart. They weren't identical within batch. High precision doesn't mean perfect conformity to specifications.

About variations in the glass. Eric Beltrando once told me that Boyer measured the properties (refractive index, Abbe number) of each lot of glass on delivery, recomputed their lenses slightly for each lot of glass before making a new batch of elements.

Cheers,

Dan

[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/read.php?f=1&i=68623&t=68599#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