rangefinder baselength?

[jano]

It is my understanding that the longer your rf baselength, the more accurate your focusing may be. It's one of the items tooted in the marketing of 35mm formats.

However.. when I see pictures of the various MF RFs, their rangefinder windows seem to be fairly close together. With apparently such great optics, wouldn't it make more sense to have more precision and accuracy and take advantage of that excellent glass? Or is this not necessary?

Jano

 

[jaapv]

The RF base is not just the distance between the windows, it is the rangefinder magnification as well. Leica varies between 45 and 65 mm approximately,depending on the camera type, although the windows are in exactly the same place.

 

[Michiel Fokkema]

Hoi Jaap,

The aperture is also of importance.
A noctilux at 1 needs more accuracy than a Fuji 100mm at 3.5.

Groeten,

Michiel Fokkema

 

[jano]

The followup question then becomes: what actually determines rangefinder baselength, if not the distance between the windows?

I don't know exactly how magnification helps with this, but I would guess the greater the magnification, then the greater the accuracy/precision you'll have for a longer lens. That doesn't really tell me anything, because I would assume the same magnification for a "normal" lens on 35mm will work for a "normal" lens on MF (although there are different focal lengths involved, e.g. 40-60mm vs 75-90mm), the field of view will be approximately the same.

Thanks, Michiel, for your response, however, I believe DOF works different for MF than 35mm.. i.e. DOF for the 100mm 3.5 will be like DOF for the 50mm at 2 or 1.7 (I'm just guess this, could be wrong!). So in that scenario, you would want a greater baselength, especially if shooting wide open, right?

 

[FrankS]

Hi Jano,

rangefinder baselength is only deermined by the distance between the 2 windows. "Effective RF baselength" is a combination of this distance and the viewfinder magnification.

"I believe DOF works different for MF than 35mm.. i.e. DOF for the 100mm 3.5 will be like DOF for the 50mm at 2 or 1.7 (I'm just guess this, could be wrong!). "

You are correct in suggesting that you could be wrong on this issue. 🙂 A MF 100mm f4 lens has the same DOF characteristics as a 100mm f4 lens for a 35mm camera.

 

[jano]

rangefinder baselength is only deermined by the distance between the 2 windows. "Effective RF baselength" is a combination of this distance and the viewfinder magnification.

Thank you! So is there a reason why the windows are closer together on MF cameras? Do they have larger magnification windows? Do they have less of a need for accurate/precise focus? If so, why? Or maybe the pictures are misleading, and the cameras really are considerably larger and thus the windows are further apart than on 35mm?

You are correct in suggesting that you could be wrong on this issue. 🙂 A MF 100mm f4 lens has the same DOF characteristics as a 100mm f4 lens for a 35mm camera.

Fair enough 😛 I would guess, however, that the DOF would function similar to crop factor on digital, where the 100mm on a 35mm frame will give you apparently less DOF than 100mm on a MF frame? Orr.. errr... does it make sense what I'm getting at?

 

[FrankS]

AS to why the RF windows are so close together on a camera: they are far enough apart to accurately focus most lenses. Those cameras with shorter RF BL will have difficulty focussing longer faster lenses. For example, my Leica CL is fine with a 50mm f2 lens, but if I wanted to use, say a Canon 50mm f1.2 lens (wish I had one) I'd need to put it on my Leica M3 that has both a longer RF BL and higher VF magnification. Same with 90mm lenses. The Elmar f4 is fine on the CL, but the Summicron f2 would be tricky.

All lenses of the same focal length will have the same DOF when used at the same aperture and focused at the same distance, regardless of the format of the camera they are attached to.

 

[ErnestoJL]

There are several factors involved with the design of a RF baselenght.
The wider the baselenght, the most accurate focusing (distance measuring) you have, but is forces the manufacturer to make a system whose mechanical accuracy is at the same level of that of the optic system.

One way to reduce the mechanical requirements is to make the distance between VF and RF mirror/prism as wide as possible, then the milling process for cams, levers or rollers would require less tight tolerances. Conversely, with same tolerances, the RF systen with a wide base should be of better accuracy than others with shorter distances.

Magnification of the finder also takes part in the design as the baseleneght is modified by it, then the effective baselenght usually doesn´t equal the measured baselenght, in fact it´s reduced at a ratio which depends on the mag factor of the VF.

That´s the reason why RFs usually do not focus at less than 1 m (3 ft). For shorter distances the mechanical accuracy must be increased.
Conversely, if the mechanical accuracy can be increased, then the baselenght can be reduced accordingly without suffering the risk of a big measuring error.

DOF takes it´s part too, as any RF must be capable of showing in focus at any distance, within the DOF for any lens at full aperture.

Note that in any RF, the helix for different lenses have the same rotation angle, and pitch of the helix is varied to bring the lens in focus and RF the right (mechanical) information for the RF assy.

Ernesto

 

[jano]

Thank you FrankS!

I understand that DOF is the same on at the same focal lengths, but I'm not certain you understood what I was referring to. And it is this issue which makes me not understand why MF cameras don't seem to have longer baselengths. I'm having difficulty verbalizing it, give me a day or two here to compose my thoughts and try it again. 🙂

Also, thank you Ernesto, that certainly makes sense! But how does it address the MF questions?

 

[ErnestoJL]

Be it either 35mm or MF, the problems are the same, but the difference is that the size of the camera isn´t the same. A 6x9 cm is twice as big as any 35, then you have more room to acomodate a greater BL. At a first glance they look the same but when they are side by side....

This also gives another "benefit": the slower lenses (a reason of production costs only)(do you think of a 75/80 mm f 1.2?) allows for a greater DOF at full aperture then the requirements of the RF are lower.

If we go back to 1932, The ZI Contax should offer some benefits over the Leica to be able to compete, then they used a prism instead of mirrors, the BL was way greater (at least twice), and the focusing helix was in the camera body instead on the lens (among many other differences between them).

But both didn´t focus at less than 0,9 m... Leicas were 1 m...

Ernesto

 

[David Goldfarb]

This is one of those things that gets rehashed on various forums from time to time, but DOF most certainly DOES depend on format, because it is calculated using the value for acceptible circle of confusion relative to the format. This value is arguably subjective and might not be satisfactory for certain uses (very large enlargements, close viewing distance, personal tastes, etc.), but it relates to print resolution, not film resolution, so it makes sense that format is a factor.

There are a few ways of calculating acceptible circle of confusion, but the simplest is the Zeiss method, which uses the value of the diagonal of the format in mm/1730. As I understand it, the constant 1730 is based on the ability of the human eye to resolve detail in an 8x10" print at typical viewing distances.

The formulas for calculating DOF can be found in many books. A handy program for calculating such things is "f/calc," which has concise help screens that explain the formulas. It can be found at http://tangentsoft.net/fcalc/

As far as the original question goes--I wouldn't rely on pictures of cameras to determine the rangefinder baselength. Those MF rangefinders often look bigger in person than they do on the B&H website, and they typically do have a longer baselength.