Rangefinder Accuracy

Is there any way to calculate how long the effective base must be in order to

accurately focus a lens of a given focal length and aperture?<br>

Or, to put it another way, how can you find out which lenses can be accurately

focused with a camera and which not?<br>

Thank you.

http://www.imx.nl/photo/leica/rangefinder_accuracy.html

Thank you.

Two points of classic RF cameras which I recently discovered, and are of interest to me is the old Zeiss Contax, with it's huge RF baselength. I always wondered how they managed to have the RF coupling in the bodies for their standard lenses, while Leica had to individually grind the cams. The answer, it turns out, was that Zeiss chose to set the RF for infinity, and didn't actually track the focusing distance. "As we used to say -- it's close enough for government work."<P>The other thing that's always bothered me is that the DOF scale for Leica lenses is about one stop more stringent than for the Contax lenses of the same focal length. Turns out that the CoC used by Leica was about one quarter the size that Zeiss allowed.<P>I was always impressed by that super-long RF base. Boy was I brainwashed! Today I'm even more impressed by the Leica quality than ever!<P>The real answer to your question is that theory doesn't matter -- it's the practice that counts.

OOps. I hit the wrong keys.

 

I just want Vlad to get the good information to his question:

 

Essentially the effective baselength of a

rangefinder is the distance between the center of the windows times the

magnification of the eyepiece. In order to accurately focus a lens this has to

be greater than the diameter of the f/stop being used on the lens. A 90mm f/2

lens wide open has an effective base-length of 45mm (90 devided by 2). The M2,

M4, M5, M6 can just about pull this off. I think their efective base length is

about 48mm."

Hope this helps.

Thanks, Todd (and Al). That's exactly what I wanted to know.

But this means that a Noctilux needs an EBL of at least 50mm, while a 75mm f/1.4 would need at least 53.57mm. So my understanding is that any 0.72x or 0.58x Leica would struggle with those, while Voigtländers (apart from Bessa T) would not even get close.<br>

Is this true? Are these fast lenses really impossible to focus accurately with anything but a 0.85x Leica or a Bessa T?<br>

Also, a 75mm f/2.5 Skopar should need an EBL of at least 30mm, but despite this I've used one wide open on my Bessa R (EBL = 25.16mm) with very good results.

There is also the M3.

 

It is true, but it depends on how close you get. You need 0 EBL to focus at infinity.

 

Best,

 

Roland.