basic question about base length and other newbie things

[mich8261]

It looks like I am on my way to acquiring a Bessa R2A or R3A and in the process I am doing as much reading on RF cameras as I can. I have been reading the Camera Quest pages and this one got my attention - http://cameraquest.com/leica.htm. Unfortunately I do not understand what the RangeFinder Base is. Is it an actual measurement of part of the camera? And how does it affect focus accuracy? Does it impact image quality?

Thanks

 

[Matthew Runkel]

It's the distance between the viewfinder and rangefinder windows multiplied by the viewfinder magnification. The greater the baselength, the greater the focusing accuracy. Rangefinder focusing is based on triangulation, and the longer the base of the triangle, the more precisely you are able to triangulate.

Baselength only affects image quality when it leads to out-of-focus results. In shallow depth-of-focus situations (longer lenses, fast 50s, close subjects) with a too-short baselength, you may not be able to achieve proper focus consistently, lowering your "hit rate." Baselength is irrelevant when your subject is at infinity.

 

[ywenz]

excellenty concise answer matthew!

 

[mich8261]

so then since the R2A and R3A are identical except for the viewfinder magnification (R2A is .7x and of course the R3A is a 1:1), the R3A has a greater effective base length. Is there more to it?

Thanks for helping a newbie.

Michel

 

[wpb]

so then since the R2A and R3A are identical except for the viewfinder magnification (R2A is .7x and of course the R3A is a 1:1), the R3A has a greater effective base length. Is there more to it?

Thanks for helping a newbie.

Michel

Not really. With higher magnification you will loose the ability to frame wider angle lenses through the viewfinder, but the flip-side is wide angle lenses don't require a very long effective rangefinder base length.

 

[mich8261]

thank you for the explanation. It brought back memories of high school math class.

So since base length is an important part of the equation, why don't manufacturers always put the rangefinder window all the way to the left (when looking at the front of the camera)? I imagine there is some technical limitation or challenge.

 

[BillBingham2]

I think it is one of those trade-offs or features that they do not want people to think about. It's kind of like being an informed consumer. For most people, not an issue. But if you play around in the fast moderate tele space (715/1.4, 85/2, 90/2, 105/2.5, 135/2.8) it can be an issue.

B2 (;->

 

[Dougg]

The actual baselength is surprisingly useful in the feel of the rangefinding. I didn't have a feel for this until I compared a Kiev 4, a Leica M2, and a Bessa T. Despite the T's high-magnification RF, the short baselength meant that the relative displacement of out-of-focus objects was smaller than with the M2 and especially the Kiev. The surprise to me was that the visual displacement in the viewfinder wasn't as useful in fast and accurate focusing as the relative displacement of the double image of the object being focused. So for me if there were two RFs with equal "effective baseline" (length x magnification) the one with longer actual baselength (despite lower magnification) wins over shorter baselength and higher magnification.

 

[lmd91343]

I think it is one of those trade-offs or features that they do not want people to think about. It's kind of like being an informed consumer. For most people, not an issue. But if you play around in the fast moderate tele space (715/1.4, 85/2, 90/2, 105/2.5, 135/2.8) it can be an issue.

B2 (;->

Bill,

How much did that 715mm f1.4 cost you? 😉

I would add a 1.0 or 0.95 50mm lens to your list of teles.

I agree. The manufacturers would assume you don't really need to know. It is not an item advertised or listed in specs. You won't find it listed at B&H. The makers and sellers would rather tell us the stand off angle of a thumb wind lever or the height, width, and depth of a camera down to a tenth of an mm!

Photographers have to be at least a little bit technical to master relationships of exposure time to lens apertures and dof to focal length. Surely we would want to know something as important as bl and ebl.
Perhaps the manufacturers are a bit embarrassed that they can't match the triangulation values reached by the Germans 70+ years ago or the Ukrainians 50 years ago? Wouldn't you like to have that kind of bl value with the modern workmanship of Leica or Cosina for your super-fast lenses?

-Lance

p.s. My Fed2 (bl = 71, ebl =71) is much easier to focus accurately with an 85/2 than my Fed1 (bl = 38, ebl = 38). That is much more important to know than the difference in width down to the tenth of an mm!