Effective Rangefinder Base length

A Quick Guide to a Rangefinder’s Effective Base Length (EBL)

I recently published my thoughts on the Voigtlander R2a and R3a two great cameras that have one major caveat – their short rangefinder effective base length. I’m now in the process of putting together my thoughts on the Leica M3 (edit: you can find my M3 review here), a camera that’s biggest technical advantage is possibly its especially long rangefinder effective base length. Short of repeating myself by explaining what rangefinder effective base length means in the M3 post, I thought I’d put together a short post to detail the basics of the subject.

Rangefinder base length is the space between the camera’s rangefinder window and the viewfinder. It is the difference between two corners of a triangle, the third corner being the subject the camera is focusing on. The longer the base length, the longer the space between two corners of the triangle, the more accurate the rangefinder is.

To make things slightly more complicated the rangefinder’s base length needs to be multiplied by the magnification of the viewfinder to give something called the “Effective Base Length” – commonly abbreviated to “EBL”. This effective base length, as a specification on paper can give a good idea to how easy it will be to focus the camera accurately. The longer EBL the more precise focusing can be achieved… though there is a little more to the story than that.

Calculating a Rangefinder’s Effective Base Length

First we need to know the two specifications required – Rangefinder base length and viewfinder magnification. The most common Leica viewfinder has a 0.72x magnification, meaning what you see through the finder is 0.72x lifesize. Largely speaking these cameras have a base length of 68.5mm – though there is some variance apparently.

So, if we take the one number and multiply it by the other:

0.72 x 68.5mm = 49.32mm

So the effective base length of the most common Leica M rangefinder is 49.32mm

Compare this to these aforementioned Voigtlander R2a and R3a cameras. These cameras have a shorter base length of 37mm and have rangefinder base lengths of 0.68x and 1x respectively.

0.68 x 37mm (R2A) = 25.16mm
1 x 37mm (R3A) = 37mm

So the R2A has an EBL of 25.16mm and the R3A an EBL of 37mm. In short, neither the Voigtlander R2A or R3A have as precise a rangefinder as most other M-mount rangefinder cameras. And this is despite the R3A having the advantage of a 1:1 rangefinder.

At the other end of the spectrum we have the Leica M3. The M3 has a rangefinder base length of 68.5mm (like most leica M cameras), but with that longer base length it also has a high 0.91x viewfinder

0.91 x 68.5mm = 62.33mm

The Leica M3 Actually has the longest effective base length of any m-mount Leica rangefinder giving it an on-paper advantage for focusing m-mount lenses.

Some cameras, the “Barnack” Leica’s included have separate rangefinders and viewfinders. The benefit of this is that the rangefinder can have a magnification greater than that of the viewfinder. Most Banack Leica’s have an rangefinder base length of 39mm, which is nearly as short as that of the aforementioned Voigtlanders. The difference with the Barnack Leica’s is that the rangefinder magnification is 1.5x.

1.5 x 39mm = 58.5mm

Which puts them close to the Leica M3 in terms of their effective base length.

The bigger picture

So what does this mean in real life? Well, in my experience, it means a little less than many out there in the world of the hyper-critical-internet would have you believe – and more importantly it means something different to different people. There is no arguing with the fact that a shorter EBL is a disadvantage when shooting longer or faster lenses – the RF patch just doesn’t seem to pop as obviously when focus is found when compared directly to a longer effective base length rangefinder camera. But, I’m just not sure it’s quite the deal breaker some people suggest.

The ability to focus and shoot long or fast lenses accurately with a rangefinder comes down to a combination of things alongside its rangefinder’s effective base length. These include: size of the viewfinder, clarity of the viewfinder, size of RF patch, clarity of RF patch, whether or not the RF patch has solid edges, whether or not the RF and VF are combined, a steady hand, appropriate shutter speed, quality of eyesight, how well lit the subject is, subject distance and subject movement etc. Yes, a long RF EBL will help, but the other pieces of the puzzle also need to be in place for success. A camera with a longer EBL will possibly increase chances of success, but so too might practice and experience. Some of these variables might also affect or be a concern to some people more or less than to others.

For my personal tastes I find focusing long/fast lenses easier with the like of a Voigtlander R3A than I do with some of the more elderly cameras in my collection like my “Barnack” Leica iiia. For me, a bigger, brighter more distinct viewfinder and passably good, solid edged rangefinder patch trumps the extra magnification in the rangefinder of my iiia. The fuzzy-ish rangefinder of my iiia works less well for me, as does having to focus with a small rangefinder then frame with a separate independent viewfinder; the sheer act of moving the camera from rangefinder to viewfinder for me feels a distraction from the process when such precision is required. I know for a fact that others would dispute this and claim the exact opposite reality for them, but this is really my point, what works for one person might or might not work for the next. The key – as always – is finding what works best for you and your shooting needs and requirements.

Cheers for reading


Links & References
Very useful reference chart on CameraQuest


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18 thoughts on “A Quick Guide to a Rangefinder’s Effective Base Length (EBL)”

  1. Great piece, and an interesting read. I’m pretty new to rangefinders, but is another EBL advantage the accuracy of close focusing at or near minimum focus distance too? As a glasses wearer, I think lower magnification, high eyepoint viewfinders will be my lot (newer Zeiss Ikon seems to be recommended), but don’t want to completely erase EBL with a super low magnification finder.

    1. Hi Matt,
      Yeah, longer effective base length is helpful wherever shallow depth of field is an issue. So with longer lenses, faster lenses and both especially so at closer distances.
      As far as magnification, rangefinders and eyepoint goes … It’s not quite as simple as it is with an SLR I don’t think.
      The Zeiss Ikon ZM – which would indeed likely be a good choice for a glasses wearer looking for a versatile camera – actually has the same magnification as the M7 et al. But it also has a longer EBL and a physically larger finder.
      In short, I can see the 28mm lines in a ZM, but not in a Leica, even though the magnification is of the finders is exactly the same.
      Of course, it also depends on how frequently, you would want to shoot 28mm … If you only want to shoot 50mm, even the M3 would suit. That has the highest magnification going, and I can still see the framelines with my glasses on.
      Another factor is of course how far your glasses are from your eyes.
      Does that help, or confuse matters? 🙂
      Sorry for the slightly delayed response … been a bit manic

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    1. Perhaps, but as I say, it’s not all about length. I can’t imagine the RF patch is as bright or has a solid edge?

    2. Michael, I am having trouble verifying the EBL of the FED-2. But you said “rangefinder base of 68mm” Is that viewfinder magnification multiplied by the rangefinder baselength or just the baslength? Several rangefinders have 60+mm baselengths but their viewfinder magnification cuts the effective baselength down. The Kiev 4 series have a 90mm baselength and .8x viewfinder, making the longest EBL of which I’m aware; 72mm. The Contax II and III are just below at 67.8mm and then, in third place is the Leica M3 at 63.73. But as Hammish states, there is more to accurate focus than just EBL. And the longer the baselength, the more precise and regularly maintained the RF needs to be. Leica M’s seem to be a strong compromise between baselength, magnification, precision of mechanism and pleasantness of view. Compare this to the early Contax’s which had Leica well beat in EBL but, as the story goes, were so difficult to keep working that they became the stuff of collectors rather than photographers. I can’t speak for the Russian bodies much on first hand knowledge but from what I gather, being that QC is a big concern, while the EBL may be greater on some bodies than Leica, reliability of this is questionable. Without fresnel lenses in their viewfinders, these other RF’s will not be as bright as Leica either.

  3. Dear Hamish,
    A very interesting read. Thanks for the effort you put in. I know how time consuming it can be to author this sort of content.
    I’m a little reluctant to blow my own horn but, as you clearly wish to delve deeper into the subject matter than most internet writers (a trait I thoroughly approve of, BTW) perhaps you might find some musings I wrote about the pre-war Contax II/III rangefinder system to be of some interest? You can see the relevant comments here:

    I suppose it’s inevitable when discussing the 35mm rangefinder that Leica features prominently. I will say, though, that in my humble opinion the Contax installation is both more accurate and technically superior. Also that the flawed, but fascinating, Kodak Ektra features a rangefinder effective base length that annihilates even the Contax with its gargantuan dimensions. Both types of cameras would be worthy of inclusion in any future discussion about the specifics of rangefinder design. For students of technical aspects of rangefinder theory and design I would refer them to the long out of print, but still, extraordinarily informative, publication Miniature and Precision Cameras, authored for Amateur Photographer by J Lipinski in the early 1950s. Good condition copies of this title are still available with a little searching.

    1. Thanks, Brett! Interesting stuff – What are your feelsing toward my thoughts about solid edged rangefinder patches vs soft edged ones. For me this is the most important differential in practice. I just don’t get on with the soft edged ones nearly as well.

      1. Hi Hamish,
        That is a good question. It actually made me get a few rangefinders out of my cabinet, revisit them, and put on my thinking cap.

        For me, I suppose the most important factors are the brightness of the viewfinder and rangefinder patch a camera has, and how contrasty the latter is. Taking a step back, an example with incredibly accurate optics and mechanicals that is very hard to see in a wide range of lighting situations, would reasonably be expected to perform worse than one with less impressive specs but easier to align, yes?

        Hence, whilst a patch with very sharp, defined edges may indeed be a plus, it’s (for me at least) just one of several factors, overall, that will determine whether or not I’m going to get along with a particular camera. If I’m using a RF with a bright finder and good, contrasty patch I probably won’t be too worried about its edges—as long as I can see some sort of edge, that is! I recall you shared an image of one finder in your relevant article about this subject with edges so indeterminate I think I would *not* be happy using.

        To, not necessarily, “prove” my point—because I don’t think there is a “right” or “wrong”, personal preference counts for so much—but, perhaps, further illustrate it, I’ll mention some rangefinder systems I had the opportunity to play with last year. Not sure how much it will inform the conversation, but as they’re so uncommon, it might at least be interesting. These items were part of a collection of Swiss-made ALPA 35mm cameras that were being sold locally to me. Actually I have been helping with the consignment of them all. Now, any ALPA is an odd camera at the best of times, but in particular the collection included an original Reflex model and its successor the Prisma (not the earliest SLR with integral pentaprism but not that much newer than the Contax S or Rectaflex incidentally).

        Quite apart from anything else, both these models are a sort of single lens reflex, but more than that, they’re that rarest of things photographic—genuine hybrid designs which incorporate both coupled rangefinders *and* reflex focusing.

        Naturally I found them very fascinating—they are each truly unique, in so many ways—and, it must be said, somewhat flawed. But quite apart from their reflex viewfinders the coupled rangefinders were intriguing. Quite unlike the later hybrid 7 and 8 series ALPAs (I own a 7) these early ones used a horizontal rangefinder either side of the reflex pentaprism (yes they are rather crowded under the hood). The rangefinder is not incorporated into the viewing finder but discrete along the lines of a Barnack. But it’s also not of the coincident type. Two rectangular parallel windows are visible through the focusing eyepiece. Edges of each are as crisp and well-defined as you could ever wish for. And it’s also one of the hardest to use RF systems I have ever looked at. The complete absence of any superimposed image means the edges, and *only* the edges of visible objects can be used to focus on, by merging them together.

        So on the one hand we’ve looked at quite conventional coincident systems with fuzzy patch edges. On the other, these ALPAs have razor patch edges but *no* superimposed Image to merge. Ideally, then, I’d say it’s the sum of all the parts which determine how effective a particular rangefinder is to use, yes? 🙂

        My own allegiance rests with the Contax II. Its patch edges are not as sharp as an M. But they’re not useless at edge alignment either. When in good repair (and I always remove the top cover of any II that I work on, and clean every single air to glass surface in the RF system, all 15 or 16 of them) the Contax has a finder and patch superior to many Eg. Japanese fixed lens RF viewfinders forty years younger. It’s good enough for me to use indoors under low light if I have to. And when I do it’s deadly accuracy means focus is rarely ever off—such is the precision afforded by its exceptional base length and use of swinging wedges. Doubtless, to some the total reliance on an accessory finder for focal lengths other than 50mm, and complete absence of any form of parallax adjustment would be untenable. And I get that. But, Leitz, or Zeiss? It was ever thus. I’ll always be a Zeiss guy.

        1. Hi Brett, thank you for taking such time to respond – fascinating stuff! It is of course a case of different stokes, but I do appreciate such a thoughtful answer. You realise though, you are in danger of me asking you to write some of this up into a more formal post about the subject – do you think I could convince you…?

      2. Just to follow up my previous comment, Hamish, you can, in this image of the rear of the ALPA Prisma see, to the left of the pentaprism, the rangefinder focusing eyepiece. And though it’s quite small you’ll hopefully be able to spot the letterbox type slit with which the completely non-superimposed reflection from the RF window must be edge aligned with the residual view directly through the remaining portion of the finder.
        To the right of the prism is located the viewing finder for composing one’s image with a 50mm lens—assuming, of course, that one is not in a mood to compose and focus with the central reflex eyepiece behind the pentaprism!

        These are shown with the removable camera back detached. In duty its light traps seal off the film gate from any errant rats sneaking through the finders and, yes, you look “through” the back of these to range-find. I did say they were odd…

        As the owner of later ALPA Alnea 7 hybrid, I feel compelled to air the possibilities that whereas the 7 might arguably offer the best of both rangefinder and reflex worlds within one camera—these older ALPA hybrids might present the case for possessing the worst, combined.

        One can forgive them much for being so exquisitely beautiful objects today. And they are of their time. 😉
        Incidentally you can see more views of the Reflex and Prisma ALPA hybrids, including the front, which may inform the layout of their unique RF window system (as well as a number of other ALPA cameras or lenses and some images made with some of them) in this album.

        1. I honestly had no idea about this feature of any camera. Alpas – perhaps for their scarcity – have passed me by. They often seem to get mixed reviews in terms of usability too – which is I guess what your talking about. To reiterate, if I could convince you to pen a few words for 35mmc, that would be great. It feels increasingly rare that I read something that I had no idea at all about when it comes to a camera or series of cameras, and most of what you have said is very new to me! (There is no doubt a lot more that I don’t know – but you don’t know what you don’t know, do you ;)) Thanks again for the in-depth comments!

          1. Dear Hamish,
            I have written a probably too lengthy email about the ALPA cameras to your “contact” email address for the site. I don’t *usually* pen much stuff for third parties, but, I *do* appreciate your efforts to delve a little more deeply into familiar (or not so familiar) topics photographic. It’s what I like to also do. So I would be happy to put pen to paper, if you would like me to. Feel free to reach me via the email address I’ve written you from.

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  6. After playing around Canon rangefinders from the P to the 7 I finally landed a Leica M7 in 1994 and it has been a real game changer. From fuzzy edged imprecise focusing to sharp edged on target focusing. While the ,72mag viewfinder of the M7 is not the best there is the brightness and lack of flare compensate so that most lenses can be focused fairly well unless you use the 135mm which in any event is pushing the limit of rangefinders.

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