What's the point of baselength?

[wintoid]

Educate me!

I've read and understood that the longer the baselength of a rangefinder, the more accurately it will focus. What I don't understand is how that fits in with what goes on in the viewfinder.

When I focus, I start from OOF, and gradually bring the images together until they coincide. Now, there's a range from just before they coincide perfectly until just after they coincide perfectly where I'm not 100% sure whether I can improve the focus. With a longer baselength, are we saying that this range is reduced?

In other words, if my eye can't tell with 100% accuracy whether I've got the right focus, does baselength really make any difference to the accuracy of focus?

 

[VinceC]

I'm pretty sure it's related to engineering tolerances. Akin to fine tuning.

The longer base length has more "travel" to make the images line up, therefore more accuracy.

A base length of just 1 cm would mean there would be only one cm of apparent travel of the secondary image between 3 feet and infinity. With a 6 cm base length, there is six times the travel. Thus, a 1mm of misfocus in the RF patch would represent perhaps three feet in the short base-length camera but just six inches in the longer baselength camera. It makes sense to me, but I'm having trouble explaining it.

 

[peterm1]

As I have always understood it, its actually got to do with the angles involved and geometry - or is it trigenometry? Not sure. But if you have ever seen the rangefinders used on old battle ships (before radar ranging became the vogue) these were identical in concept to those used in a camera but scaled up hugely - the rangefinder "tube" / base length was several feet long in fact. That means that at extended distances small changes in the angles involved for the rangefinder mirrors / prisms could be read accurately off the distance scale and in fact this large size allowed the rangefinders to accurately predict distance out to 20,000 yards and more, the range of the big capital ships' main armoury. Without this, it would be difficult to discern the differences caused by even quite large changes in distance to the "target." In short a longer rangefinder allows more accuracy in principle. This is also why you need a wider rangefinder baselength for long tele lenses - these have narrower depth of field so the focussing has to be more accurate to take account of this. The other factor is magnification. If the viewfinder magnifies the image more its possible to be more accurate - its the equivalent of an increase in ranegfinder length. So Leica M series like the M3 are more accurate (and hence more suited to longer tele lenses like the 135mm ) than some later versions of the camera which had smaller magnification even if the actual physical baseline is identical.

Try mounting this one on your Leica standard.

http://www.usstexasbb35.com/All-Photos/Bailey-Harold/Bailey-Harold-A-Bailey-RangeFinder.jpg

 

[OldNick]

As I have always understood it, its actually got to do with the angles involved and geometry - or is it trigenometry? Not sure. But if you have ever seen the rangefinders used on old battle ships (before radar ranging became the vogue) these were identical in concept to those used in a camera but scaled up hugely - the rangefinder "tube" / base length was several feet long in fact. That means that at extended distances small changes in the angles involved for the rangefinder mirrors / prisms could be read accurately off the distance scale and in fact this large size allowed the rangefinders to accurately predict distance out to 20,000 yards and more, the range of the big capital ships' main armoury. Without this, it would be difficult to discern the differences caused by even quite large changes in distance to the "target." In short a longer rangefinder allows more accuracy in principle. This is also why you need a wider rangefinder baselength for long tele lenses - these have narrower depth of field so the focussing has to be more accurate to take account of this.

Peter nailed it. The wider the base length, the more accurately the RF can determine the height of the triangle, which is the distance to the subject.

 

[Doug]

Yes, Peter gave a good explanation. The wider the baseline, the more angular change for a given change of subject distance.

The visible outcome for the user is that for a given amount of focus change on the lens, a wider-baseline RF will move the patch more, and this makes it easier to focus.

Surprisingly, baseline trumps viewfinder magnification (which does help too), because the patch moves more in relation to the image being focused.

A couple years ago when this question arose here in RFF I dug out my Kiev 4a with its very long baselength, and several other shorter-baselength RFs including an M2 and a Bessa T. A direct hands-on comparison really gave an excellent feel for the situation for the user. I wrote up a basic explanation of how an RF works in that thread that I think was made into a sticky...

 

[tripod]

The bigger the better, the tighter the sweater, and the more accurate the focusing. Like the previous posts said.

 

[Solinar]

With a longer base-length, you get repeatability with regards to setting the focus on a fast lens. I had a Canon VI-L for a bit and what I noticed with my 85/2 Nikkor using an object at 2 meters sometimes the focus mark was a tad to the left, occasionally centered on the 2m mark and other times a tad to the right.

With M3 or my IIIf it's was bang on the mark on the focus scale. The big Nikkor has a long focus travel. So, you could really see, if you were off. On some more modern lenses with only 45 degrees of travel it ain't as easy to see the variance.

 

[BillBingham2]

Let me try my two cents. Effective Base Length (EBL)is a combination of both the magnification of the optics you are looking through (in most modern cases the viewfinder along with the rangefinder patch) and the base length of the rangefinder (the distance between the exit pupils which combine to make the rangefinder magic).

Almost any rangefinder camera does a great job of focusing wide angle through normal lenses. In fact, Leica state years ago that rangefinders were more accurate at wide angels than SLRs (even their SL2). I find it much easier to focus my S3-2000 with the 25/4 on her than my F2 Photomic with a 24/2.8. Because of the wide depth of field of most wide angle lenses, other than with very slow wides in very dark places, both work well. Put an f4 lens on an SLR in existing darkness and try focusing it, not a lot of fun.

Where EBL starts to make a difference is with longer faster lenses. 75/1.4, 85/2, 105/2.5 nice lenses that have a very shallow depth of field and demand focusing accuracy. You will notice that CV does not make fast moderate telephotos. I know they could but the Bessa line, other than the T, does not have a long EBL. So you see 75/2.5 and 90/3.5, where you have sufficient depth of field to cover inaccuracies. Tolerance of design, materials and build are several parts of the picture, but EBL does matter. I love my Nikkor 105/2.5, but I could not expect to shoot wide open up close with her not on a rangefinder with a long enough EBL (e.g. Bessa T, M4-P, M3). The further apart those exit pupils are along with the magnification the more accurate you can focus.

If you like fast moderates inside you need something with a longer EBL than a Bessa R4M. I'm not even sure the 35/1.2 will work well wide open up close. If you are shooting outside, don't worry be happy. You stop down to f11 and you probably are OK. I used a 135/2.8 up close outside on a Bessa T and never got a bad shot.

While he can be long winded (perhaps the mirror should move to me too?) there a lot of good info here:

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

Here is some more good info:

http://nemeng.com/leica/031b.shtml

Hope this helps.

B2 (;->

 

[wintoid]

Thank you all for taking the time to explain this. The bit that really nailed it for me was this:

The visible outcome for the user is that for a given amount of focus change on the lens, a wider-baseline RF will move the patch more, and this makes it easier to focus.

Thanks Doug, if that's true then I understand perfectly now.

 

[Roger Hicks]

Unfortunately, the bit about the rangefinder oatch is not true. The distance the patch moves is controlled by the mechanical leverage between the coupling and the finder patch, and is completely independent of EBL.

Try this explanation instead:

Think of levering up a heavy rock with a short lever, and a long lever. You want the rock to rise 6 inches/15cm so you can get another lever under it.

With the short lever, let's say you have to push the lever down one foot/30cm. With the long lever, let's say you have to push it down three feet/90cm.

The short lever is harder work (irrelevant in this case) BUT the important part is that you have more control with the longer lever: push it down one foot and the rock rises 2 inches, not 6.

This is much the same as why a longer RF base is more accurate: it's simply easier to make the mechanism with the necessary precision (= control). A small error with the short base is much more apparent than a small error with the long base.

To a limited extent, you can make a short base more accurate by magnifying it, BUT you then magnify any errors as well. With a long base and a magnification of less than unity, the errors are reduced. Thus, a 40mm base with a 1.5x magnifier has the same EBL as a 75mm base with a 0.8x magnifier (60mm) but the 75mm requires less precision and is therefore easier to make both accurate and robust.

Cheers,

R.

 

[Sparrow]

I have to agree with Roger on this.

One disadvantage of a longer EBL that no one ever considers is the fact that at closer distances the image in the VF and the patch become dissimilar due to the steeper angles required to make them coincide. Consider that 12ft warship RF trying to range an object at 6ft distant.

 

[FrankS]

The focus throw of the lens also needs to be considered in this equation.

 

[Kim Coxon]

Just to expand on this slightly. The focussing accuracy of a lens is largely dependant on it's depth of focus. This is governed by the angle of the light hitting the objective plane and the size of the circle of confusion. The wider the angle of the objective light rays and the smaller the circle of confusion, the smaller the depth of focus. In general terms a wide angle lens has a much narrower depth of focus than tele one. This is offset slightly in SLR lenses because most wides are retrofocus designs which reduce the light angle behind the lens. However, there is a problem in getting the lens to it's correct point of focus. With an SLR you rely on the depth of field to get the lens to it's optimum depth of focus and the wider the lens and the slower the lens, the more difficult this is.

With an RF, the problem is slightly different. The depth of focus is not relevent but depth of focus still will be. It is for this reason that if you need to check the film plane/flange setting on an RF, it is better to use a fast wide lens rather than a long one as this is, if anything, more critical than the RF accuracy. As has been said elsewhere, in theory the "wider" toe base length of the 2 viewing windows, the greater the angle for a given distance. Have a look at any sin/cos tables. The differences for narrow angles are very small amd more importantly so are the differences. So the further away an object is, the less accuracy in determining the difference. In effect with a long lens, the lens is correctly focussed with regard to it's depth of focus but at the wrong distance. With a wide angle this is not so important because the depth of field covers the difference.

Magnification will make it easier for the user to use to to be able to focus accurately but it will not affect the accuracy of the RF itself which is governed by the mathmatics of the angles. In reality, unlike a projected image in an SLR, the accuracy of an RF is purely depenant on the base length and is independent of the angle of the lens in absolute terms.

Kim

Almost any rangefinder camera does a great job of focusing wide angle through normal lenses. In fact, Leica state years ago that rangefinders were more accurate at wide angels than SLRs (even their SL2). I find it much easier to focus my S3-2000 with the 25/4 on her than my F2 Photomic with a 24/2.8. Because of the wide depth of field of most wide angle lenses, other than with very slow wides in very dark places, both work well. Put an f4 lens on an SLR in existing darkness and try focusing it, not a lot of fun.


B2 (;->

 

[Roger Hicks]

Magnification will make it easier for the user to use to to be able to focus accurately but it will not affect the accuracy of the RF itself which is governed by the mathmatics of the angles. In reality, unlike a projected image in an SLR, the accuracy of an RF is purely depenant on the base length and is independent of the angle of the lens in absolute terms.

Kim

Dear Kim,

Of course, you are absolutely right, and I was being sloppy in my earlier post about the use of 'accurate'. And very few understand your earlier point about depth of field/depth of focus, which again is central.

Cheers,

R.

 

[Solinar]

Magnification will make it easier for the user to use to to be able to focus accurately but it will not affect the accuracy of the RF itself which is governed by the mathmatics of the angles. In reality, unlike a projected image in an SLR, the accuracy of an RF is purely depenant on the base length and is independent of the angle of the lens in absolute terms.

Kim

Where the 1.5x magnified view helps is that brings the RF image closer to the photographer. Thus any line or contrasting object in the RF patch is easier to discern through the eye piece.

Similarly, the brightness of the RF patch versus the viewing image is of consequence as well. I found that an older Canon IV with a bright RF patch easier to focus than the later VI-L or P. The best upgrade in my opinion that one can perform on an old Leica IIIc or IIIf BD is to have the beam splitter replaced.

Neither of the above really increases the accuracy of the RF apparatus itself, but they do diminish user error if the RF apparatus is properly calibrated. Just my opinion, YMMV.

The M3 is about as good as it gets with a vintage RF with its largish, clear crisp VF/RF images, coupled to a 60mm ish base-length.

 

[Al Kaplan]

The effective base length is the distance between the windows multiplied by the viewfinder magnification. The old thread mount Leicas had a magnification of about 1.5, giving an effective base length 1.5 times the measured base length. The M Leicas have more distance between the windows but the M-2 viewfinder has a "magnification" of 0.72, so the effective base length is a bit less that 3/4 of the distance between the center of the windows.

Lenses also have what might be called an effective base length, which is the actual diameter of the lens, or of the f-stop being used. As long as the "effective base length" of your lens is less than the one of the rangefinder you should have no trouble achieving good focus. With a 135mm f/4 lens the "base-length" is 33.75 mm. For a 50mm f/1.4 it's about 35.7mm, and the 90mm f/2 Summicron, at 45mm, is an easier lens to focus accurately than the 75/1.4 at 53.57mm.

In many situations it is difficult to find a good edge to focus on, or your eye sight might not be what it once was. Try focussing by judging contrast in the rangefinder patch. Even something with no hard edges suddenly seems to "pop" when it's in focus, as the contrast of the image increases.