effective vs actual rf base- does it matter?

[jpberger]

You often hear about "effective rangefinder base" as in the physical distance between the rf mirror and the view finder divided by the magnification of the viewfinder. Since a wider absolute base length implies greater mechanical precision, does a high magnification view finder with a short base length really have the same focus accuracy as a low magnification finder with a long baselength? Case in point Bessa r3 and .58 Leica finder have similar "effective" base length-- but are they really about the same in terms of real world focus accuracy?

 

[rxmd]

You often hear about "effective rangefinder base" as in the physical distance between the rf mirror and the view finder divided by the magnification of the viewfinder. Since a wider absolute base length implies greater mechanical precision, does a high magnification view finder with a short base length really have the same focus accuracy as a low magnification finder with a long baselength? Case in point Bessa r3 and .58 Leica finder have similar "effective" base length-- but are they really about the same in terms of real world focus accuracy?

Short answer: Yes, they are.

Long answer: Focus accuracy depends on how much the split image in the rangefinder moves when focusing from X to Y. This is a function only of the base length and the magnification.

Mechanical precision is nice to have, but then the question how far two optical systems are apart does not, by itself, have any implications on mechanical precision. There are other, more important factors here, such as how the mechanism itself is constructed.

 

[maddoc]

From my understanding the physical base length determines the accuracy of the two overlapping RF images while the magnification only enlarges the RF patch so that minor differences are more easily to see.

 

[rxmd]

From my understanding the physical base length determines the accuracy of the two overlapping RF images while the magnification only enlarges the RF patch so that minor differences are more easily to see.

But when the magnification of your RF patch is enlarged, so is the movement of the split image within it.

 

[maddoc]

But when the magnification of your RF patch is enlarged, so is the movement of the split image within it.

Yes and no. I compare it to the situation of looking at a - grainy - print with and without using a magnifying glass. With magnifying glass you will see larger grain but not more details.

 

[rxmd]

Yes and no. I compare it to the situation of looking at a - grainy - print with and without using a magnifying glass. With magnifying glass you will see larger grain but not more details.

You're right, but in your example I don't see the comparative equivalent of having a larger RF base. That would be more detail with less grain, but the analogy breaks down.

The benefit of a larger RF base is that it gives you more angular movement of the split image (I think on that we all agree). Then again, a larger magnification also gives you more angular movement of the split image, because it enlarges the angular movement along with the image. Either gives you more angular movement, and in turn more focusing accuracy.

Arguably with a larger magnification you can also see more detail inside the RF patch itself. If that has an influence on focusing accuracy, you could argue that magnification might be even more important than baselength.

 

[aizan]

Yes and no. I compare it to the situation of looking at a - grainy - print with and without using a magnifying glass. With magnifying glass you will see larger grain but not more details.

except you're not looking at a print.

 

[Doug]

...The benefit of a larger RF base is that it gives you more angular movement of the split image (I think on that we all agree). Then again, a larger magnification also gives you more angular movement of the split image, because it enlarges the angular movement along with the image. Either gives you more angular movement, and in turn more focusing accuracy...

This is a reasonable argument, but something is missing in practice. The question came up a few years ago, and I determined to look for myself. I cleared some space on the table and got out my Bessa T with very short base but high magnification. And the CLE with short base and low magnification. And the M2 with longish base and medium magnification. And the Kiev 4a with long baseline and 1:1 magnification.

It was interesting to see the clear progression to easier focusing as the baseline lengthened, despite changes in magnification. With a longer baseline, the patch image moved more, for a given focus distance change, in proportion to the size of the object focused upon. And that made it easier to be more precise about finding the best focus point. Even getting a fast approximation was improved.

 

[Sparrow]

This is a reasonable argument, but something is missing in practice. The question came up a few years ago, and I determined to look for myself. I cleared some space on the table and got out my Bessa T with very short base but high magnification. And the CLE with short base and low magnification. And the M2 with longish base and medium magnification. And the Kiev 4a with long baseline and 1:1 magnification.

It was interesting to see the clear progression to easier focusing as the baseline lengthened, despite changes in magnification. With a longer baseline, the patch image moved more, for a given focus distance change, in proportion to the size of the object focused upon. And that made it easier to be more precise about finding the best focus point. Even getting a fast approximation was improved.

Now I did the same sort of thing and concluded it was magnification and contrast that mattered from a subjective point of view, through the VF I saw pretty much the same thing regardless of base length, obviously as an engineering problem the shorter is the greater challenge.

A long base length is better over longer distances but by then dof covers the discrepancy of course

 

[jpberger]

This is a reasonable argument, but

It was interesting to see the clear progression to easier focusing as the baseline lengthened, despite changes in magnification. With a longer baseline, the patch image moved more, for a given focus distance change, in proportion to the size of the object focused upon. And that made it easier to be more precise about finding the best focus point. Even getting a fast approximation was improved.

Aha! this was my impression comparing a contax IIa and a Bessa r3a.

 

[Gabriel M.A.]

effective vs actual rf base- does it matter?

If it matters in Mathematics...

 

[Pico]

Effective and actual only differ when considering a persons eyesight.

FWIW, rangefinders are more accurate at medium-to-close distances rather than distant focus.

 

[jmkelly]

Every rangefinder design has an absolute accuracy limit that is related to the physical baselength and the magnification - and the resolving power of the human eye. The value of this error is related to the range to target. Regardless of the range to target, doubling the physical baselength OR the magnification halves the error. In practice what this means is that an RF with a 40mm baselength and 1.0x mag has the same error as an RF with 80mm baselength and 0.5x mag. An 80mm baselength and 1.0x mag has HALF the error of 40mm and 1.0x mag. These theoretical errors are measured in mm so they are not insignificant.

Also assumed is perfect calibration of the lens, coupling to the RF cam, and registration and movement of the RF optical surfaces. Mechanical errors here - even those within manufacturing tolerances - will compound the theoretical error. These errors are moot, though - so long as they are covered by DOF.

 

[ampguy]

hmm, where to start ...

hmm, where to start ...

Agreed, the print magnification example is not suitable, neither are CoC factors in effective baselength.

Higher magnification of VF is equivalent to a longer physical baselength, but as mentioned also has qualitative issues like flare, accuracy, quality of the magnifier.

DOF plays no role here, your VF should be accurate to a fixed distance when paired with a long throw lens on a properly adjusted camera body. This is where some RFF'ers have gone wildly astray...

except you're not looking at a print.

 

[Roger Hicks]

Consider an analogy with photomechanical reproduction/paste-up. When creating original artwork, it is common to work 'two up' or 'three up' (2x to 3x the final repro size). Any errors or ragged edges are thereby reduced at the final repro size.

It's much the same with RF bases/EBLs (Effective Base Lengths). A 100mm base, reduced to an EBL of 70mm (0.7x), makes fewer demands on mechanical and optical precision than a 35mm base multiplied to 70mm (2x).

Then again, the limit to magnification is normally set by the viewfinder, if the RF is incorporated. A 1x finder can't include a 35mm frame, which is why so many finders are in the 0.70 range.

Cheers,

R.

 

[Sparrow]

Well yes, but ... that only holds true if the camera is level. If one points it down at an angle then the parallax kicks-in and causes the verticals to start converging or overlapping. In that circumstance the larger EBL is a disadvantage.

I imagine a lot of this size matters thing comes from the importance of EBL in gunnery ranging before radar came along, ranging a ship at 10,000 yards is much more challenging than a pet cat at 2

 

[rxmd]

A 1x finder can't include a 35mm frame, which is why so many finders are in the 0.70 range.

I've always wondered why that is. There must be a relation between a finder's eyepoint, the magnification ratio, and the angle of view, but I've never understood how it works.

Beyond the trivial reply of "it would have to be so big and heavy that it wouldn't fit in the camera", what is the theory here?

 

[Roger Hicks]

I've always wondered why that is. There must be a relation between a finder's eyepoint, the magnification ratio, and the angle of view, but I've never understood how it works.

Beyond the trivial reply of "it would have to be so big and heavy that it wouldn't fit in the camera", what is the theory here?

I dunno either, but I suspect that the trivial answer is really all you need. Look at a Linhof Tech 70 finder, for example, which is about the size of a Leica.

Cheers,

R.

 

[ampguy]

I don't understand the question

I don't understand the question

By 1x, is that 100% viewable area (RF?) at a given eyepoint distance, or 1x magnification.

I have the answer, but just need the question clarified ... :angel:

I've always wondered why that is. There must be a relation between a finder's eyepoint, the magnification ratio, and the angle of view, but I've never understood how it works.

Beyond the trivial reply of "it would have to be so big and heavy that it wouldn't fit in the camera", what is the theory here?

 

[t6un]

"A 1x finder can't include a 35mm frame, which is why so many finders are in the 0.70 range."
-This must have something to do with the need to hold the beamsplitter in the viewfinder at ca 45 degree angle. But the higher the refraction index of the beamsplitter prism is, the thinner it looks and the wider the view is.