Baselength and magnification

[Lauffray]

RF newbie back again for another dumb question
My question is how does magnification affect final focussing capabilities? I thought it was just a property of the viewfinder, but as I keep reading on the web I keep seeing weird calculations involving the baselength and the magnification.
Let's say we have the exact same system with different magnifications in the finder, what practical differences will the shooter notice? how will this impact accuracy if they share the same baselength?
I probably should note that I'm using the 1:1 R3M finder, which may explain my ignorance about this

 

[Roger Hicks]

Well, start out with a 70mm base, and a magnification of 0.7x. Effective baselegth 49mm. Now start out with a base length of 35mm, and a 1.5x magnification. Effective base length 52.5mm.

With the first one, the effect of any mechanical errors is reduced. With the second, it is magnified, so even though the EBL (Effective Base Length) of the second is greater, its accuracy is probably less -- though a lot depends on mechanical precision. Fast lenses at close distances are the most demanding: there is no such thing as 'the shooter' because people have different lenses, eyesight and demands.

Does this answer your question?

Cheers,

R.

 

[payasam]

Correct. Effective base length is based on arithmetic, while physical base length is an immutable reality. The principle of triangulation is a simple one.

 

[johannielscom]

Lauffray,

this page the Head Bartender has up on his site explains it all (well, at least to me it does...?)

EDIT: That page does not take into account that maginifaction isn't always the best solution, because mistakes are also magnified. I had not thought of that. Also, newer RF cameras aren't included, but the principle stands, so you can calculate these yourself I guess.

 

[Lauffray]

Yes I sort of get it now, so better to have longer base length and say a 0.7 magnification than a shorter base length and 1.0 magnification?
Focus from 5m-infinity will be less impacted by a smaller EBL than at 1m at f/2, correct?

 

[payasam]

Base length will affect accuracy at all distances, as a function of the mathematics involved: but there is greater depth of field at greater distances. Higher magnification helps us to see the R/F images at larger apparent size. However, higher magnification also magnifies errors, as has been said.

 

[Paul T.]

The other advantage of physically greater base length, as opposed to greater magnification, is that the VF will be able to incorporate framelines for wider lenses; the Zeiss Ikon, IIRC, has more readily visible 28mm frame than, say, the M7.

 

[Roger Hicks]

The other advantage of physically greater base length, as opposed to greater magnification, is that the VF will be able to incorporate framelines for wider lenses; the Zeiss Ikon, IIRC, has more readily visible 28mm frame than, say, the M7.

Dear Paul,

There's absolutely no necessary relationship on that one, as far as I can see.

Cheers,

R.

 

[payasam]

Paul T, the angular coverage of a viewfinder is related inversely to its magnification. The rangefinder does not enter into this.

 

[Paul T.]

If two cameras have the same effective baselength, but one has a greater actual baselength, then this camera will have a VF of lower magnification.

Lower magnification in the VF equates to a wider FOV for the same size finder, which is why the R2M finder, with .7 magnification, can accommodate wider framelines than the R3M's 1:1 finder.

 

[Sparrow]

A longer base length is easer to manufacture but the two images are less alike, especially close up , one can’t see base length in the VF only magnification

 

[Roger Hicks]

A thought experiment:

Build a viewfinder of any magnification.

Add a rangefinder.

The rangefinder may be of any base length.

Therefore, viewfinder magnification is independent of base length.

Cheers,

R.

 

[Sparrow]

exactly …..

 

[fuzk]

Lauffray, I hope you don't mind me jumping in here because I have the same difficulties in trying to understand this.


Roger: When you say mechanical errors are either reduced or maginified, do you mean when one looks through the viewfinder itself? Or do you mean that it is more obvious after you take a picture, i.e. it will show on your prints?

Why is it easier to focus a wide lens (e.g. 28mm) using a .72x magnification as compared to 1x magnification? Erm. or at least that's what I think I read.

Thank you in advance for any help!

Jy.

 

[Paul T.]

A thought experiment:

Build a viewfinder of any magnification.

Add a rangefinder.

The rangefinder may be of any base length.

Therefore, viewfinder magnification is independent of base length.

Cheers,

R.

True indeed. But add the words "for a camera of a given effective baselength" at the beginning, and then VF mag is inversely proportional to actual baselength.

If a designer is aiming for a minimum effective baselength, then a layout which has bigger actual baselength, and lower magnification, will - as well as being more accurate for the reasons outlined above - also accommodate wider VF frames.

The OP asked how VF mag affects focusing capability. It was stated that a larger actual baselength, and smaller mag, is more tolerant of errors. I added that this combination (as opposed to small baselength, high mag) also has the benefit of wider framelines, if desired.

 

[Sparrow]

taking Roger’s thought experiment a little further, I do love received wisdom …

 

[Roger Hicks]

Lauffray, I hope you don't mind me jumping in here because I have the same difficulties in trying to understand this.


Roger: When you say mechanical errors are either reduced or maginified, do you mean when one looks through the viewfinder itself? Or do you mean that it is more obvious after you take a picture, i.e. it will show on your prints?

Why is it easier to focus a wide lens (e.g. 28mm) using a .72x magnification as compared to 1x magnification? Erm. or at least that's what I think I read.

Thank you in advance for any help!

Jy.

Imagine a mechanical coupling error of 0.1mm.

With a 0.75x magnification, its significance is reduced to 0.75mm. With a 1.2x magnification it is magnified to 1.2mm -- a 50% greater error than with the reduced image.

This makes a difference to the focusing when you take the picture. Afterwards, it doesn't matter: either the picture is in focus or it isn't.

Cheers,

R.

 

[Roger Hicks]

True indeed. But add the words "for a camera of a given effective baselength" at the beginning, and then VF mag is inversely proportional to actual baselength.

If a designer is aiming for a minimum effective baselength, then a layout which has bigger actual baselength, and lower magnification, will - as well as being more accurate for the reasons outlined above - also accommodate wider VF frames.

The OP asked how VF mag affects focusing capability. It was stated that a larger actual baselength, and smaller mag, is more tolerant of errors. I added that this combination (as opposed to small baselength, high mag) also has the benefit of wider framelines, if desired.

Dear Paul,

Sorry. I was thinking backwards, because one does not normally approach the problem from this direction. As far as I can see -- and I'm not much good at this sort of visualization -- you have to be right.

Cheers,

R.

 

[Paul T.]

I was thinking backwards, because one does not normally approach the problem from this direction....

True indeed, if you're redesigning a camera, it's a bit tricky to change the physical base length. All it means is that there are diminishing returns in simply increasing VF magnification.

And, as you point out, the experience is in any case subjective, depending on your eyesight etc.

 

[payasam]

When both variables are brought in and one is kept constant, Paul T's first formulation is correct. I have used an M3 and then an M2, and am now back on an M3, partly because there are no diminishing returns to higher magnification.