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M9 Frame Lines

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Is there actually anyone that knows what the optical path looks like in a M viewfinder? Surely that is a key element in determining what is possible.

 

Indeed. There are some good illustrations here: rangefinder issues. You will see that the key elements in the optical path of the viewfinder (as distinct from the rangefinder patch and the framelines) are simply a concave (negative) lens behind the viewfinder window and a convex (positive) lens in the eyepiece: i.e. a reversed Galilean telescope. The concave lens forms a virtual image of the subject, and the eyepiece focuses the virtual image for the user. The virtual image is located a little way in front of the concave lens. The viewfinder window is just a window, and the beam-splitter prism is - in this regard - just a parallel-sided block of glass in the light path.

 

The magnification is nothing more than the relative angle of a incoming collimated beam compared to the angle of exit of a collimated beam, both with respect to the optical axis. There is nothing in there that neccesarily links the physical size, or the distance from the front to the back element, to the angle of acceptance. Take a microscope objective with NA=0.8, this allows light to enter and be imaged at sin(theta)=0.8 so about 106° field of view, nevertheless the front lens element is only about 1 mm diameter and the rear element (the ocular) is 160 mm away.

 

The optical system in a compound microscope is quite different from a viewfinder. The objective is a compound positive lens with a very short focal length that when focused creates a real aerial (i.e. not cast on film or screen) image towards the top of the microscope tube, and the eyepiece is another compound positive lens which focuses and magnifies the aerial image.

 

So I do not believe that the thickness of the body and the size of the front element (the rangefinder window) determines the possible magnification value and or the size of the visible frames. Which frames are visible depends only on the magnification and the angle of view that the eye can take in at a single glance.

 

In some of my posts I have specifically referred to the Leica M finder and its simple reverse Galileian construction (plus some very sophisticated optics for the rangefinder and framelines), and I think everyone else has been thinking in the same terms. Once you get away from the reverse Galileian then other things are possible - but it would mean a total redesign of the M range/viewfinder and it would be a huge challenge to fit it in the same space.

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No cameras here with me to measure - but do I not recall that the main VF window on the M5 was larger, in part to provide backlighting for the metering scale on the bottom?

 

Nor here. IIRC the windows were the same but there was a translucent plastic ridged thing above the VF window for meter illumination.

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No cameras here with me to measure - but do I not recall that the main VF window on the M5 was larger, in part to provide backlighting for the metering scale on the bottom?

 

I think, but am not sure, the M5 was slightly thicker, front to back, then M2-M4 and again with the M4-2-M7. This would call for a bigger front window to keep the same FOV and framelines as other M's at any given magnification.

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...So I do not believe that the thickness of the body and the size of the front element (the rangefinder window) determines the possible magnification value and or the size of the visible frames. Which frames are visible depends only on the magnification and the angle of view that the eye can take in at a single glance.

On the size of the finder window as well. The larger window of the R-D1 allows viewing 40mm FoV with 1:1 magnification. Even the .91x M3 is beaten here. The M9 will be unable to compete i guess since a larger window would not fit the favourite Frank's tool --i mean the Visoflex of course

-- or lenses with goggles.

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M6ttl/M7/M8 actually have slightly larger holes in the metal top plate (now that you mention it) - filled in by an extra strip of mirror coating across the VF top for cosmetic reasons. The front of the mechanical RF/VF inside is not bigger, however.

 

M5 I wouldn't know - last held one in 1976.

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Is there actually anyone that knows what the optical path looks like in a M viewfinder? Surely that is a key element in determining what is possible.

 

The magnification is nothing more than the relative angle of a incoming collimated beam compared to the angle of exit of a collimated beam, both with respect to the optical axis. There is nothing in there that neccesarily links the physical size, or the distance from the front to the back element, to the angle of acceptance. Take a microscope objective with NA=0.8, this allows light to enter and be imaged at sin(theta)=0.8 so about 106° field of view, nevertheless the front lens element is only about 1 mm diameter and the rear element (the ocular) is 160 mm away.

 

So I do not believe that the thickness of the body and the size of the front element (the rangefinder window) determines the possible magnification value and or the size of the visible frames. Which frames are visible depends only on the magnification and the angle of view that the eye can take in at a single glance.

Yes I have. It shows the body thickness is irrelevant, unfortunately there are copyright laws.

There is one in G.Osterlohs book PP 32.

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Yes I have. It shows the body thickness is irrelevant, unfortunately there are copyright laws. There is one in G.Osterlohs book PP 32.

 

Then please in your own words explain why the M8, with the thicker body then film M's, has a x.68 magnification viewfinder and can only fit in 24mm framelines, Effective Field of View of 32mm, and a film M has a x.72 finder, higher magnification, and can fit in 28mm framelines.

If it isn't because of the thicker body then Leica could of put in a x.80 finder and still showed the cropped 24mm framelines. x.80 being slightly less magnification then the film M's x.85 finder which only include the 35mm framelines as the widest lens for that finder.

 

Until you can prove this wrong with specifics facts I have to believe you don't know, and don't want to learn, what you are talking about.

 

Now after saying all that, Jaapv I think you are a nice guy and I don't want to get on your bad side but you really need to look at this from a logical point of view with some real common sense. It just does not make any sense that Leica would reduce the VF magnification by .04 but yet only be able to include the largest framelines for a 32mm FOV.

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If one wants to keep a consistent maximum field of view using the type of finder that we find in the M cameras then the thicker the camera body (viewfinder front glass to eyepiece) the lower the magnification must be.

 

It's true that the M8.2's widest frame lines (for example) show a 32 mm EFOV and it is also true that the .68 mag. comes because the M8 body is a bit thicker than the .72 mag. M7 which can (just barely) show a 28 mm field of view.

 

Naturally, one can change the mag. of a finder even with various body thicknesses but the relationship of finder depth (tunnel depth in Andy's analogy) to magnification (for a given physical finder size) will dictate the widest field of view that finder can show.

 

The M8.2 could use a .72 mag. finder but it would then no longer be able to show 32 mm (EFOV) frame lines successfully. The natural widest frame lines for a .72 mag. finder on the M8 would probably be matched for a 28 mm lens (37 mm EFOV).

 

Cheers,

 

Sean

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If one wants to keep a consistent maximum field of view using the type of finder that we find in the M cameras then the thicker the camera body (viewfinder front glass to eyepiece) the lower the magnification must be.

 

<snip>

 

The M8.2 could use a .72 mag. finder but it would then no longer be able to show 32 mm (EFOV) frame lines successfully. The natural widest frame lines for a .72 mag. finder on the M8 would probably be matched for a 28 mm lens (37 mm EFOV).

 

One more person who thinks clearly. But it will be a shame if people believe Sean because he's a guru when they wouldn't accept the same logic from Shootist and adan and me.

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OK, so instead of telling everyone else that they are not thinking clearly I decided to check the equations for a reverse Gallileian telescope (RGT). You can do this yourself with some elementary high-school optics.

 

1 if you have a front lens with (negative) focal length F1 and an ocular with (positive) focal length F2 then the magnification M of the RGT is M=F1/F2 (note F1 has to be smaller than F2 to keep an upright image). The magnification is therefore less than 1 unless both objectives go to infinite focal length (flat glass), then M=1.

 

2 the geometry of a RGT is such that the front focus of the positive lens coincides with the (imaginary) focal point of the negative lens i.e. the body thickness B=F2-F1 (assuming that the lenses are mounted flush with the body which strictly speaking is not neccesary, they could be slightly recessed or slightly extended).

 

3 The cone of light entering the eye is governed by the body thickness (

and the size of the front lens of the rangefinder (Dr), approximately as theta-eye-max = Dr/2B, this is the "tunnel-vision" effect referred to before. (You should actually use an arcsine but the error is rather small).

 

4 the cone of light that is accepted by the front lens of the rangefinder is therefore given by theta-rangefinder-acceptance = theta-eye-max/M = Dr/2BM.

 

Result of this is:

The body thickness is irrelevant for the magnification factor and only depends on the focal length ratio. The maximum field of view that the rangefinder can cover is governed by the body thickness (to some extent), the focal length ratio, the acceptable field of view at the eyepiece and the physical size of the front rangefinder element.

 

So I would propose that the body thickness is only one of the design factors, and certainly not the only (limiting) factor.

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...So I would propose that the body thickness is only one of the design factors, and certainly not the only (limiting) factor.

My feeling as well but each millimeter counts. It is difficult to view the widest framelines in .72x (28mm) and .85x (35mm) film VFs yet. So as long as the physical size of the viewfinder and the mechanical base length of the rangefinder will remain the same, the least body-thickness increase can force Leica to lower the VF magnification with the associated problems we know with the M8: smaller effective base length of the rangefinder, thus poorer focussing accuracy.

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OK, so instead of telling everyone else that they are not thinking clearly I decided to check the equations for a reverse Gallileian telescope (RGT). You can do this yourself with some elementary high-school optics.

 

1 if you have a front lens with (negative) focal length F1 and an ocular with (positive) focal length F2 then the magnification M of the RGT is M=F1/F2 (note F1 has to be smaller than F2 to keep an upright image). The magnification is therefore less than 1 unless both objectives go to infinite focal length (flat glass), then M=1.

 

2 the geometry of a RGT is such that the front focus of the positive lens coincides with the (imaginary) focal point of the negative lens i.e. the body thickness B=F2-F1 (assuming that the lenses are mounted flush with the body which strictly speaking is not neccesary, they could be slightly recessed or slightly extended).

 

3 The cone of light entering the eye is governed by the body thickness (

and the size of the front lens of the rangefinder (Dr), approximately as theta-eye-max = Dr/2B, this is the "tunnel-vision" effect referred to before. (You should actually use an arcsine but the error is rather small).

 

4 the cone of light that is accepted by the front lens of the rangefinder is therefore given by theta-rangefinder-acceptance = theta-eye-max/M = Dr/2BM.

 

Result of this is:

The body thickness is irrelevant for the magnification factor and only depends on the focal length ratio. The maximum field of view that the rangefinder can cover is governed by the body thickness (to some extent), the focal length ratio, the acceptable field of view at the eyepiece and the physical size of the front rangefinder element.

 

So I would propose that the body thickness is only one of the design factors, and certainly not the only (limiting) factor.

 

Yes and no. It's only one of the factors but it is a limiting factor. The equations M=F1/F2 and B=F2-F1 can be rewritten as

 

F2 = F1/M

F2=F1 + B

and give only one solution F1, F2 for each pair of M, B. In other words the magnification and the "body thickness" - the distance between the two lenses - determine the focal length ratio.

 

And it's not as simple to change the effective thickness as you imply. Moving the concave lens further back from the front window makes it necessary to enlarge the window or reduce the field of view; moving the positive lens deeper into the eyepiece reduces the clearance between eyeball and eyepiece, which is tough on people with glasses.

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Or you will make yourself seem not to smart, as you already have.

 

It is best to let people think you are a idiot then to open your mouth and prove it.

 

You know, I have a sneaking suspicion that you would not take this tone with me in person, but I guess some people become bolder than they are in daily life, when typing from the apparent safety of their keyboard.

Edited by thrid

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Yes and no. It's only one of the factors but it is a limiting factor. The equations M=F1/F2 and B=F2-F1 can be rewritten as

 

F2 = F1/M

F2=F1 + B

 

and give only one solution F1, F2 for each pair of M, B. In other words the magnification and the "body thickness" - the distance between the two lenses - determine the focal length ratio.

 

And it's not as simple to change the effective thickness as you imply. Moving the concave lens further back from the front window makes it necessary to enlarge the window or reduce the field of view; moving the positive lens deeper into the eyepiece reduces the clearance between eyeball and eyepiece, which is tough on people with glasses.

[Dunce hat]Good point! I had missed that.

 

Eliminate F2 gives F1/M = F1+B or F1 = M.F1 + M.B or F1(1-M) = B => F1=B/(1-M)

 

So Indeed the body thickness imposes the magnification (and vice versa) contrary to what I wrote before. [/Dunce hat]

 

But the acceptable field of view is still susceptible to other factors. The size of the rangefinder front element is not cast in stone, nothing stipulates that the lenses could not be slightly recessed. Also, by using a compound lens, or a 'thick lens' you can move the effective location of the lens plane (or planes for a thick lens). The body thickness is a factor but there are more tricks to consider.

Edited by SJP

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The body thickness is a factor but there are more tricks to consider.

 

Agreed - but the tricks are very hard to play without requiring major changes to the rest of the RVF. If it was easy, the M8 would be available in 0.85, 0.72 and 0.58 versions!

 

A factor that we haven't taken into account yet is compatibility with goggled lenses. This certainly constrains the vertical and horizontal location of the RVF windows in relation to the lens mount - but it might also constrain the fore-and-aft location of the concave lens of the viewfinder (and some equivalent on the RF side).

 

Maybe it's time, as Mark Norton says, for a new RVF system.

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You know, I have a sneaking suspicion that you would not take this tone with me in person, but I guess some people become bolder than they are in daily life, when typing from the apparent safety of their keyboard.

 

Don't be so sure about that.

I'll give you my address if you like because I doubt I'll ever be anywhere near here and there.

 

Errr, no.

 

They went to .68 because of the x1.33 mag factor. The body thickness has absolutely nothing whatsoever to do with the magnification of the rangefinder.

 

See analog M bodies of .58/,72/.85/.91 magification. All the same thickness and size. Even the M5 with it's oddball shape is .72.

 

This was your first reply to me and this thread. The cropped sensor factor of the M8 makes framelines for any given focal length lens smaller, they cover less field of view, not bigger then those same lenses on a film M.

No where did I say the M8 COULD ONLY have the x.68 magnification. It could and can have any magnification Leica chooses to put in it.

The body thickness along with the magnification of the viewfinder determine the widest framelines that can be easily seen by the user.

 

Sorry, but the thickness of the body has nothing to do with the magnification of the viewfinder.

 

Again, there are four versions of the viewfinder out there with magnifications of .58, .72, .85 and .91.

All of these analog bodies are the same size (except the M6ttl is 2mm taller, because of the extra electronics).

 

They went to .68 to make the 24mm framelines fit in the viewfinder. That's all there is to it.

 

No as it has been explained several time the cropped 24mm framelines in the M8 are SMALLER, narrower and shorter, then the UN-cropped framelines for a 28mm lens in a film M.

 

On an M6 with the .72 finder the 28mm framelines are all the way at the edge of the viewfinder.

If you look through an M6 with .58 finder there is a comfortable amount of space between the 28mm markings and the edge of the viewfinder.

 

Now, if Leica had put a .72 finder in the M8, the widest lens you would have had framelines for was a 28mm, which gives you a 37mm due to the x.133 crop.

 

But many M shooters wanted the equivalent of a 28mm in 135 format (x1.0) photography on the M8.

 

So, you have a choice.

 

24mm x 1.33 = 31.92mm

21mm x 1.33 = 27.93mm

 

The 21mm essentially gets you the equivalent of a 28mm on the M8. But in order to make 21mm framelines visible in the fiewfinder they would have had to lower the magnification (.50ish?) to an unacceptable level. As a result the markings for the other focal lengths would have been tiny and overall focusing accuracy would not have been high enough.

 

The choice of the 24mm is a compromise, because you only have to lower the magnification to .68 to fit the framelines for that focal length in to the viewfinder. The 24mm turns in to a 32mm on the M8, which is not perfect, but a reasonably compromise.

 

I'm going to state the obvious here, but it's just like zooming in or out with a zoom lens or raising or lowering a magnifying glass. Leica simply reduced the magnification to the point that they could just barely slip in the 24mm framelines.

 

As for the thickness of the body and increased length of the 'tunnel', that something any engineer worth his salt can work around to maintain a given magnification.

 

OK I've had enough.

Yes I can be this bold in person and that all depends on how someone rubs me and you sir have rubbed me the wrong way.

But you are right. I may not of said those words to you in person because at the point I wrote them here in person I would of figured you a lost cause and no matter what I said or how I said it you just would not get it and figured I'd just be waisting my breath. As I'm now waisting keyboard strokes.

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Agreed - but the tricks are very hard to play without requiring major changes to the rest of the RVF. If it was easy, the M8 would be available in 0.85, 0.72 and 0.58 versions!

 

A factor that we haven't taken into account yet is compatibility with goggled lenses. This certainly constrains the vertical and horizontal location of the RVF windows in relation to the lens mount - but it might also constrain the fore-and-aft location of the concave lens of the viewfinder (and some equivalent on the RF side).

 

Maybe it's time, as Mark Norton says, for a new RVF system.

 

I think the real reason the M8 was never offered with different magnifications was simply Leica knew the M8 was not the last of the line in digital M's and would not be around long, in Leica years, before they came out with a better digital M that came closer to the original specs of film M's.

 

That new model, it may be the M9, would be around for a longer time and come closer to film M specs so they could easily offer different viewfinders in it.

 

Look at the film M's. They didn't offer different magnification viewfinders untill the M6. And even then I doubt the x.58 or the x.85 models are ordered as much as the stock x.72 finder even when people were still ordering new film M.

 

Even if Leica comes out with some new and improved RF system there will still be the need for a mechanical linkage between the lens and whatever you focus through. That is unless they come out with all new lenses.

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I take your point - but mine remains: if it were easy to give the M8 a 0.72 magnification Leica would have done it.

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One more person who thinks clearly. But it will be a shame if people believe Sean because he's a guru when they wouldn't accept the same logic from Shootist and adan and me.

 

Thanks but I'm not a guru by any stretch of the imagination. This subject is just something I've had to look into in the past because I write about these cameras.

 

Cheers,

 

Sean

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I take your point - but mine remains: if it were easy to give the M8 a 0.72 magnification Leica would have done it.

 

John if this was a reply to my last post then simply Leica could not offer the x.72 finder as stock because then the EFOV would of been 37mm using a 28mm lens. I think they knew if they did that the camera would not of sold as well as it did.

 

Sure they could of but for reason stated many times in this thread they had to lower the mag to make the camera more usefull.

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