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Calculating the correction for rear coated reflex mirror thickness!

Klaus Esser

pro member
Any thoughts about a 4x5 SLR? Your input would be most welcome..........

http://homepages.ihug.com.au/~razzle/4x5_SLR/4x5_slr.html

Great stuff, Dean!
There were some SLR in 4x5" or even 5x7" on the market in the last 80 years.
All very clumsy . .
Doesn´t a simple mirror attached to the filmholder almost do the same job? A 4x5" you wouldn´t typically use to shoot action pictures anyway, do you? ;-)
What about shift/tilt?

btw.: a GREAT site!!

best, Klaus
 

Asher Kelman

OPF Owner/Editor-in-Chief
Congratulations Dean!

Razzle_4x5_SLR_side.jpg


This is a wonderful project and we want to keep abreast of every step on the way. I hope you will make this thread into a running blog on your efforts to tame this beast!

Any chance you will get some movements?

Asher
 

Dean Jones

New member
It actually has interchangeable twenty four inch bellows too..... I'm just working on how to keep them under control. No rotating back as the body is not square. I'm hoping it will do more than portrait, currently it is set up to focus at 8-12 feet with the 210 and much the same with a 180 Xenar...what I really need to incorporate the bellows is a 240mm.

Let's also consider the mirror aspect...this camera uses a mirror that's coated on the upside, same as all SLR's. These things are mighty expensive, so what compensation do I make when using a cheaper mirror that is coated on the underside like the one on your bathroom wall?
Say it's 2mm thick, does that mean I must space the film plane 2mm farther back than normal, or if it's on a 45 degree angle is that 1mm?

If I decide to make another camera of this type, obviously my prime consideration would be keeping the price to a minimum and the mirror is probably the most expensive part.
Has anyone gone down this road before? I have posted this same question on the Large Format Photography forum, but I figure Asher would love a scenario like this to think about!

Cheers, Dean.

P.S Asher you are a world of knowledge.........!
 

Asher Kelman

OPF Owner/Editor-in-Chief
Dean,

Somehow your question was overlooked!

A lightweight reflex 4x5, able to take ready loads and Polaroid film, would be attractive. The one made by Wisner may or may not be available. So Dean, your new one might be the only choice! Sounds great.

It seems to me that the extra path length of light reflecting from the back surface of a 2mm mirror would be close to twice the hypotenuse of a 45-degree Isosceles triangle i.e. 5.66 mm.

Any other ideas?

Asher
 

Doug Kerr

Well-known member
Hi, Dean,

Let's also consider the mirror aspect...this camera uses a mirror that's coated on the upside, same as all SLR's. These things are mighty expensive, so what compensation do I make when using a cheaper mirror that is coated on the underside like the one on your bathroom wall?
Say it's 2mm thick, does that mean I must space the film plane 2mm farther back than normal, or if it's on a 45 degree angle is that 1mm?

If you could confirm the situation, I can show how to make the calculation. (I came into the story late.)

I suspect we basically saying that we have a reflex camera designed to use a front-surface mirror, but we don't have the original mirror, and are going to put a rear-surface mirror in it (for economy or whatever), keeping the mirror and focusng screen locations the same? Then we need to know how to shift the film plane to compensate for the disruption caused by the mirror change?

I'll proceed as if that were so.

If we put a transparent glass plate of thickness t into the path of an image being focused by a lens (the plate being perpendicular to the lens axis), the point of focus is moved farther away from the lens by the amount n-1(t), where n is the index of refraction of the glass. For typical kinds of glass, the index of refraction might be about 1.4. Thus for a 1 mm thick plate, the point of focus is "pushed back" by about 0.4 mm.

Now in the situation I am assuming, with regard to focus via the mirror on the ground glass, the ray bundles travel twice through the glass, and at an angle of 45 degrees to boot. Thus the effective distance traveled through the 1-mm thick plate is about 2.8 mm (1.4 mm going, 1.4 mm coming.)

That means that, if the camera is focused so that with the mirror up, the image would fall on the standard film plane, with the mirror in place it will fall about 2.8 mm beyond the ground glass.

But of course we will focus so the image falls on the ground glass. This means that when focus on the ground glass is achieved, the lens will be farther from the mirror (forward) by about 2.8 mm (assuming we are speaking of an object at some reasonable distance) than with a front-surface mirror.

Then, when we flip the mirror up to take the exposure, the image will fall about 2.8 mm in front of the standard film plane.

Thus we need to move the film plane forward (toward the lens) about 2.8 mm to compensate for the different mirror situation.

These numbers would all be doubled for a 2-mm thick mirror, if course.
 
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Doug Kerr

Well-known member
Oops!

Oops.

I wrote:

If we put a transparent glass plate of thickness t into the path of an image being focused by a lens (the plate being perpendicular to the lens axis), the point of focus is moved farther away from the lens by the amount n-1(t), where n is the index of refraction of the glass.

And I meant to write "(n-1)t". But that's not correct anyway.

In fact, I should have said:

. . . the point of focus is moved farther away from the lens by the amount (1-(1/n))t . . .

If n= 1.5, then the displacement of the point of focus would be 0.33 mm, not 0.5 mm.

Follwing the trail to the final answer (for a mirror at 45 degrees), and using a 2 mm thick mirror plate this time to begin with, the amount by which we need to move the film plane forward (toward the lens) would be about 1.88 mm.

Sorry for the error - I should have followed my first instinct and calculated it later!

I don't know how the "Razzle" 4x5 SLR fits into this story, but of course it is apparent that in it the mirror angle isn't quite 45 degrees.
 

Dean Jones

New member
4x5 has been rested!

Hi Asher, I'm still here, but have rested the SLR for a spell due to Polaroid Razzle commitments!
The camera currently has a 210mm focal length lens fitted, but can only focus between 8-12 feet. I await a 240mm so I can not only utilise the bellows, but gain infinity focus also.
I thought that if I were to build a similar camera from the ground up, I would need a readily available mirror.

I have experimented with a highly polished stainless steel plate, but light falloff is too much, as with a normal mirror..........perhaps the light transmission is further reduced a by having to travel through the glass itself, something that doesn't occur with the mirror that's coated on the upside?

Brightness is the key issue with an SLR, especially when checking focus at the corners, even with a bright f5.6 lens it's still a little dim when ambient light is bright. What I really need is a 240mm f2.8, if anyone has one?

A further problem will ensue with falloff should I attempt macro, as the neccesary bellows draw will also rob me of valuable brightness. It's a pity I cannot intensify the light somehow, as does a digital CCD, by increasing pixel sensitivity, guess I'll be thinking more about that one.

For now this SLR is a means of experimentation.....when I get some more time. The shots I have taken have been extremely sharp every time, so I think the mirror/ground glass/filmplane relationship must be spot on, now about that f2.8.....

Cheers, Dean.
 

Ray West

New member
Hi Bart,

I don't know, but the eec is bringing in/brought in legislation re selling mercury - to the extent, I believe that you can not sell your barometer. So, the chances of buying the kits you could once get for 'resilvering' mirrors are probably non- existent. Anyway, iirc, paint stripper to remove the protective coating, then nitric acid (but, if you are silvering the front, why worry about the back?). I think, if I can remember back to my school-days, simply clean the glass well, and 'rub on' the mercury.

Mercury was fun stuff. Apparently, many years ago, they ran engines on it. Probably why its being banned now, a viable dyno oil alternative. ;-)

Best wishes,

Ray , not quite 'mad as a hatter', but working on it
 
Hi Bart,

I don't know, but the eec is bringing in/brought in legislation re selling mercury.

That's correct, mercury thermometers are forbidden AFAIK.

Anyway, iirc, paint stripper to remove the protective coating, then nitric acid (but, if you are silvering the front, why worry about the back?).

Yes, paint stripper might work on a regular mirror. I haven't tried it since 30 years ago, so it could be a fun experiment, useful for a homemade single shot stereo device as well.

Bart
 

Doug Kerr

Well-known member
Effect of a glass plate

I have just completed a very rough draft of a new tutorial paper with the working title, "The Effect of Inserting a Glass Plate into the Optical Path Downstream from the Lens". It derives, by paraxial ray tracing, the expression for the amount of shift of the plane of image focus caused by the insertion of an erect flat glass plate into the path downstream of the lens, as has been discussed earlier in this thread.

The article has not yet been indexed on The Pumpkin (it is in fact not atually "released"), but it is available directly here:

http://doug.kerr.home.att.net/pumpkin/Glass_Plate-v01a.pdf

Please keep in mind that this is a very rough draft, done on a "stream of consciousness" basis, with all the algebra done "on the fly", and hasn't even had the benefit of The Red Fox's copy editing to unsplit my infintives and relax my tenses. I ask that you not circulate it further at this time. I hope to have it properly released soon, but the upcoming days are very busy, with a wedding in the family (my step-grandson), so I am a bit short of working time.

Best regards,

Doug
 

Doug Kerr

Well-known member
Tutorial article released

Earlier in this thread, I provided a direct link to the preliminary version of a new tutorial article describing how we can determine the shift in the focal plane resulting from the insertion of a glass plate into the optical path.

I have just relased to my technical information site, The Pumpkin, the released version of this article, now entitled, "The Effect of Inserting a Flat Glass Plate into the Optical Path Downstream from a Lens". It is available here:

http://doug.kerr.home.att.net/pumpkin/index.htm#Glass_Plate

I have pulled the preliminary article from the site.

Best regards,

Doug
 
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