Mamiya RZ 67 Screens for Large Format Groundglass?

[zarrir_junior]

<p>I have a couple of 8x10 view cameras with very dim groundglasses and i have been offered a lot of 10 Mamiya (very bright) medium format focusing screens. The lot is pretty cheaply priced. My question is, glued together, would they make a bright ground glass replacement for a large format camera?</p>

[rodeo_joe1]

<p>Err..... NO!</p>

<p>A fresnel viewfinder lens has to be pretty much co-axial with the camera lens, so the screens at the edge of the frame will almost certainly be partly blacked out. Of course that situation will change if you shift the lens at all, but there's always going to be parts of your proposed "fly's eye" focusing screen that are dark.</p>

<p>No offence meant but this has got to be the most off-the-wall suggestion I've yet seen on this forum. Whatever you're on I'll take some Zarrir!</p>

[zarrir_junior]

<p>Joe, i have re-read my post entirely and found no suggestions there. I only found a QUESTION which seems pretty much reasonable. What makes you think that the screens would be joined together irregularly, not making a perfectly plain board? Maybe you are not familiar with home made works or newest adhesion products? Maybe you think Mamiya produced defective or bowed screens for professional photographers? Do you have a faint idea on how bright these screens are? Have you ever seen one?<br>

Ok, i will take the same product you are on and let us both wait for another contributor to this thread.</p>

[indrek_r_nkla]

<p>IF the Mamiya ground glasses do not contain a fresnel lens, this could be theoretically possible. Practically I would doubt the sense of it. If You use an additional glass to hold the pieces together you will produce about 4 extra surfaces (2 on glass and 2 on glue) of reflection behind Your ground glass. If You claim that there is an adhesive that holds the glass by edge then I have no arguments.<br>

But if the fresnel lens is integrated in the Mamiya gg, then You will have more trouble than You expect as Joe stated above. The issue of fresnel lens centers for first and that of their focal distance for second. As the 6x7cm gg fresnel would be optimised for lenses far shorter than those meant for 8x10'' the effect of the lens would not be evening the illumination but <em>au contraire</em> . Then instead of an image on gg You would end up with a collage.</p>

[darkroommike]

<p>You can probably sell the focusing screens and buy a fresnel for your 8x10.</p>

[zarrir_junior]

<p>Thanks guys! Any possible online source for <strong>affordable </strong>8x10 fresnels?</p>

[bob_salomon]

<p>For which camera?</p>

[rodeo_joe1]

<p>Jeez! Remind me never to try and inject a little humour into this forum again. Or to give a perfectly reasonable answer to someone who's obviously determined to ignore one.</p>

[bob_salomon]

<p>This would really be totally impractical. First there is a significant issue of getting all those screens flat. Then there is the problem of what to do with the join lines. Then there is the issue of the screen itself. A Mamiya 6x7, since it has a revolving back, has a calibrated screen area with format markings or a grid of 7x7cm + there is a boarder around that 7x7cm area that isn't designed for focusing or composing. Then are the corners of the 67 screen clipped or 90°? If clipped when you place them together you have a square hole everywhere the screens butt together.<br>

Then you have the problem of centering the fresnel on 8x10". Each of those 67 screens, if they do have a fresnel, have a circle centered on the 7x7cm image capture area. Butt a bunch together and you will have an image with multiple bright circles. Lastly a fresnel screen is a lens and has a focal length. The fresnel used in a 6x7cm camera has a focal length that will be compatible with most lenses used on 67. The fresnel for an 810 camera has a totally different focal length as the normal range of focal lengths used for 8x10 is much longer then those used with 6x7cm. So at some focal lengths the fresnels would black out and at others be unusable.<br>

Best advice? Buy a fresnel that fits your camera. Since there is no standard size for an 8x10" camera's fresnel or ground glass one size will not fit all. So before you waste money on any fresnel/gg make sure it will fit. One that is too large can be ground down - each edge equally so the fresnel is optically centered on the gg. But you can't make one that is too short or too narrow or both fit.</p>

<p> </p>

[vijay_nebhrajani]

<p>Hate to be a fly in the ointment here, but it is a misconception that the focal length of the Fresnel has anything to do with the focal length of lenses used or the format provided that the Fresnel sits between the loupe and the ground glass.</p>

<p>The simple point is that the ground glass forms a diffuse image - sort of like putting a Fresnel on top of a print - and just as the focal length of the lens used to make that print has nothing to do with the focal length of the Fresnel, the focal length of the taking lens has little to do with the focal length of the Fresnel.</p>

<p>Thus, Sinar for instance has only one Fresnel screen for each format - not one per lens focal length.</p>

<p>Also, centering the Fresnel has nothing to do with anything. You can shift and raise/drop your front standard all you like; you don't have to move the Fresnel to center it with the taking lens.</p>

<p>I do agree that there are several other reasons why making an 8x10 bright screen by joining several Mamiya screens is impractical, but these two aren't among them.</p>

 

[bob_salomon]

<p>What you are missing is joining several small screens together to make one big one. That results in multiple centers over one big area, won't work.</p>

<p>And yes, fresnel focal lengths can matter. With extreme wides the wrong fresnel make it very difficult to focus, with very long lenses the wrong fresnel may make the image black out depending on your eye placement. Yes a single fresnel can be used off center, in fact that is how the fresnel for the Linhof M679 system is made. You shift it up and down for maximum brightness as you change the camera movements. But the OP is not asking for that reason. His idea just won't work.</p>

[zarrir_junior]

<p>Thank you Bob, i gave up. To which point grinding my own groundglass with aluminum oxide (there is a nice HOW TO on the NET), will produce a good , bright usable product?</p>