Mamiya c3 Focusing Screen & Ground Glass

[wclavey]

I have, among my Mamiya TLRs, a c3. I wanted to add a clear glassine grid to it as I have on my 4x5 to help me with composition. I removed the housing on the focusing screen and found that it has both a fresnel lens and a ground glass screen. BTW, my c220s only have the ground glass.

What is interesting is that the fresnel lens is in the light path between the viewing lens and the ground glass, whereas the fresnel is outside the ground glass on my 4x5. On my 4x5, the glassine grid is between the ground glass and the fresnel, but the fresnel is abousolutely outside the ground glass.

The screen arrangement on the c3 is held in place with some dried glue in each of the corners, so my assumption would be that this is the arrangement that came from the factory, but it seems contrary to everything I understood about the way fresnel lenses worked and the position of the image forming surface. In addition, even though the c3 has the fresnel lense, my experience with is is that it has been harder to focus than the c220s, which I would now say was counter-intuitive.

Can anyone offer some clarity on this? Should I just put the clear grid between the 2 lenses and put it back the way it was? I would be afraid that switching the two screens (putting the ground glass inside the fresnel, as I would expect it to be) would now place the ground glass too close to the viewing lens and alter the focus point, so I do not want to do that.

On the other hand, if this fresnel lens was an "after market" modification, I would have no qualms about rigging some type of clip and putting it outside the ground glass on top of the focusing screen housing so that the focusing screen is still in the right place.

Anyway, I'd like any insight that anyone could offer. Thanks.

 

[Dave Parker]

Actually most, but not all 4x5's the fresnel is in fact mounted first before the ground glass, there are however a few that were mounted on the outside and especially if it was an aftermarket addition, but most of the time, the mounting is fresnel first, then ground glass..

Dave

PS, what 4x5 do you have?

 

[wclavey]

Dave:

I have a Crown Graphic. You made the replacement ground glass for it, but I reassembled it the way it was when I received it, with the ground glass in the light path first, then the fresnel. My understanding was that the image-forming surface of the ground glass was the first thing that the light needed to hit. Of course, it looks like I was wrong... I should check my son's and see what his has.

Thanks.

 

[Dave Parker]

Hi Wes,

If the crown was made from the factory with the fresnel, it would mount in front of the ground glass, however many of them were equipped after market in which the fresnel would be behind the ground glass, you can normally tell by looking at the bosses the screens set on, I have both non and pro fresnel crowns in my collection, One thing for sure is there don't seem to be a standard in the business...Are your images out of your Crown Sharp? If so, then no need to change things around..

Dave

 

[grahamp]

Put the grid on top of the C3 screen. Unless you had focus issues before, it is safe to assume that the installation has been correctly shimmed. If you reposition the fresnel and ground glass that will no longer be true.

The C3 may need the mirror cleaned, and I wouldn't be surprised to find some dirt between the fresenel/screen sandwich. Unless the camera is unusable I wouldn't split it myself.

 

[kb244]

Just so you know, the focus screen on the C3 was never meant to be interchanged, though I'm sure you could, just make sure any replacement is of the same thickness as the original. If you want a grid, it cant hurt to print a transparency and place it on top. A fresnel should be the last thing the light hits, you want to make sure that the ground glass is the first thing the light hits, so do not put the fresnel under the ground glass.

So if we have the camera standing up, on the bottom should be the ground glass with the frosted side facing down, just above that would be the fresnel with the ridges facing down, then just above that a transparency with the grid.

I have a Mamiya C3 Pro at home, and I don't think it ever needed anything other than maybe a grid to make it look nicer. Also if I am not mistaken it was just a single groundglass peice, I don't remeber seeing anything in layers on mine which I know is the way it came from the factory. In my opinion you may have an after-market modification.

 

[Dave Parker]

Karl,

I have to disagree a bit here, I have made screens for many of these cameras and many MF cameras, there is a misconception going on here about fresnels, there are actually many cameras made that the Fresnel goes first in the light path....the bronica has the fresnel first, as do many other models of MF format cameras as well as large format cameras..I actually have a C330 screen in my hands as I type this and it don't even have a "Ground Glass" is it a fresnel with a frosted side and you can still see the concentric rings in the fresnel...But you are right the C3 was not designed with an interchangable screen, the easiest way to add grid to any camera that does not have it is to get overhead transparancy material and print then cut and mount on the outside of the screen, which is what I have been doing for years now on everything up to 8x10

Dave

 

[kb244]

Karl,

I have to disagree a bit here, I have made screens for many of these cameras and many MF cameras, there is a misconception going on here about fresnels, there are actually many cameras made that the Fresnel goes first in the light path....the bronica has the fresnel first, as do many other models of MF format cameras as well as large format cameras..I actually have a C330 screen in my hands as I type this and it don't even have a "Ground Glass" is it a fresnel with a frosted side and you can still see the concentric rings in the fresnel...But you are right the C3 was not designed with an interchangable screen, the easiest way to add grid to any camera that does not have it is to get overhead transparancy material and print then cut and mount on the outside of the screen, which is what I have been doing for years now on everything up to 8x10

Dave

I was speaking in respect to those cameras that did not come with one out of the factory, and I have a C3 at home, so I was also speaking from experience. I originally looked at a C330 cuz could interchange to have a split/micro prism in the middle, but the C330 body felt too cheap to me compared to either the C3 or C33 I have. What I was saying was strictly in regards to the C3. Because if it did not come with a fresnel screen to start with, then putting anything in front of the groundglass or equivilent will only cause focus inaccuracies with the film.

 

[grahamp]

All the Mamiya TLR finders (at least those made for the C33 and later) have a recess in the base. The only thing Mamiya made for it was the modified scale plate for the 55mm lens, but it works as a place to put a grid screen. Since it is not in contact with the screen there is a small parallax effect, but nothing serious for a grid. It's a good way to try out various effects.

I like a grid with the centre open for the focus spot on my C330. Hence this item: Dead Link Removed