Bronica Focusing Screen Diagonal Prism vs Ground Glass (Special Situation)

[Fragomeni]

Hi all. I work with a lot of homebrew and experimental optics as many people here already know. I'm working on a project which makes use of my wonderful Bronica S2A with it's tilt shift bellows. Anyway, with the optic I'm using I'm noticing a discrepancy between the focus on the diagonal prism vs the surrounding ground glass. When I focus using the prism the area on the ground glass closest to the diagonal prism doesn't actually look focused. When the area around the diagonal prism looks focused the two parts of the image in the diagonal prism don't line up as they should if they were focused. Basically I don't know which to believe, the ground glass or the prism. This is a unique situation and seems to be due to the optic I'm using, it is not a standard problem with the camera and any other lens. I know the best way to figure it out is to shoot two rolls, focusing with the ground glass for one and the diagonal prism for the other, and compare the two. Just looking for any insight into this. I know some people here work with some hombrew optics and I'm wondering if anyone can offer me any insight in the meantime? Thanks!

Also, is there a ground glass focusing screen for the Bronica S2A that is purely ground glass with no focusing prism in the center of the glass?

 

[Trond]

The ground glass in the S2A doesn't have split image focusing, so it sounds to me that you have a third party screen installed. Perhaps a Kiev screen? The screen isn't user changeable, and Bronica didn't offer alternative screens for this model.

I tried the Kiev screen myself once, but I changed back to the original screen.

Trond

 

[Fragomeni]

Yes, it was a third party screen. It works very well when the camera is used with more conventional lenses but it isn't the best for some of my homemade optics which produce extremely shallow DOF. Being right in the center often causes a distraction and difficulty focusing. Many of my homebrew lenses also have fall off from the center so being able to focus in the center of the ground glass is critical since that area offers the sharpest focus. I took the camera out yesterday after I cut the new ground glass and its much easier to work with now that it doesn't have the micro prism in the center. Like I said, the ground glass I used is older and slightly used so I'm sure the image would be even better with a new lean ground glass. So I'm still looking for the original matte ground glass or a equivalent replacement that will fit.

 

[Trond]

Do you still have the fresnel lens installed?

Trond

 

[Fragomeni]

Yes. The fresnel sits atop the first set of spacers/springs and the glass sits atop the second. From what I understand, this along with the downward pressure of the retaining plate is what maintains the proper placement. I've checked it and its spot on.