Tachihara 4x5 focusing screen

[Christian Olivet]

"The Velveteen View Screen is permanently shimmed as part of its design.The fresnel lens is located behind the diffusion layer so it does not affect focus it only acts as a collimiter of light, helps to brighten. It is of proper focal length which means it does not affect focus at all.You take out Your old gg and put in the Velveteen View Screen with the smaller glass towards the lens.The shimmed area sitting on the camera back mounting area. This screen is designed to be used without a loupe, just like a tv set if You get too close You defeat the design." This is the email I've got from Greg. Does it help?

[Alan Davenport]

"The Velveteen View Screen is permanently shimmed as part of its design. <SNIP!> The shimmed area sitting on the camera back mounting area."

That addresses most of my questions, it seems that they have tried to address the problem.

[kw]

After much discussing with Greg over email, I have finally ordered the screen for my Tachihara.

I'll inform you guys once I've tested it out on the camera.

[kw]

Finally tried the screen today and the following list my observation:

1) The image on the screen looks rather grainy, as compare to my original Tachihara GG and fresnel lens combination. It really takes some time to get use to the new way of focusing.

2) I compare both type of screen under low light condition (EV 5, at ISO 100)on my SA 90mm, f8. The result with the Velveteen screen is awesome! I can easily see all 4 corners of my composition. With the Tachi GG, only the centre of the image on the GG is viewable, all 4 corners cannot be seen.

3) I perform the same test on a Sironar-N 210mm, f5.6. The difference between the 2 is not so much. To certain extent, I actually prefer the Tachi GG. As it's far less grainy as the Velveteen, which is crucial when performing table top selective focusing shots.

4) As far as film registration, I don't find any problem so far.

[Robert A. Zeichner]

"The fresnel lens is located behind the diffusion layer so it does not affect focus it only acts as a collimiter of light, helps to brighten. It is of proper focal length which means it does not affect focus at all".

If, by behind the diffusion layer, the manufacturer means between the diffusion surface and the viewers eyes, then the Fresnel will not affect focus. If, by behind he means between the diffusion surface and the lens, it will affect focus. Let's just assume for a moment that the maker meant the first option. Wouldn't the proper position for the diffusion surface simply be against the mounting pads that support the standard ground glass? If so, why is there any need for shims? Shims would just move the focusing surface rearward, unless the diffusion surface were trimmed to the size of the smaller (inner) sheet of glass, in which case I suppose you would need to compensate for its thickness with shims of equal dimmension. If the the maker had the second option in mind, then focus would be shifted rearward by an additional amount roughly equal to one third of the thickenss of the Fresnel screen. I suppose that as long as both Fresnel and diffusion surface are hanging in front of the original mounting pads, that this could be compensated for as well by adding an additional thickness of shim material. And, of course some additional shim thickness would be needed in either case for the focus shift generated by the inner sheet of 1/16" glass.

Assuming all of the necesary shifts in position and focus have been accurately compensated for, why would one opt to trade brightness of the gg image for a grainy focusing surface that you can't get close to?