Shim thickness to replace fresnel

[kw]

On my Tachihara 4x5, the fresnel thickness is 2mm. I'm going to use another GG without the fresnel. So, would like to seek advice on the thickness of shim to be added on the mounting pad. Should it be 2mm plus 1/3 the thickness, resulting in total of 2.667mm or just 1/3 of the total 2mm i.e. 0.667mm.

The Tachihara is designed with the fresnel on the lens side. The purpose of using just GG without fresnel is to enable me to focus better without seeing the fresnel grain.

[Jon Shiu]

Hi, maybe my tachihara is different, but the ground glass side on my tachihara faces the lens, and the back side of the glass is a fresnel. So, maybe check your camera again. Just take the back off and look at the inside. It should be a "ground" surface with no ridges.

[Gem Singer]

Hi KW,

I checked the viewing screens on my 8X10, 5X7, and 4X5 Tachihara backs, and all of them have the ground surface facing the lens, where the film plane will be located after a film holder is inserted. The fresnel and the cover glass are on the outer surface. Therefore, no need for shims as long as the replacement groundglass also has it's ground surface inside, facing the lens.

By the way, how strong is the magnification of your focusing loupe? The fresnel pattern has never been a problem for me, and I'm using a 6X Silvestri loupe.

[Michael S. Briggs]

A Fresnel lens in the converging beam, e.g., in the beam in front of the ground surface of the ground glass, has the effect of shifting the focus farther from the lens by about 1/3 of the thickness of the lens. The exact amount depends on the refractive index of Fresnel lens. So, if you remove the Fresnel lens from a configuration like this (assumed to be correct with the Fresnel in place), you want to move the ground glass closer to the lens by about 1/3 of the thickness of the Fresnel. If the Fresnel was resting against the reference surface of the camera, you should replace it with a shim on the edges of the ground glass of about 2/3 of the thickness of the Fresnel lens that was removed.





Obviously, you should carefully test the focus accuracy after doing this. First, it is always a good idea when changing a focusing screen. Secondly, since your camera has the Fresnel in front of the lens, and other users are reporting differently for their Tachihara cameras, it is possible that your camera is currently misconfigured. If used, perhaps a former owner misassembled it.





After working out the equation, I found a webpage with a good diagram of the optical effect: http://astro.martianbachelor.com/CB245/OWFS.html. Only the first sentence and the figure are useful for this question -- most of the webpage is about the spherical abberation that is created for a fast lens.

[kw]

Thanks very much for the reply so far. I would like to clarify the fresnel and cover glass setup on my Tachihara.

Looking from the lens side, I can see the grounded surface of the fresnel. The ridge side of the fresnel is facing the cover glass. So the cover glass is actually resting on the fresnel, which in turn is resting on the mounting surface of the camera back.

Therefore, the shim should be 2/3 the thickness of the fresnel, since the effective focus is drifted 1/3 the thickness of the fresnel away from the grounded surface.

[Gary Frost]

Per your description of the GG surface position, which is the same as my tachihara and all the others who have posted, you will not need to shim the GG at all. The GG surface is resting on the proper position now as the fresnel is not between the GG surface and the lens. Simply remove the existing cover glass and GG/fresnel plastic, and replace these with the new GG only, making sure the GG surface rests on the same place as the current GG. If you want to verify focus is correct, setup the camera and carefully focus with the existing GG/Fresnel. Leave the camera set up and remove the back, replace the GG and put the back on the camera. The focus should not have shifted.

[Robert A. Zeichner]

I'm used to making this kind of adjustment by measuring stuff in thousandths of an inch. The last Graphic focusing screen I did required about .040" of shim for roughly a .060" Fresnel. So, 2/3 of the Fresnel thickness is nominally correct. I always run film tests using a system I described in the article I wrote for ViewCamera back in 1996. The results indicated some fine tuning was in order and after making those tweaks, I was able to get the gg to pass the test perfectly.

[kw]

I've run a test base on Gary's advice using a 210mm (@f5.6), focusing on a bar code on a typical table top setup. It seems like there is no drift in focusing difference, as far as I can see with the help of a 4x loupe.

What I did was simply remove the entire original cover glass and fresnel of the Tachihara and replace a satinsnow ground glass. I will run the same test on infinity focus with a 90mm and update you guys.

Now, I have another question. The original fresnel of the Tachihara has one side of its surface grounded and facing the lens side. Am I right to say that this is the focusing plane? If so, what about the fact the fresnel lens has an effect of converging the image 1/3 its thickness?

[Gem Singer]

Tachi,

Since you can see the ground surface of the viewing screen from the lens side, it is in the proper position and has not been misconfigured.

Essentially, the configuration of the Tachihara viewing screen is the same as it would be if a removable-style plastic fresnel lens was placed against the back (outside) of the groundglass. The thin cover glass is merely there to prevent damaging the grooves on the surface of the the soft plastic fresnel

There is no need to use shims when replacing the viewing screen, as long as the ground surface of the new glass is in the same position, facing the same direction, as the ground surface of the previous viewing screen.

The image that is focused on the ground glass is on the grounded (rough) surface. A fresnel that is positioned against the opposite surface of the glass will not effect the position of the ground surface of the glass. Focus will not shift.

Geez, this is difficult to explain before the first cup of coffee in the morning!

[Gem Singer]

Hi, again Tachi,

Evidentally, you posted at the same time that I did. The 1/3 thickness shift that you mention will only take place if the fresnel is placed on the inside, between the lens and the ground surface of the groundglass.

Please let us know if the Satinsnow groundglass makes a difference in the brightness and ease of focusing. It's certainly a lot less expensive than the other types of bright screens. (Now, for that first cup of coffee this morning).

[kw]

Hi Eugene,

Thanks for the clarification! I'll perform the infinity focus on my 90mm with both ground glass and will inform you on the results.

As for focusing on the satinsnow ground glass, its far better than the original Tachihara cover glass and fresnel lens combination. The brightness is about the same, although its just a piece of ground glass, but more important is its ability of achieving a tack sharp image without the fresnel lines! That was my initial discovery with the 210mm lens. I'll update you , once I've tried out on my 90mm.

Besides that, the glass is very well make, even if I compare it with velveteen screen, which I own one. I think Dave at satinsnow has done an excellent job! Just finished my first cup of coffee...

[kw]

I've just finished running the tests with both 90mm and 210mm, focusing at infinity. Interchanging the original Tachihara cover glass/fresnel combination and the satinsnow ground glass in both cases.

The final conclusion, is that the focusing is right on in both cases, therefore no shim is required. For Tachihara users, you may like to check out the satinsnow ground glass, I have no regrets changing to it, for its price and performance.

Thank you for the advices given...