Removing Fresnel from Toyo 45A

[jonathan_lipkin]

I would like to see what it's like to focus my Toyo 45 field (an A I think) without the fresnel. The gg and the fresnel are held together with clips, and this assembly is then screwed into the back with another set of clips. Will I affect the accuracy of focusing if I remove the fresnel and install the gg without it? I'm afraid I forgot the order they were assembled in (ie gg in front of fresnel or the other way around. I think it's fresnel then gg.

[Jack Dahlgren]

I would like to see what it's like to focus my Toyo 45 field (an A I think) without the fresnel. The gg and the fresnel are held together with clips, and this assembly is then screwed into the back with another set of clips. Will I affect the accuracy of focusing if I remove the fresnel and install the gg without it? I'm afraid I forgot the order they were assembled in (ie gg in front of fresnel or the other way around. I think it's fresnel then gg.

If the fresnel is in front of the gg then yes. If not, then no.

[jonathan_lipkin]

Grr, now I'm having trouble getting the camera back together. It appears that the fresnel goes in front of the gg, at least from the size of the flanges where they appear to have sat. The flange for the gg is about 5mm back from the flange where the film holder sits, and the film sits about 5mm back in the holder. Is it correct to assume that the gg should sit in the camera in the same plane as the film?

So, having put the fresnel in front (the words 'toyo field' are engraved into the fresnel, so I'm certain it's pointing in the right direction) and the gg behind it, I point at a distant object with the front standard against the infinity stops for the 150mm that I'm using. The object is out of focus. In order to get it in focus I have to rack the front standard out (farther from the gg) by about 3mm.

If I put the fresnel behind the gg, I get roughly the same error in focusing.



Any ideas what's going on? Is it possible I set the infinity stops incorrectly?

[DuncanD]

Mamiya America Corp. gave me the Toyo specs for camera back to ground glass as:

with Fresnel, 5.10mm. Fresnel is on the lens side of the ground glass, with the concentric ring etching away from the lens - sandwiched touching the ground glass.
with no Fresnel, 4.75mm

MAC also said that while these are the Toyo factory specs, actual dimensions vary considerably.

And they do. I put my micrometer on six or eight Toyo backs as they went through my hands (a complete 45G was $200 on theBay) and found the measurements varied within any single back by as much as .25mm from corner to corner. Then I selected a set of backs for myself which measured 4.70 to 4.85 on all four corners, gave away the Fresnel glass sets, and replaced them with fine, plain ground glass from Steve Hopf (http://hopfglass.com/).

The plain single sheet ground glass (i.e., without Fresnel) sits directly on the machined flats and is secured with the two retaining flanges, each with three screws.

The result was clear ground glass images in the same plane (+- .05mm) as the spec for my film, all of which runs in Grafmatics loaders as they are reputed to be the flatest method.

[jonathan_lipkin]

You're a lifesaver, Duncan. Many thanks.

[Policar]

Very interesting. Are you saying there's a .25mm variation between focal plane and film plane (plus or minus whatever is introduced by the film holder) with the 45a? That's petty egregious. Is there any way to test for poor tolerance and then any way to fix for them?

[Jack Dahlgren]

There are a few ways to check. One is to lay a flat bar against the face of the back and use a dial gauge to check the depth between that bar and the face of the glass. Another would be to remove glass and check distance from face to flange with a micrometer caliper. If you just want to check that the glass is parallel to the face of the back, you can place it on a level surface and set a steel ball on the glass, if it rolls you know that one side/corner is lower.

There are probably a bunch of other ways to check if you get creative about it.

[DuncanD]

Glad to help, Johnathan. I am no expert but common sense and my available workshop guided my methods.

Here is the procedure I used to check each of my Toyo 45 backs, which, by the way, are interchangeable between 45A field cameras and 45G monorails, both rotating and non-rotating:


Remove the Graflock back from the camera.
Remove ground glass, and Fresnel, if present, from the back by removing the two retaining clips, each held with three screws. Set the glass aside safely (not as I have done, carelessly breaking some!).
Place the back flat on a granite reference block (see, e.g., http://www.woodcraft.com/Product/2004864/7535/Granite-Surface-Plate.aspx) with the lens-facing side of the back down on the granite surface. The back will rest on four small bare metal ground flats - one at each corner. The two milled strips which support the ground glass will be facing up.
Now use a depth gauging micrometer to measure the distance (depth) from the granite surface to the bare metal milled surfaces which support the ground glass. Rest the body of your depth micrometer on each corner, in turn, (and more sample points, if you like) of the glass supporting surfaces. Record the depth measurements, being careful to maintain the micrometer perpendicular to the granite.
Record each measurement and hope that you have found no more than, say, 0.10mm variance from corner to corner. And yes, I did find over 0.25mm variance on some backs. Needless to say, I threw those back into theBay.
Reassemble the ground glass into any back which survives measurement. Remember that Fresnel (if any) is inside, closer to lens, with its groved side facing away from the lens.
Optinally, label the average measurement of each back with a small bit of masking tape somewhere on the outside. I did it on the 3/8" ledge which surrounds the glass.


With this procedure, I was able to identify four backs which each averaged between 4.70 and 4.85mm, with each being consistent within .05mm. That made a big difference in my ground glass focus being matched on the film.

[DuncanD]

I forgot to mention that, aside from the actual measurements ranging widely, the Toyo backs I obtained revealed no strong correlation between the presence, or absence, of Fresnel lens and the approximate average measurement. I found Fresnel in backs of 4.60mm and plain glass in backs of 5.00 and 5.30.

So apparently some folks are replacing the ground glass without understanding that, in Toyo backs, the Fresnel lens shifts the focus point away from the lens about .35mm (5.10mm Fresnel spec - 4.75mm plain gg spec).

[jonathan_lipkin]

Fascinating. I shot a test last night after re-assembling the back and it seemed pretty sharp edge. Shot a bunch of film today and hopefully it's in focus...

I'll get a micrometer shortly and test the back.