Position of ground glass on Toyo 45A

[ganesh_ramachandran]

Hi, I have a toyo 45A - while cleaning the Ground Glass (GG) recently I found that the Fresnel screen (FS) faces

the Lens. After reading up a lot on this I am now convinced of the idea that the GG should be in front of the lens

and the FS at the back. After removing the back I measured, using a vernier calipher, the distance from the back

to that of a loaded fidelity film holder and it was 0.7mm less of that of when I measured with the GG only in place.

The GG edge sits on a raised ridge and if I have to make the corresponding changes I would have to machine off

0.7 off the ridge on which the GG edge rests on. Originally, the FS stays flush with the GG by means of a narrow

folded metal holder which holds it together and the FS projects out towards the lens. So my question is- If I

machine off 0.7mm off the ridge and put the GG there and put the FS at the back, i.e. towards the eye, would I

achieve what I wanted. I have to say here that I shoot mostly at f/22 and more and I haven’t so far found any

focusing problem in the results as such but by doing this I feel it will improve the focusing. Will I be doing anything

wrong- sorry for this long post- any ideas welcome so that I want to get the best out of the system both in

focusing acuity and accuracy. Thanks a lot in advance for any of your ideas which will help in my problem

[emil_ems5]

Ganesh!

 

Why would you like to go through all this trouble? I have a Toyo 45A that I am perfectly happy with, groundglass and all. I would not advise trying

to remachine the groundglass holder. The factory (being Japanese) has done a nice job, keeping, I trust, all specifications within calibrated

standards. Why risk that through remachining? I have never found the groundglass solution chosen by Toyo to be to the detriment of accurate

focusing, not even when using extreme lenses, such as, the Rodenstock 65 mm lens.

 

As I understand it, the suggestion to put the Fresnel screen on the back of the groundglass stems from the need to add such a screen to a

camera that comes without it. By placing it behind the groundglass, one avoids the need to remachine the holder. But in your case, the holder is

perfectly adapted to the solution with the fresnel facing the lense!!!!

[User_503771]

First off, have you run a focus test with the glass and fresnel in their current configuration? If you come up with good focus at a few different distance settings, then you're all right.

 

These ground glass holders have been manufactured by various companies in different ways over the years, according to what kind of glass/fresnel configuration was going to be used. It always kind of irritates me when it comes from the factory with the fresnel facing the lens because what if I have to replace the glass and the fresnel? But then, it's highly unlikely that the fresnel would ever need replacing, and if the glass needs replacing, it would thus sit at the correct distance.

 

Have you measured the thickness of the fresnel?

 

Also, bear in mind these figures: The ANSI standard for this distance on the film holder is 0.197", with a margin for error of +/- 0.007". Typically, ground glass panels are manufactured to hold the focusing surface of the glass at 0.190", to compensate for the thickness of a "standard" sheet of film. But if you're within a few thousandths of an inch of that figure at the ground glass focusing surface, you don't need to worry. Just put the whole thing back together the way it was.

 

When measuring this stuff, do be sure to put the panel (without glass) on a very flat surface, and measure from the points where the ground glass would sit, down to that flat surface, because sometimes the bottom of the panel is machined so that only a few points rest against the camera. You can also take the opportunity to check to make sure that the bottom of the panel is truly flat -- does it "rock" back and forth from one corner to another? If it rocks on two corners to any pronounced degree, then you would have a good reason to do some machining work on it.

 

But, really, if the thing works well -- if your focus is good and accurate shooting at moderate apertures and various distances, why fix it if it ain't broke?

 

Good luck, and let us know what happens!

[michael_briggs2]

As the others are saying, "If it ain't broke, don't fix it". In more standard English, if your camera focuses correctly, why work on the focusing? My preference is to have the Fresnel screen on the photographer's side of the ground glass, because that allows the photographer to remove the Fresnel screen if the photographer doesn't like it without requiring any adjustments in focus. It is also simple to figure out the register of the ground glass for this configuration: the ground surface of the ground glass should be at the same position as the film. Since the depth of a 4x5 film holder is 0.197 +/- 0.007 inches (5.00 mm), the ground glass should be a few thousands of a inch closer than this to allow for the thickness of the film. But the other configuration, with the Fresnel in front of the ground glass can be made to work and many camera manufacturers have done so. In this configuartion the register of the ground glass needs an offset which is different from the thickness of the Fresnel lens, so you can't simply remove the Fresnel lens if your camera has this configuration.

 

It isn't clear why you are convinced that the ground glass should be closer to the lens than the Fresnel lens. So my advice is not to bother changing your camera. But if you decide to go ahead, if you only have air in between the ground glass and the taking lens, with the ground surface of the ground glass facing the taking lens, then you want the ground surface to be at the same position as the film. For your approach, the simplest way would be to measure a film holder with a sheet of film in place, the figure the change to place the glass at the same position. But again, why do this work if your camera is focusing correctly?

[ganesh_ramachandran]

Thanks to all of you for all your response.

 

Emil- i agree with your post: it is a perfectly machined Japan made camera and i hate to do some alterations like

this but my i was tempted to do it only on seeing the FS on the inside and more so when i found the distance

between one reference point of the back to the GG register vary with that of a loaded film holder! I like the visual

satisfaction of seeing my film sitting precisely at the place where the GG is positioned. I understand that the

company have done this arragement taking into account the FS's nature and ability to bring about the change in

position of the focus.

 

And Michael-the panel does sit flat and everything is made to fit properly with projecting machined ridges mating

with the opposite faces accurately and i have measured it also and it was just that i was unable to come to accept

that the focusing GG register and the offset has been arrived at taking into account with that of FS in place and

also seeing that the FS is not a 'ground' piece of glass and only a plastic sheet.

I see all of your point of view of not tampering with a perfectly working setup and have decided to put things back

again the same way and get back to taking pictures.

 

Thanks a lot all your response which has helped in convincing me to do the right thing- I have to say that this is

one great forum with a lot of knowledgeable people and always willing to offer and share ideas.