Strange fruit : focus problems on Norma

[Bob Salomon]

"The fresnel rests unattached on the bottom of the Norma frame (ie, the lens side) "
Put the Fresnel on your side of the GG not the lens side.

Are you using a Sinar Fresnel or a do it yourself one?

[Sevo]

On the contrary, I'd think that all those backs and all those lenses exhibiting the same problem would indicate that it has to be the camera.

You never had a Sinar, I assume? On Sinars, you swap the entire back frame assembly in its (lens board sized) standard attachment - replacing everything that could possibly cause a focusing difference between the GG and film holders. There is a small chance that could be the holder, but he stated he had used several of them as well.

[Axel]

@ Bob
The three fresnels and GGs I have are all standard Sinar parts : nothing DiY or custom.

The back with "built-in" fresnel is standard Sinar (never opened or touched by anyone) and the one I've been using since 1990 ; if there had been a problem with it, I should have had this focus problem since 1990... but didn't.
On either backs, with or without fresnel, the GG does rest on exactly the same spots so with or without front- or back-fresnel, the focus point is exactly the same.

[Struan Gray]

Your symptoms are exactly right for a situation where the optical thickness of the Fresnel is not being compensated for.

I have an original Norma Fresnel for my 5x7 back which goes between the ground glass and the lens. The manual indicates that the 4x5 one is mounted the same way. Crucially, there are small wings at the edges which go between the ground glass and the milled reference surfaces on the metal frame. This moves the ground glass backwards (by about one third of the total physical thickness of the Fresnel) and compensates for the way the Fresnel shifts the plane of focus.

What worries me is that you said the Fresnel on your Norma was removable. That implies that when you put it in place there is no compensation of the ground glass position. That will lead to exactly the effect you have found: the ground glass is optically too far forward, so when you focus on it the film ends up too far back, and thus is focussed on something closer.

As to why things worked before, I don't know. My 5x7 Fresnel is brittle with age, and the tabs are thin and friable. Something may have snapped, or was already detached and simply finally worked loose from under the clip that holds the ground glass in place, allowing the ground glass to shift from where it has been since you bought the camera.

If the Fresnel really is easily removed, try focussing with it in place, and then remove it. You should see the same shift in the focus point.

The solution is to make sure any bits of old Fresnel tab are removed so that the ground glass locates directly onto the reference surfaces, and then to use one of the Fresnels from your other backs on the outside - i.e. between the ground glass and your eyes.

[Sevo]

What worries me is that you said the Fresnel on your Norma was removable. That implies that when you put it in place there is no compensation of the ground glass position.

He was talking of three backs - a early one (fifties vintage) with a built-in lens side fresnel (presumably distance compensated at the factory). The other two will be regular Sinar style, where a snap-in viewer side fresnel can be attached for viewing (but is not used for focusing).

[Struan Gray]

I could be wrong, but

The back with "built-in" fresnel is standard Sinar (never opened or touched by anyone) and the one I've been using since 1990

Yet the posted image shows a back with no ground glass or Fresnel. And Axel's postings imply that he has not yet realised that a Fresnel between the ground glass and the lens *requires* an adjustment of the ground glass position, especially with wide angles.

On either backs, with or without fresnel, the GG does rest on exactly the same spots so with or without front- or back-fresnel, the focus point is exactly the same.

This is not right.

[Doremus Scudder]

Axel,

It is more than likely not your eyes. Even with presbyopia (age-related nearsightedness caused by the eye's lens losing its flexibility), if something looks sharp, then it's sharp. Presbyopia does NOT change where you perceive sharp focus, it just keeps you from focusing closely with your naked eye and needs to be compensated for using glasses or, in your case, a loupe.

The focus problem you relate means that the film plane and the ground-glass plane are not registered correctly. If Fresnel placement is not the issue (do test this to eliminate or confirm it) and your loupe is working well for you on other cameras (i.e., is in focus for your eyes, but test this too to eliminate it), then there are really no other choices.

If the actual focus is closer to the camera than where you focus on the ground glass, then the film is positioned slightly in front of (i.e., closer to the lens) than the ground glass. The reverse if the actual focus is farther from where you focus on the ground glass.

Time to examine all components carefully to see what the problem is. I'm not familiar with the Sinar system, so I'm just brainstorming here: Look for anything that would prevent the filmholders/roll-film backs from seating correctly. Look for anything that may have caused the ground glass to be out of position.

... The back with "built-in" fresnel is standard Sinar (never opened or touched by anyone) and the one I've been using since 1990 ; if there had been a problem with it, I should have had this focus problem since 1990... but didn't.
On either backs, with or without fresnel, the GG does rest on exactly the same spots so with or without front- or back-fresnel, the focus point is exactly the same.

There is a real problem with the above: If a Fresnel screen is placed between lens and ground glass, there WILL be a focus shift compared to using ground glass alone. For cameras designed to have the Fresnel so positioned, this focus shift has been compensated for. Try your Fresnel back with and without the Fresnel screen, as Struan suggests. In any case, If you have identical Norma backs, with the ground glass positioned in exactly the same place, but one has a Fresnel screen on the lens side, then the focus should NOT be the same between the two. One should exhibit the focus shift caused by the Fresnel.

Edit: The more I read Struan's post above, the more I think he is right on.

Good luck finding your problem,

Doremus

[Axel]

I've added three more images of the back that I actually use :
split up
viewing side
lens side
This a full-factory back : never changed anything.

The other is the same, except it has no fresnel on the lens side (but the GG rests on the same surface so I could add a back fresnel if I wanted to) ; with this one I use the Sinar snapon front fresnel for clearer viewing when using a wide-angle lens.

Again, whichever back I use, the focus point remains exactly the same to my eyes.

If you look at the DaYi sample, focus isn't off by a minimal margin due to a slight thing somewhere but completely out in the woods : zoom in on the image's center.

Other tests done with the Horseman show the same : to focus that much off limits, I'd have to be drunk and high :)

Or in need of glasses ?

[Axel]

Doremus,

Just tried (again) now with a 300, a tiny little delicate thing far away to focus on and swap backs : absolute same.
On the main back I see the added fresnel concentric circles, of course, on the other just the GG's grain, of course again, but the focus point doesn't change between the two.

If there were any focus changes which my eyes weren't able to detect (anymore), it would be of the tiniest kind, something stopping at f22 would entirely cover.

But the samples uploaded show enormous differences : something like 5cm at a 60cm distance or 10m at a 40m distance ! If my eyes were that bad, I couldn't even use my iPhone !
There must be something else - but what ?

[Sevo]

Doremus,

Just tried (again) now with a 300, a tiny little delicate thing far away to focus on and swap backs : absolute same.
On the main back I see the added fresnel concentric circles, of course, on the other just the GG's grain, of course again, but the focus point doesn't change between the two.

If there were any focus changes which my eyes weren't able to detect (anymore), it would be of the tiniest kind, something stopping at f22 would entirely cover.

But the samples uploaded show enormous differences : something like 5cm at a 60cm distance or 10m at a 40m distance ! If my eyes were that bad, I couldn't even use my iPhone !
There must be something else - but what ?

1. Forget the main back for a while - fresnels may have their own issues regarding focusing, you should work this out with a bare ground glass.

2. (Bright) ground glass focusing often is partially diffuse, partially off an aerial image. If presbyopia moves the ground glass out of your sharp viewing range, you will lose the visual anchor on the diffuse image and ground glass texture, and may start to focus on the floating aerial image somewhere to the lens side of the ground glass. Aerial images have no depth of field, regardless of aperture, so stopping down does not improve focusing in that particular case.

3. The described focus errors are not enormous, at least for the images you have shown so far - for a 55mm lens, the extension is around 5mm at 60cm distance, so a 5cm focus shift would be caused by a mere 0.5mm register error at the back.