Critical focus : ground glass vs infinity

Paul Ozzello · Aug 2, 2022

I have a Horseman SW617 with several lenses and often use a ground glass back for focus with a 20x loupe. On all my lenses when I use the ground glass and focus on an object very far away (basically infinity) - all my lenses register just shy of infinity (and if I do set them at infinity the focus is a little off). I have a spare ground glass and the result is the same. Is this normal? Could it be that the focus plane is slightly different when focusing (at 5.6) compared to exposure (f22-f32) Should I reset the infinity stops on my lenses to match the ground glass? What should I rely on for focus? Is the focus plane the same between the ground glass and film back?

Confused..

Eff64 · Aug 5, 2022

It has been a long time since I knew anything about this, but in the years when all lenses were manual focus, the stop at infinity was not always right on.

Unless you have reason to think your ground glass is not aligned with the film plane, I would trust it.

Maybe set up a film test, just as you described, using your loupe, exposing, then changing the lens to the infinity stop and exposing. Then check your film.

Alex Varas · Aug 5, 2022

Hi Paul,

The ground glass should fit the film perfectly and you have infinity stop that one should be the one to trust unless you focus with ground glass.
If I remember correctly there were some lenses that it was advised to focus at f/ used but I might be wrong.
I would check the film back if it’s pressing correctly the film.
Two test with different film stock would help, let’s say Ektar 100 and Delta 100, back paper feels different thicknesses to me.
One shot at infinity stop, then focused by ground glass. Another shot (close focus) using external rangefinder using the distance lettering (metering for real the distance) and then againg using the ground glass.
Not only f/22 or f/32 but something such f/8 would help better to see if there is any discordance between the ground glass and the film back (less DOF).
The close focus will show clearly if the ground glass is before or after the film back.

Maybe you tried all this already and of course I can be wrong.

Cheers!

Sirius Glass · Aug 5, 2022

Move the infinity stop to agree with the infinity focus on the ground glass. Besides the infinity stop is easier to move and is moved around for each lens. The infinity stop in its present position is just not correctly placed.

Besk · Aug 5, 2022

I have read in the past that for wide angle lenses the manufacturers intentionally set up the lens to not quite reach infinity. I would leave it as it is.

brbo · Aug 5, 2022

Bob Eskridge said:
I have read in the past that for wide angle lenses the manufacturers intentionally set up the lens to not quite reach infinity. I would leave it as it is.

His lens goes past infinity.

Paul Ozzello · Aug 5, 2022

Eff64 said:
Maybe set up a film test, just as you described, using your loupe, exposing, then changing the lens to the infinity stop and exposing. Then check your film

Alex Varas said:
One shot at infinity stop, then focused by ground glass. Another shot (close focus) using external rangefinder using the distance lettering (metering for real the distance) and then againg using the ground glass.
Not only f/22 or f/32 but something such f/8 would help better to see if there is any discordance between the ground glass and the film back (less DOF).
The close focus will show clearly if the ground glass is before or after the film back.

Hi Alex ! I went out this afternoon and shot a test roll, 2 wide open (f5.6) set to infinity mark on lens and another focused on the gg, a third at 10m, and the 4th again focused on the ground glass but at 2 stops from fully stopped down (so f32) on a roll of Delta 100. I will check the results tonight.

Sirius Glass said:
Move the infinity stop to agree with the infinity focus on the ground glass. Besides the infinity stop is easier to move and is moved around for each lens. The infinity stop in its present position is just not correctly placed.

Yeah that's my initial reaction as well. But I'll test a few rolls before adjusting the lenses in case they are correct.

Bob Eskridge said:
I have read in the past that for wide angle lenses the manufacturers intentionally set up the lens to not quite reach infinity. I would leave it as it is.

I remember hearing something similar, do you remember why ? I wonder if the plane of focus moves between f5.6 (wide open for focus) and f32 (stopped down for exposure)

I checked a few of my other cameras and I'm noticing the same thing

My SW612 is also off by roughly the same amount, and also my SWC/M with ground glass - perfect focus is just shy of infinity.

gone · Aug 5, 2022

I had a Leica R 90 Summicron that couldn't get to infinity. In fact, it looked to be quite a bit off in the viewfinder. Being a Leica lens, I had no desire to tangle w/ disassembling that thing. So I went out and shot a test roll....by f5.6, everything was fine. That was 35mm though, LF may need a little more stopping down to resolve the focus error.

Dan Daniel · Aug 5, 2022

Two things-

Field curvature is real. Not certain about lenses that you are using. Here's a fun discussion of the issue and how it can play out in real life-

Field Curvature, A Tricky Problem in Photography

The above photo is taken at 28mm at f/11 on an ancient Minolta 28-85mm lens, the focus was set to about 2.5 meters and according to the depth of field

petapixel.com

Second, it could be that the film is not sitting at the same plane as the ground glass. At least once you get away from the edges along the rails. I've loaded film and then looked at the film from the front of a roll holder and seen a bit of bowing back from what would be the ground glass film plane. Minor, but worth considering. Maybe the ground glass should be ever so slightly bumped off the film rail plane? Like Scotch tape thin shim.

brbo · Aug 6, 2022

Paul Ozzello said:
Could it be that the focus plane is slightly different when focusing (at 5.6) compared to exposure (f22-f32) Should I reset the infinity stops on my lenses to match the ground glass?

Yes, there is a thing called focus shift. Plane of focus will drift backwards as you stop the lens down. This is most noticeable in fast under-corrected lenses. This might not be the problem with your lens, but if your lens was suffering from focus shift it would make your situation even worse, not better. So, yes, you should set the infinity stop on your lens according to what you see on the ground glass. I know nothing about the camera, but it would seem very odd for manufacturer to provide ground glass that is not placed in the correct film plane.

Dan Daniel said:
Second, it could be that the film is not sitting at the same plane as the ground glass. At least once you get away from the edges along the rails. I've loaded film and then looked at the film from the front of a roll holder and seen a bit of bowing back from what would be the ground glass film plane. Minor, but worth considering. Maybe the ground glass should be ever so slightly bumped off the film rail plane? Like Scotch tape thin shim.

Worth testing in reality. But if that's the case it would be better to check if something is wrong with film back not holding film in proper position than playing with the ground glass position. Landscape photography is typically not about getting just the centre of the picture in focus.

Crazy thought, is it possible that the ground glass was placed with the wrong orientation into the ground glass holder?

Alex Varas · Aug 6, 2022

It’s possible but unlikely in my opinion with your camera, I have seen this only in plate cameras using homemade sheet holders so reversing the ground glass would hit the current film plane.

Nitroplait · Aug 6, 2022

Film is not held flat like ground glass. It is likely moving inward in the center. If all your lenses agree, the manufacturer probably took this phenomenon into consideration.

brbo · Aug 6, 2022

Nitroplait said:
Film is not held flat like ground glass. It is likely moving inward in the center. If all your lenses agree, the manufacturer probably took this phenomenon into consideration.

If the manufacturer would take that into consideration wouldn't then ground glass be positioned accordingly? I mean, camera manufacturer knows film is not held properly flat (which is most probably not the case at all!) and then still positions ground glass in the plane that most of the film's surface WON'T be at. How is that taking this phenomena into consideration?

Nitroplait · Aug 6, 2022

brbo said:
If the manufacturer would take that in consideration wouldn't then ground glass be positioned accordingly? I mean, camera manufacturer knows film is not held properly flat (which is most probably not the case at all!) and then still positions ground glass in the plane that most of the film's surface WON'T be at. How is that taking this phenomena into consideration?

Lens infinity hard stop is taking film curvature into consideration- ground glass is not. You can’t have everything.

brbo · Aug 6, 2022

Nitroplait said:
Lens infinity hard stop is taking film curvature into consideration- ground glass is not. You can’t have everything.

Sure you can. Place the ground glass into proper position. That's how every camera is doing it. Or at least tries to do.

grat · Aug 6, 2022

Nitroplait said:
Lens infinity hard stop is taking film curvature into consideration- ground glass is not. You can’t have everything.

Then it's a terrible camera, because you're claiming it has two different focusing points depending on whether the ground glass or film is being used. That's not how cameras with ground glass work. Ground glass and film plane should be identical.

Now, the OP is using a very high power loupe to focus-- and I'm personally wondering if he's focusing on the smooth side of the ground glass, instead of the rough side (where the image is actually being projected). Could explain a persistent slight difference between the lens and the ground glass.

brbo · Aug 6, 2022

grat said:
Now, the OP is using a very high power loupe to focus-- and I'm personally wondering if he's focusing on the smooth side of the ground glass, instead of the rough side (where the image is actually being projected).

Now, that would be one terrible ground glass (one that formed two images)...

Nitroplait · Aug 6, 2022

brbo said:
Sure you can. Place the ground glass into proper position. That's how every camera is doing it. Or at least tries to do.

I have never seen ground glass that curves like 120 film.

Nitroplait · Aug 6, 2022

grat said:
Then it's a terrible camera, because you're claiming it has two different focusing points depending on whether the ground glass or film is being used. That's not how cameras with ground glass work. Ground glass and film plane should be identical.

I am not claiming that - read what I write.
And yes 120 film curves and if several lenses consequently focus slightly in front of the ground glass plane when set to infinity hard stop, I would trust the lenses, not the ground glass- at least until the opposite is proven.

grat · Aug 6, 2022

brbo said:
Now, that would be one terrible ground glass (one that formed two images)...

It's one image, but it's being refracted through the thickness of the glass, and then passed through another optical system (the loupe).

I have an adjustable loupe, and I can shift (slightly) the plane of sharp focus by adjusting the loupe while holding it against the ground glass. Most LF photographers will tell you for critical focus, make sure you're focusing on the grain.

grat · Aug 6, 2022

Nitroplait said:
I am not claiming that - read what I write.
And yes 120 film curves and if several lenses consequently focus slightly in front of the ground glass plane when set to infinity hard stop, I would trust the lenses, not the ground glass- at least until the opposite is proven.

But what you're describing, while theoretically wonderful, isn't how cameras with ground glasses work. The lens manufacturer would have to know what camera the lens was going onto, and exactly how much the film curves. A Graflex knob operated RH10 120 back has significantly more film curvature than a lever-operated Singer RH10 120 back. But both fit on the Mini Speed Graphic-- so which do you adjust the lens for?

A large-format camera (which is what the OP has, really) is even crazier-- sheet film doesn't curve, but 120 film can, so which do you design the lens for? No. The lens is designed to focus at a specific point, usually called the focal flange distance, and that does not take film into account-- it projects onto a film plane, and every camera I've heard of with a ground glass, the ground glass is set at that distance (well, the grain side is).

The OP should be able to look up the optical data for his lens, find the flange-focal-distance, or whatever Horseman calls it, and measure the distance from the flange of the lens to the ground glass, and determine whether infinity is set correctly-- but I have encountered multiple lenses that go slightly past the infinity mark.

maltfalc · Aug 6, 2022

try again, but focus on the moon.

RalphLambrecht · Aug 6, 2022

Paul Ozzello said:
I have a Horseman SW617 with several lenses and often use a ground glass back for focus with a 20x loupe. On all my lenses when I use the ground glass and focus on an object very far away (basically infinity) - all my lenses register just shy of infinity (and if I do set them at infinity the focus is a little off). I have a spare ground glass and the result is the same. Is this normal? Could it be that the focus plane is slightly different when focusing (at 5.6) compared to exposure (f22-f32) Should I reset the infinity stops on my lenses to match the ground glass? What should I rely on for focus? Is the focus plane the same between the ground glass and film back?

Confused..

Ideally, ground glass and film plane should be at the same Distance from the lens' Odal pointIf too far off, they must be calibrated. Resetting the infinity stops may be a first good attempt to do so.

nicelynice · Aug 6, 2022

Focus calibration issues are one of the most obnoxious things to diagnose using film, and one of the most disappointing things when they're off. Horseman SW617 and the film it uses are not cheap. If it were me, simply for peace of mind, I would simply have it checked and possibly calibrated by a technician familiar with the camera, like the poster above.

Sirius Glass · Aug 6, 2022

nicelynice said:
Focus calibration issues are one of the most obnoxious things to diagnose using film, and one of the most disappointing things when they're off. Horseman SW617 and the film it uses are not cheap. If it were me, simply for peace of mind, I would simply have it checked and possibly calibrated by a technician familiar with the camera, like the poster above.

Bingo! We have a winner!

Critical focus : ground glass vs infinity

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