Check Your Ground Glass ~ Film Holders for Registration! Simple, Important and Fast

[Jim Galli]

This afternoon I put together a 5X7 back out of junk parts that were laying around. It's for an old Kodak 2D that found it's way to my house. It had a nice 4X5 back but no 5X7. So I put one together from Stuff that was in the junk piles.

After dinner I thought, I'd better check this assembled gobble-d-gook back and make sure things are lined up. Takes about 5 minutes, and it cracks me up that you guys will spend a gazillion dollars for some silver bullet outfit and never spend 5 minutes checking the most important thing. The film MUST be on the exact plane that the ground surface on your glass is at.

I used a rubber band, a torpedo level, and a thick business card. A credit card might be even better.



Lay the level across the back surface. Tap the biz card down until it sits flat on the glass. Carefully take it off and set it aside without bumping the card.



Insert a film holder with a piece of film in place, and set the level back on the same surface you started with. That card should just brush that film if everything's perfect. Both of the ones I checked were spot on. Remember, it doesn't have to be pretty to make pretty pictures.

[Paul Ron]

easy peezy!

nice

[Tin Can]

Yes, very good advice and 'how to'

A level is useful when using a LF box, any straight edge will work for this

[Corran]

Good tip!

What about cameras with the ground glass reversed, or with a fresnel?

[Tin Can]

Ha, stole my upcoming post!

Basically add the GG thickness if reversed, fresnel is different, not my topic yet

I recently got 3 'cycle' camera types that definity used GG ground side to the rear

I will document the 4 I have soon

Good tip!

What about cameras with the ground glass reversed, or with a fresnel?

[Jim Jones]

. . . Basically add the GG thickness if reversed, . . .

. .. . and allow for the refractive index of the ground glass.

[Tin Can]

I'm no genius at that, but is seems negligible for 1.5mm soda green as I measured one from 1902 at 3 days ago

https://refractiveindex.info/?shelf=...oda-lime-clear

Please elucidate

. .. . and allow for the refractive index of the ground glass.

[reddesert]

I'm no genius at that, but is seems negligible for 1.5mm soda green as I measured one from 1902 at 3 days ago

https://refractiveindex.info/?shelf=...oda-lime-clear

Please elucidate

The converging beam from the lens to the focal plane converges less strongly as it travels through the glass. That means it forms an image on the back side of the glass, that is slightly farther away than where the image will be formed when the glass is removed. The effective thickness of the glass is equal to (actual thickness / n), where n is the index of refraction. Normal glasses that would be used for a ground glass usually have a refractive index very close to n=1.5.

What this means is that if your ground glass is 2mm thick and the ground side is on the back, the image on the GG will be 2mm behind the front side of the glass, but the image in air will be (2mm / 1.5) = 1.3mm behind where the front side of the glass was. This is enough to matter.

If the ground surface is on the front side, you don't need to worry about this, unless there is a Fresnel on the front side, and then you have to do the same calculation but with the thickness of the Fresnel.

I don't have a perfect illustration of this, but it is the same effect as looking through the surface of a pond at a fish - the light bends at the water/air interface and changes the apparent depth of the fish, see https://www.physicstutorials.org/hom...pth-real-depth

[Jim Galli]

Good tip!

What about cameras with the ground glass reversed, or with a fresnel?

Don't know, and since I don't I removed a glass and fresnel from the 4X5 and went with traditional straight glass. Someone will chime in. I've been spooked by them ever since the huge long discussion about first generation Chamonix that didn't quite get it right?

[Corran]

Yep and I have one of those Chams!