Slight focus error

[Shen45]

Critically focussed on the ground glass almost wide open aperture but focus in the print is slightly in front of the point of initial focus [nearer to camera] Do I shim between the ground glass frame and the ground glass which moves the GG closer to the object? Thanks

[Leigh]

If the actual focus is closer than shown on the GG, it means the film is farther from the lens than the GG.

Negative shim. :eek: or remove an existing shim.

- Leigh

[pasiasty]

BTW: some small format lenses are prone to focus shift - i.e. changing slightly focus plane while stopping down. From obvious reason this can be detected only in case of very fast lenses, that gives shallow DOF even, when stopped down one or two stops. I don't know if this may also cause focus errors with large format. Anyway, unequal registration of a ground glass and a film seems to be more probable cause.

[Shen45]

Thanks Leigh, sadly no existing shim. Ah well a wood work project down the road by the looks of things.

[E. von Hoegh]

Critically focussed on the ground glass almost wide open aperture but focus in the print is slightly in front of the point of initial focus [nearer to camera] Do I shim between the ground glass frame and the ground glass which moves the GG closer to the object? Thanks

Shen, before you start drilling and blasting, check you filmholders - or roll back if you used one.

[E. von Hoegh]

BTW: some small format lenses are prone to focus shift - i.e. changing slightly focus plane while stopping down. From obvious reason this can be detected only in case of very fast lenses, that gives shallow DOF even, when stopped down one or two stops. I don't know if this may also cause focus errors with large format. Anyway, unequal registration of a ground glass and a film seems to be more probable cause.

Ever heard of a Goerz Dagor? They aren't fast, they aren't small format, and they're famous for growing longer as you stop them down.

[vinny]

Thanks Leigh, sadly no existing shim. Ah well a wood work project down the road by the looks of things.

After you've measured* the depth to make sure it's incorrect, I'd return the back to wherever you bought it. Have them fix it.

*in the book-Way Beyond Monochrome

[Chauncey Walden]

Shen, if the film is farther than the GG from the lens, you need to shim the ground glass out not in.

[Ari]

If you aren't a machinist, have a machinist do the work.
It may be spendy, but it will be done properly with a minimum of frustration.

[Jac@stafford.net]

Shen, if the film is farther than the GG from the lens, you need to shim the ground glass out not in.

I don't think so.

[Light Guru]

Critically focussed on the ground glass almost wide open aperture but focus in the print is slightly in front of the point of initial focus [nearer to camera] Do I shim between the ground glass frame and the ground glass which moves the GG closer to the object? Thanks

I had a similar issue with some graphmatic film holders. After measuring them I found that they did not have the same distance to the film plane that my regular film holders did.

[Chauncey Walden]

Jac, the farther the lens is from the ground glass, the closer the focus. If the image on the film was focused closer than it was on the ground glass then the film was farther away from the lens than the ground glass was. The solution is to move the ground glass farther away from the lens. Maybe it was shimmed to be correct and at some point the shims have been lost.

[Shen45]

Just in case I have explained things incorrectly.

Focussed object on GG, distance from GG to exact point of focus 100 cm, when printed the actual point of focus appears very marginally closer to the camera. Perhaps .25 to .5 of a cm.

I have three other 5x4 cameras and they all focus [and print] at the exact spot. All the holders I have are within 1 or 2 thou of each other.

So shim or gouge??

I will add a shim of unexposed film and see where that takes the focus. As soon as I do this I will report back.

[Doremus Scudder]

Just in case I have explained things incorrectly.

Focussed object on GG, distance from GG to exact point of focus 100 cm, when printed the actual point of focus appears very marginally closer to the camera. Perhaps .25 to .5 of a cm.

I have three other 5x4 cameras and they all focus [and print] at the exact spot. All the holders I have are within 1 or 2 thou of each other.

So shim or gouge??

I will add a shim of unexposed film and see where that takes the focus. As soon as I do this I will report back.

Shim!

In order to get a closer focus, the film plane has to be farther from the lens than the ground glass. Move the ground glass back to align it with the film plane to correct the error.

However, your focus error is so small, I would recommend repeated testing before doing anything. There are a number of other factors (including temperature, human error, an inadvertent tweak...) that would cause a very small error once. Unless you are really sure that this is a consistent and quantifiable error, I'd do nothing.

Also, before doing any shimming, make sure the grooves that your filmholder rests in are clean and free of debris and things like curled or wrinkled baffle material/velvet. Check for irregularities and warping as well. A very small error here can affect focus.

A light sanding of the filmholder seat may do the trick as well, eliminating the need for shimming the ground glass.

Also, check to make sure the error is the same over the entire focus plane. I had a camera once with too-thick replaced light-trap material (don't asked who replaced it...) that was out-of-focus on only the end of the film slightly displaced from normal by the thicker material.

And, if you get around to shimming, your shimming will be quite small. You can test with thicknesses of Scotch tape or the like before making permanent changes.

Hope this helps,

Doremus

[jp]

What's the lens at issue?

[Shen45]

Shimmed the GG and took a sheet and it is now as close as I care to fiddle. Aperture was wide open to eliminate DOF. To everyone that responded to this query thank you.

Steve