What a difference half a millimetre makes...

[barnacle]

I've posted previously regarding the difference in focal plane between my ground glass and the film holder - the glass is 0.45mm behind the film - the net effect of which is to move the focus further from the camera than expected. Here's a lovely and extremely annoying example... the wall and door are pin-sharp, but the headlight of the moped is where I focussed. Meh.

https://i.imgur.com/zdznYfs.jpg

Neil

[Ari]

I hate when that happens.
If you're not a technical kinda guy, best to send it to someone who can adjust the GG precisely to line up with the film plane.
What camera are you using?

[johnmsanderson]

What f stop ?


I've posted previously regarding the difference in focal plane between my ground glass and the film holder - the glass is 0.45mm behind the film - the net effect of which is to move the focus further from the camera than expected. Here's a lovely and extremely annoying example... the wall and door are pin-sharp, but the headlight of the moped is where I focussed. Meh.

https://i.imgur.com/zdznYfs.jpg

Neil

[barnacle]

I'm a techy sort of guy and the only solution is a replacement ground glass carrier, sadly. It's designed (they're not exactly difficult) and just looking at getting it made now, Ari. It's an MPP press camera, 1940s vintage, and it's not right for modern film holders. It *could* be machined off, but it's tricky to get right and I'd rather not risk butchering the original.

John, that would have been f16 or f22.

Neil

[sdynes]

I ran into this problem with my Cambo. I used a toothpick rigidly attached to some straight, unbending rod (I must confess I cannot remember what it was), and adjusted the 'depth' of the toothpick so it *just* kissed a sheet of film in a holder. (I tried this with multiple holders, and the film plane was surprisingly consistent between holders, and across the plane of the film). I then did this with the GG, and the GG was below the plane of the film. I shimmed the GG (using thin cuttings of film, of course) until the toothpick *just* kissed the GG.

Now, what I focus on is actually in focus on the film.

[ericantonio]

I ran into this problem with my Cambo. I used a toothpick rigidly attached to some straight, unbending rod (I must confess I cannot remember what it was), and adjusted the 'depth' of the toothpick so it *just* kissed a sheet of film in a holder. (I tried this with multiple holders, and the film plane was surprisingly consistent between holders, and across the plane of the film). I then did this with the GG, and the GG was below the plane of the film. I shimmed the GG (using thin cuttings of film, of course) until the toothpick *just* kissed the GG.

Now, what I focus on is actually in focus on the film.

THIS!! I think LF'ers are original McGuyvers.

[Doremus Scudder]

Wait...

If the actual focus on the film is farther away than the focus on the ground glass, then the film is closer to the lens than the gg; meaning that the ground glass needs to be moved closer to the lens, not shimmed farther away from it. This would involve machining the ground-glass seat; not so easy as shimming.

@OP: One alternative, if you could get it to be light-tight, would be to shim filmholders back by the appropriate amount. Maybe some dedicated holders with adhesive shims or a shim on the filmholder mating surface? Alternately, one could accurately measure the right distance to compensate and simply adjust focus by the correct amount after inserting the filmholder, much as is done with IR film. Of course, having the camera machined to the correct measurements would be most desirable, but down-and-dirty might get you shooting more quickly.

Doremus

[Luis-F-S]

Probably wouldn't see it at f/45!

[ic-racer]

You can use the simple lens equation to figure out the required ground glass displacement for coincidence with the film plane.
1/p + 1/q = 1/f

For example if 'p' is the distance your focus is off at the subject plane and 'f' is your focal length lens, then you can solve for 'q' which would tell you how far your ground glass needs to be shifted.


Also, a 1mm focal error at the film plane is can be, in theory, absorbed by an aperture of f16 assuming this, that, and the other thing, etc...

[Bernice Loui]

Half millimeter is HUGE in optical terms. This illustrate the significance of front and rear camera standards alignment, film holder area on the rear standard to ground glass alignment and all related. Problem increases with shorter focal length lens and increasing film format size.

As an adder, this illustrates the importance of film flatness inside the film holder. Problem of film flatness grows increasingly significant with increase in sheet film size.

Keep in mind the lens has a very small point of actual focus and depth of field_depth of focus is an illusion of being in focus. This is why if high definition images are desired, camera and film holder precision can make all the difference along with using the largest lens aperture possible for a given image creation need. The belief that "stopping down" is the focus cure-all does not always apply and is not always the ideal solution to the problem.

Optical performance of a lens becomes less an less relevant if the camera, film flatness (this is where a Solid State imager has an advantage, these devices are quite flat and stay that way), overall camera image system alignment is not of proper precision and accuracy.



Bernice

[barnacle]

Hence my initial post here: http://www.largeformatphotography.info/forum/showthread.php?141861-Out-of-position-ground-glass-plane-MPP

Doremus, a set of shims for the film holders was something I considered but there are a number of issue that persuaded me against it:
- the shims would have to be fixed to the holders, which would mean I couldn't use them with anything else
- the difficulty of getting the shims made flat (thickness is easy)
- the difficulty of fixing the shims to the holders - glue thickness and lumps could be an issue

So I have a design for a new focus holder which I am looking to get made in Delron. I'd rather make it myself (just for the fun) but I can't mill that large or that precisely on the depth; this needs a professional setup. I'll post some pictures when I get it organised (might be a while; it's a complicated time of year).

I'll probably be dragging the camera off to Berlin over xmas, so hopefully I'll have something sorted; if not then the focussing forward approach will have to do, guesswork and all.

Neil

[Jac@stafford.net]

As mentioned earlier, .5mm is huge.

If focus is short of the GG, then shim the GG on its underside,
placing shims between the GG and lens.

Only four are necessary. Good flat shims are available. You can trim their
length and width with a razor. I will post sources if you wish, but a good
start is to Google "feeler gauge shims"

I have found couple wood backs had apparently been refinished, reducing
GG set-back - a bad thing but fixed as above.

Moving the GG towards the lens is a matter of first checking if there are
already shims in the place mentioned above. Remove them and replace
with thinner shims. I've found one such case.

If none, then machining the place where the GG holder seats (the back)
is necessary. IMHO that is not a hand work operation. A professional
machinist would find it trivial. I am not one.

[barnacle]

No existing shims, Jac; it was designed for glass plates and wooden holders. The gg casting has four 'feet' that are machined to provide the correct offset, but they're only 0.3mm above the casting. There are six contact points on the inside, on which the glass sits; that's where the machining would have to be done. Possible, but as you say, not a home job.

Neil

[Bernice Loui]

Suggest using brass shim stock cut then rolled smooth, deburred to size as needed. Shim stock is a common machine shop consumable that comes a a variety of thickness from 0.001" to about 0.032" thick:

https://www.onlinemetals.com/merchant.cfm?id=1188&step=2


Measure the distance from the film holder seating area to the GG on all four corners, edges and center, then shim or remove material in say 0.001" increments as needed to achieve overall uniformity of flatness across the GG to film holder seating area.


Do keep in mind adding a Fresnel lens for GG viewing can alter where the lens focal point ends up on the GG-Fresnal to film holder seating area.



Bernice

[Bernice Loui]

Double post. deleted.