Constructing 4x5 back and rear frame

[Tyler Payne]

Hi all. I've recently put together a rear frame for 4x5, and I'm just
wondering if it might be a little too big. It's square, and 8.5 inches on a side. I built it this big out of fear of the rear frame casting a shadow on the image
area when using movements. Is this something to worry about in any serious way, or will this size make it
harder to insert and remove film holders?

Second, now that I have film holders on hand, I'm ready to make the
back itself. Does a film holder need to
fit so that it is in direct contact with the outside edges of its seating, or should
there be some leeway? (The dimensions of the holders I received differ
slightly from the standard.) Also, they are the old variety, though in good condition.
Are there differences between the old and new film holders that will impact how a
springback must be constructed?

Thanks a lot everyone.

[Emmanuel BIGLER]

Tyler
I do not think that 8.5 inches is that big if you mean the outside dimensions of the frame.
As an example Arca Swiss F-classic cameras with 171mm boards have an identical opening of 171mm at rear, i.e. 6.7 inches ; add about one inch for frame thickness, you are not far from 8 inches.
I think you should be more concerned by the bellows casting shadows on the image, but if your rear standard is slighty oversized, so will be your bellows and you do not have to worry about cutting useful light rays.

About light leaks and proper seating of the film holder, you should have a look at a good ol' 4"x5" camera and do some reverse engineering ;-) It is difficult to explain things in pure ASCII text ;-)

ANSI standard 4x5 holders have pre-defined dimensions as explained in this document.
home.earthlink.net/~eahoo/filmhold.html (http://home.earthlink.net/~eahoo/filmhold.html)

You can check whether your film holders are ANSI or not ; my recommendation would be to avoid building a film back that would not be compatible with modern film holder dimensions. Used 4x5" film holders are not that expensive.

The most critical dimension is the film depth specification, the distance from where the holder seats to film plane. This dimension is 5mm = 0.197" for 4x5. Check with your holders ; if you do not find this dimension probably your holder is not ANSI, in any case be careful to place the ground glass at the right place.

[John Layton]

Tyler - can't tell about your frame size without seeing your design - but if its reasonably shallow it could probably be a bit smaller - even 7" X 7". And there should be a little "wiggle-room" between the edges of your holders and the track into which they slide. This is important for dynamics, but also to help compensate for minor variations in film holder dimensions as well as the fact that the film back itself may shrink and swell a bit.

And I'll second the comment above about the "most critical dimension" of where the holder seats to the film plane - and suggest that you ignore the published specs for this and either perform this measurement yourself or have this done by a competent machinist. I've had lots of experience with this - and have found the best method for arriving at this measurement is to gather a variety of film holders (different brands) and load them with a few different emulsions - then take measurements from the plane defined by the film holders flanges to the surface of the film itself - and do this in several places on the film (midline, one-third to edge, two thirds to edge) and take an average of these. Also note any tendency for the various films to bow out slightly, but make sure its not due to the heat/moisture from your hands. I've always noted that if a ground glass frame is spec'd out for simply the distance from the film holder flange to the inner plate - minus the average thickness of film, this is rarely accurate enough.