Size Difference between GG and film

Following on thishttp://www.largeformatphotography.info/forum/showthread.php?t=34237&page=3 thread I just measured my 8x10 back and my holders and found a 4/16 inch difference along the long side, where the film's held down in the holder's guides. That's a significant difference between wys and wyg, so now I have to figure out a good way to crop my back. Any ideas? It's a magnesium C-1, fwiw.

GB

I built the groundglass holder of my 8x10 to exactly (well...) fit the exposed area of the film

G

If you want to determine what part of the GG image makes it to film, try this:

- Tape a single news paper page to a wall.
- Setup your tripod and camera, and compose an image that encompasses the newspaper - no blank areas!
- Expose a sheet of film.
- Don't move the camera, tripod, or newspaper - leave everything alone.
- Develop the film
- Using the film as a guide, look through the GG and find the corners that made it onto the film.

You can then mark your GG.

I did this for a 6x9 rollfilm holder on my 4x5 camera, and it works really well. Using the exposed negative, I made a wooden template that quickly allows me to mark out the corners of a 6x9 image when my felt markings wear off.