1) THE POWDER CAMERA
Powder cameras vary in quality and precision of manufacture. If the camera(s) that were used to prepare the films are available, they should be checked using a standard such as Si powder to ascertain if they are indeed of the proper diameter. A standard reference such as Klug and Alexander (H.P. Klug and L.E. Alexander. 1954. X-ray Diffraction Procedures. John Wiley & Sons, New York. 716 p.) should be consulted on methods of sample preparation and sources of error.
2) THE FILM
Film is subject to many variables during exposure and processing. Inclusion of a standard, such as Si powder, in the sample will provide an internal check of film shrinkage; however, in some cases it may not be possible to do this (for reasons of peak interference, etc.). One technique is to run a pure standard and check shrinkage using a controlled set of development and processing steps. As long as the film is subjected to the same conditions, shrinkage should be the same.
3) INHERENT LIMITATIONS
A standard Debye-Scherrer powder camera has limited resolution and cannot be pushed beyond certain limits. I have found that specimens mounted as a sphere mount with a diameter of only 0.1 mm can be used to obtain good diffraction patterns. Using these tiny mounts increases the resolution that can be obtained, but at a cost of exposure time and difficulty in preparing the sample for mounting. A rotated sphere mount essentially eliminates preferred orientation and is the main advantage of using this technique.