PhotoModeler depends on the fact that the internals of the camera do not change from photograph to photograph. The internals are focal length, lens distortion characteristics and lens position relative to the imaging media (film or CCD). For this reason, the user:
• should not use a zoom lens on the camera unless the focal length can be locked securely,
• should not use interchangeable lenses unless one lens is left securely on the camera during the shoot for one Measurement Project,
• should not refocus the camera because in many consumer cameras, the lens focal length and lens distortion characteristics change as the lens is focused, and
• should not depend on autofocus cameras since the lens is focused out of user control.
The user must follow dictates 1 and 2 above and should take care with 3 and 4 but not be unnecessarily worried about focus effects unless the focusing is changing considerably (i.e. some shots are focused at 1 foot and some at infinity).
Note: When using a macro lens, adjusting focus usually causes relatively large focal length changes, which should be kept in mind when taking photographs.