High resolution film, such as Kodak's Technical Pan film, combined with a high resolution optical system and good seeing and tracking require much more critical focusing. Film Construction Film is composed of several layers. The emulsion, containing the light sensitive compounds, is suspended in a gelatin that is coated on top of a base layer that is made of acetate or polyester. Color film has three or more emulsion layers sensitive to different colors. Film may also include an anti-halation layer and for color films, a masking layer.
Films come in different thicknesses.
Tech Pan is a very thin film. Fuji's Velvia is a very thick film. As discussed previously, the thickness of the film must be taken into account during focusing. A knife edge focuser constructed with the film itself used as a knife-edge, or calibration by star trail testing on film can compensate for film thickness variations. I see this problem quite frequently in the humid conditions I photograph in on the east coast.