Acutance in film photography

One of the main physical measure of sharpness is acutance.

It is however not the same as resolution as many would have assume.

Acutance is actually

  • The measure of the ability of film to delineate critically between adjacent tones contained on a negative or transparency.
     

  • The clearer, finer, and more precise this delineation is, the higher the acutance of the film.
     

  • The actual process of measuring a film's acutance level involves bringing an opaque knife edge in direct contact with the film surface being tested and then exposing the film to a well-collimated light source. The resulting image on film should be an easily observable edge between dark and clear portions of the film.
     

  • Comparing edge widths of different films of the same general type produced under identical conditions gives us some idea of different films' relative ability to distinguish edge boundaries sharply.
     

  • High acutance does not always go along with high resolution. In the case of some films, there actually seems to be an inverse relationship. Some high-resolution films appear to exhibit substantially less acutance than similar film of lower resolution.
     

  • New thin-emulsion films (films that are thinner from front to back) are able to generate higher levels of acutance than their conventional counterparts.

Logically, one would assume that always choosing a high-acutance film that also displays high resolution is the thing to do. This isn't always possible. Film design involves a series of compromises, and creating a film that is fast but still has all the desired sharpness qualities may not be within the film-maker's ability. It is a tribute to the state of the technology that we even come close to the ideal combination of sharpness, acutance, and speed.
 

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