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Film vs Digital Imaging
This series on digital and traditional film imaging was written by LOGIC member Ken Nelson. This is but one of the many postings on the LOGIC BBS - logon to see what more we offer!Table of Contents
Film - Part I - Background
Photographic film is simple in concept but the details are important. In it's simplest form it is a clear substrate coated with a gelatin emulsion of sensitized silver salts. The substrate can be any optically clear substance: glass, nitro-cellulose, or plastic. Glass is still used in scientific imaging where dimensional stability is really important for measurement purposes. Nitro-cellulose is no longer used because it is very flammable and because it deteriorates into a very unstable explosive ooze. The films we are familiar with today are a form of relatively dimensionally stable plastic: for instance you wouldn't use polyethylene because as a thin film it would stretch and distort. The reason of course for using an optically clear base is so that light can pass through it to facilitate printing and enlarging.Emulsions can be made of several substances but gelatin has been standard for most of the history of photography. Gelatin is that same substance we eat as Jello. In the late fifties and early sixties because of nuclear tests a problem arose with the Strontium 90 content in gelatin which caused fogging in film (overall exposure from the radiation which reduced the contrast of films ... made them look grey). I can't recall the remedy but a solution was found.
The main component of the emulsion is the sensitized silver salts that undergo a change when exposed to light. This change is not visible until the film is ÒdevelopedÓ by being brought into contact with chemicals that cause the crystals contacted by light to be turned into metallic silver. The film is then ÒfixedÓ by another chemical which dissolves the unexposed silver salts leaving a negative image of the light that fell on the film (darkest where most light fell). The film is then washed thoroughly to remove the chemicals and the metallic silver image is a very long lived structure.
There are several "secrets" of film but the most important is consistency (repeatability). This, in fact, is one of the main "secrets" of photography in general. So a great deal of trouble is taken so that the roll of Tri-X you buy this year will perform exactly like all the rolls you bought before (if you handle and process them alike). There are two variables in the emulsion: the size of the sensitized silver salt crystals and the thickness of the emulsion. Larger sized crystals make the film more sensitive or faster (as well as more grainy ... because you can see the individual crystals) and smaller sized crystals make the films less sensitive or slower but much more fine grained. The thickness of the emulsion influences how effective some development tricks are and the sharpness of the image (thinner films are sharper). Most modern films are thin emulsion films but some eastern European and Russian films (having thick emulsions) have a following because they are better for development manipulation.
We have of course been talking about black and white film. Colour film is more complicated because it uses three emulsion layers and this wil be the topic for the next segment.
Posted July 19, 2004
