Learn Photography, Part 1 – 35mm Film

Learn Photography” is a new series of articles from www.photographySPY.com.  This series will cover a broad range of topics all relating to photography, including digital photography as well as film processes.  Come back and visit this site often for future posts.  Thanks for visiting.  Here we go!

Before the digital age cameras used film to record images, and post-production in a darkroom setting to process the negatives into a permanent state using a chemical process.  Once processed, the negative images became “fixed” with the aid of more chemicals.  Those fixed negatives were used to print a positive image onto photographic paper.  Going back even farther, images were recorded onto glass negatives, and even before that, they were recorded onto tin.  For the purpose of this article, we will focus on 35mm negative film.

The basic principle behind film negatives is that light is captured onto the film with the use of microscopic crystals in a compound called silver bromide.  They are suspended in a transparent gelatin; this mixture is called emulsion.  It is spread onto a strip of plastic – the base.  Anyone who wants to learn photography needs to know a little chemistry.  So, here we go.  When a solution of silver nitrate is mixed with an alkaline bromide solution in a solution of gelatin, silver bromide is formed.  What happens next is called a double decomposition and since the silver bromide is insoluble, it is precipitated.  Here’s what the chemistry part looks like:

AgNo3 + KBr -> AgBr(Silver nitrate + alkaline bromide -> silver bromide)

So the emulsion is really a suspension of silver bromide crystals held in the gelatin and becomes the layer of film that is light sensitive.  If you were to peel apart the film layer by layer you would get:

Anti scratch layer of plastic
Silver bromide Crystals in Emulsion
Acetate Base
Anti Halation Backing

Got it?  Good.  Let’s move on.

The sensitivity of a film to light is determined by the size of the silver bromide crystals.  The larger crystals need less light to produce a well-exposed image than the smaller crystals.  That comes with a price, though.  Larger crystals make the image appear grainy.  Smaller crystals produce a finer image, but require more light to obtain a proper exposure.  If you wanted black and white film that was a fast film, you would get what is called, Tri-X-Pan film.  A medium speed black and white film is called, Plus-X-Pan, and slow speed b&w film is called Panatomic-X film.  A standard was developed and a number assigned to specific speeds.  This number is called the ASA, which stands for, “American Standards Association.”  If the film has a high ASA number, it will be a fast film.  A film rated with a low ASA number will be a slow film.  Tri-X-Pan has an ASA of 400; Plus-X-Pan has an ASA of 125 and Panatomic-X film has an ASA of 32.

I hope you enjoyed this article about 35mm photographic film.  Please visit this website www.photographySPY.com often to read future installments in the Learn Photography series of articles.

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