1 PHOTOGRAPHY Part 2: PHOTOGRAPHY Part 2: The Chemistry of Photography

Entry questions: 1- You want to take several pictures that will have consistent exposure, as you increase the shutter speed what must happen to aperture? 2- List two parameters you can change on a camera to reduce camera shake (a blurry image). 3- The numbers on the lens (18-55mm, 70mm, 100mm, etc.) refer to… 4- When shooting landscapes, the best type of lens to use would be a…

What are “Silver Halides”? Silver Halide is a compound made of silver (Ag) atoms and atoms from the halide group (Cl,Br,I) of the periodic table. The material property of a silver halide crystal changes after absorbing photons.

Silver and halide atoms within the periodic table of the elements Silver (Ag) Halide group

Black & White Film Black and white film is composed of 4 layers. 1- An upper protective coat. 2- A layer of gelatin that contains silver halide (AgBr, AgCl, or AgI) crystals. (The type and proportions of the different silver halides determining thespeed of the film) 3- The film base, usually made from a flexible polymer. 4- An anti-halation backing to prevent light from reflecting back onto the emulsion.

The emulsion layer is ~12% silver halide Grain size affects contrast and light sensitivity 0.1 ~ 3 microns Big grain means film more sensitive to light, but grains are seen in pictures. More photons = more reduced silver

Image formation

Effects of light on the film Within a crystal, the Silver atoms have a positive charge and the halide atoms a negative charge. Light (photons) striking the halide atoms within the grains causes excitation of electrons which move within the crystalline structure. Those electrons are attracted to the sensitivity centers (edge of crystal). Ag + Br - (crystal) + photon  Ag + + Br + e -

Latent Image Formation The silver ions are attracted to the negative charge of the electrons at the sensitivity center. As more light (photons) hit the halide atoms silver ions build up on the sensitivity centers. The silver ions acquire and additional electron and become metallic silver. (silver is reduced) These sites form development centers and make up what is called the “latent image”. Ag + + e -  Ag 0

Exposure, Development of Black and White Film - Overview A. Unused film in camera B. Exposure of film to light (photons) C. Formation of silver ions (latent image) D. Development changes silver ions to metallic silver E. Fixing – removes unreacted silver halides from the emulsion. F. Wash – rinsing with clean water. Removes all by-products of development process.

Silver Halide Process Chain A latent image is formed after exposure (invisible to human eye). After processing, the latent image is turned into a visible, stable image. ExposureProcessing DevelopStopFix Latent Image Visible (Stable) Image

Step 1: Development Photographic developers are electron donors. (They reduce positive silver atoms) The silver ions are reduced to silver metal. The developer donates electrons to the positive silver ions. Only silver ions around clusters containing a number of atoms larger than a critical value behave as electron acceptors.

Hydroquinone is a reducing agent because it gives electrons and reduces silver to metallic silver. Sodium hydroxide and sodium carbonate added as accelerators Reaction must be timed

Step 2: Stop Bath Photographic developers are generally basic (pH greater than 10). A “Stop bath” usually made from a weak acid such as acetic acid is used to stop the development, and prevent fogging of the unexposed silver.

Step 3: Fixing Undeveloped silver halide crystals remaining in your film will darken with time if exposed to light. To prevent this, film is “fixed” or has the undeveloped silver halide crystals removed from the film. Sodium Thiosulfate, is one of the most common fixing agents. The chemical reactions of the fixing process follow this general form:

Step 4: Washing The final wash of a photographic negative needs to be lots of fresh clean water to remove any residual developing agent, fixative or silver complexes as these can cause degradation of the image with time.

Step 5: Wetting agent Removes water from your negative.

Film vs Print Paper B&W papers however are not sensitive to red light so when printing B&W prints you can work under a red light. Color papers are sensitive to all light and must be handled in complete darkness.

Enlarger / Reducer Light Negative Photographic Paper Optics are used to produce an image of the negative on photographic paper.