Week 2 - Practical Notes [Roll Film]*

Roll Film

Roll Film Photography

Before digital cameras came along, photography involved capturing light rays on silver-based film. Light entered the front of the camera through the aperture and lens and hit a piece of film wound out from the spools at the back. Originally, photography was a very specialised and skilful business— until this little invention came along and allowed anyone and everyone to take photos.

Remember that light is a form of energy; what a camera actually does is permanently capture the energy falling on a small, (two-dimensional) surface inside it.

In a digital camera -  There's an electronic light-detector chip called a CCD (charge-coupled device) immediately behind the lens, which converts the light energy into electricity.

In a traditional camera - There's no CCD; instead, the incoming energy is captured by a piece of plastic that is sensitive to light, better known as the film. The light energy leaves a permanent trace by causing a chemical and physical transformation of the film. (Incidentally, light doesn't just mean visible light: you can, in theory, make a photograph from any kind of incoming light: infrared, ultraviolet X-rays, or whatever you wish.)

Key Feature of a Roll Film Camera

There are four key features to a roll film camera, these are:

1) The Metal Casing

The case must protect the film and be completely light-tight.

2) The Aperture

Minolta XD11 SLR camera

Also known as the diaphragm, this is a small circular hole in the case that lets in light for the short period when you want to take a photo. There is a ring on the lens which you can manually adjust for most film camera lenses.

3) The shutter mechanism

This part of the camera is responsible for the shutter speed. Its a spring-loaded set of overlapping blades, like the ones you see at the start of a James Bond film, that open to let light in through the aperture for a precise amount of time before closing up again.

4) The roll/piece of film

Located on the back wall of the camera directly opposite the shutter, this is what your image is captured on and developed from.

Most film cameras also have a viewfinder, so you can see how your photograph will appear, a xenon flash lamp, which adds enough extra light energy to activate the film, even in dark conditions, and self-timer mechanism so you can photograph yourself without anyone's help.

How SLR Cameras Work

Inexpensive cameras generally have a viewfinder mounted to one side and above the main lens, so the image you compose is only an approximation of what you'll see on the final photograph. Professional cameras use a system called SLR (single lens reflex), in which prisms and mirrors allow you to look through the actual lens of the camera and see an exact replica of the photo you'll take.

How an SLR mechanism works

1) Light enters at the front and passes through the lenses.

2) and iris diaphragm 

3) which has metal blades that open and close to let in more or less light. 

4) Inside the camera, the light bounces off a hinged mirror 

5) Light then shoots up into a penta-prism (5, five-sided prism)

6) The light bounces it into the viewfinder (6) and your eye. 

When you press the shutter (not shown), the mirror (4) flips down out of the way and the light from the lens (dotted line) passes straight through to the back of the camera, hitting the film instead (7). 

This type of design ensures that the image you see through the viewfinder is exactly like the image captured on the film.

Exposure with a Film Camera

Film is very sensitive to light: only a tiny amount of light energy is needed to make a photograph and too much light will destroy it. 

To produce a perfect photo, you have to let exactly the right amount of light hit the film, which is called the exposure. The exposure depends on two factors:

- How long the shutter is open (the shutter speed).

- How widely it's open (the aperture). 

Shutter speed is measured in seconds (anything from about 1/10,000 second to 30 seconds); aperture is measured in units called f-stops (or just "stops" for short), such as f/4 and f/8.

Automatic, Compact, "Point-and-Shoot" Cameras 

These cameras produce a reasonable image with the click of a single button: they use photocells (electronic light sensors) to automatically adjust the shutter speed and aperture and fire out invisible infrared or ultrasound beams to set the focus automatically as well. Although sophisticated professional cameras often have automatic controls, they also allow completely manual operation: before you can take a photo, you have to adjust the focus, set the exposure time, and adjust the size of the aperture. With manual cameras, you have to adjust the exposure time and aperture setting to compensate for one another, because both of them affect the amount of light reaching the film.

How Does Photographic Film Work?

Kodak 35mm Porta Film ASA 400

Photographic film is plastic (or sometimes paper) that's coated with an emulsion made from microscopically tiny crystals of silver salts suspended in gelatine (a jelly-like substance found in sweets such as wine gums). The silver salts are compounds of silver and halogens such as chlorine, iodine, and bromine, also called silver halides—and their useful feature is the way they begin to change into pure, metallic silver when light falls onto them. 

If lots of light hits them, they change much more dramatically than if less light hits. This is how the two-dimensional pattern of light rays entering through the lens of a camera from the world outside forms a kind of invisible, chemical trace (called a "latent" image) on the surface of photographic film.

Developing and Printing Photographic Film

A light-sensitive slice of plastic film with an image invisibly imprinted on it isn't much use to anyone. To turn it into a recognisable photo, you have to develop the film in a darkroom (usually lit with red or green light that doesn't affect the film). This involves dipping the film in a series of chemicals, which convert the latent image captured by the tiny silver halide crystals into a visible image formed of larger silver particles, and also makes that image permanent. Here is a brief step-by-step process of how it is done:

Photographic Process Stages

1. The film is dipped in an acidic solution called developer, which encourages more of the silver halide to convert to metallic silver and renders the latent image visible. To stop this process continuing indefinitely, and ruining the photo, the film then has to be dipped in an acidic solution called a stop bath to neutralise the developer. 

2. The image is then made permanent by dissolving any remaining silver halide using a chemical solution known as hypo (or fixer), before being rinsed clean in water and hung up to dry. At this stage, the image, though visible, is still in a negative pattern, with light areas looking dark and vice versa. That's why developed pieces of film are called negatives.

3. Once the film is developed, it's printed: broadly speaking, you shine a light through the negative so it casts a shadow onto photo-sensitive paper and turns the negative film into a recognisable photograph called a (positive) print. You can make any number of prints from a single negative, which is one of the great advantages of this slightly laborious, "positive-negative" photographic process. 

By adjusting the distance between the negative and the paper you're printing on, and using lenses, you can also enlarge or reduce the size of a an image. The piece of equipment you use to do this is called an enlarger. 

You can make any number of prints from one negative. It's possible to develop and print films yourself, but most photographic laboratories have large electronic machines that automate the process completely, threading the film through a series of tanks filled with chemicals in the correct sequence, at just the right speed. Those big photo-printing machines you still sometimes see in the back of drug stores typically use a method of developing colour film called the C-41 process.

Types of Photographic Film

Most film is sold in light-tight cartridges that you snap into your camera. Inside, the cartridges contain a long reel of plastic film separated into one, two, or three dozen rectangular frames that measure 24mm x 36mm - this standard size is called 35mm film. The top and bottom of the reel is punched with little holes so each section of the film can be wound out of the way after a photo is taken, releasing an unexposed frame ready for the next photo.

There are numerous different kinds of film designed for taking different kinds of photo. 

Black and white film 

This type of film is sensitive only to the presence or absence of light, so it shows images only as shades of grey. 

Color film 

This type of film effectively works the same way as black and white only with three separate layers, one sensitive to blue, one sensitive to green, and one sensitive to red light. 

Films are also designed to work in widely different light conditions: 

Generally speaking, you need to use a fast film (one that forms an image with relatively little light) in dark, indoor conditions and a slow film (one that needs more light) in bright, outdoor conditions. 

The Film Speed

This is indicated by a number called the ISO or ASA rating: ISO numbers of 100 are slow, 400 or more are fast, and 200 are good for general-purpose photography. Fast films generally produce grainier, more blurry images than slow ones so, as a rule, photographers never use a faster film than is absolutely necessary. 

Modern cameras automatically detect film speed using a system called DX coding, which simply involves the camera reading a barcode printed on the film container. For no real reason other than convention, good digital cameras also tend to use ISO ratings to indicate how quickly photographs are taken in different light conditions.

Sources:

1. http://www.explainthatstuff.com/how-film-cameras-work.html

2. https://www.google.com/patents/US388850