The creation of a photograph is fundamentally a mechanical and chemical (or digital) process of capturing light. A perfectly exposed picture is a careful balance of functions that dictate how much light hits the sensor or film, and how sensitive that medium is to the light.

The Mechanics of Exposure

Exposure in a camera is controlled by balancing depth of field,¹1. The distance between the nearest and furthest elements in a scene that appear to be “acceptably sharp” in an image. Controlled primarily by the lens aperture (f-stop). motion blur, and light sensitivity. Generally, a photographer hones in on the aperture they want to achieve a specific depth of field before choosing their shutter speed or altering sensitivity.

Light Sensitivity

The baseline foundation of any exposure calculation is determining how receptive your recording medium is to light.

ISO: A numerical rating representing a camera’s sensitivity to light, pertaining to either the film stock or a digital sensor.

• A lower ISO value (e.g., ISO 100) means less sensitivity to light and is ideal for bright lighting situations.
• A higher ISO value (e.g., ISO 800 or 1600) means more sensitivity, making it suitable for low-light situations.
• Pushing ISO too high creates digital noise or heavy film grain.²2. On analog film, higher sensitivity requires physically larger silver halide crystals in the emulsion, which become visible as a gritty texture (“grain”) when developed and printed.

Example: Film Grain Structure

Microscopic view of silver halide particles embedded in gelatin and spread across a polyester base. Shows the difference in particle size/density between Low ISO and High ISO film.

Example: ISO Brightness Comparison

Exposure Blunders

When the balance of light, time, and sensitivity fails, the image breaks down in one of two directions.

Overexposure: A state where too much light is allowed in during the exposure, resulting in an overly bright, washed-out photograph.

• Highlights lose all detail and become pure white.

Underexposure: A state where too little light is allowed in during the exposure, resulting in an overly dark photograph.

• Details are completely lost in shadow areas, rendering them pitch black.

The Camera Obscura

Long before chemical photography was invented to permanently fix an image, artists used optical principles to project images of the real world for observation and tracing.

Camera Obscura: An optical device (literally translating to “dark room”) that projects an inverted image of its surroundings onto a screen or wall.³3. The fundamental optical principle where light rays from an illuminated scene pass through a small hole in a darkened enclosure, crossing paths and projecting an upside-down, reversed image on the opposite wall.

• Originally used as an aid to drawing and painting.
• Smaller tabletop versions were eventually developed with lenses to help the user focus their subject onto a glass plate on top of the device for tracing.

Example: Historical Camera Obscuras

Early camera obscuras were literal rooms that an artist would stand inside. Later, portable wooden boxes were developed with a mirror inside at a 45-degree angle, projecting the image up onto a piece of frosted glass so the artist could lay paper over it and trace.