Film & darkroom
Film speed rating and the exposure index are different
A box speed describes a standard; a meter setting describes your chosen exposure practice.
In a film-negative workflow, the camera and light set the exposure. Some analogue booths expose paper directly, so a film-speed label does not describe every booth. Where film is used, the stock’s ISO speed still cannot guarantee each negative’s density.
Separate the two numbers
The film’s ISO speed is determined under a specified sensitometric method and development condition. An exposure index is the rating a photographer chooses for a particular workflow. In a booth archive, record the film stock and the machine’s exposure system separately; do not infer one from the other.
Evidence: Manufacturer technical workbook
Read the negative as evidence
Compare frame density with edge markings and with another frame from the same strip. If every picture is thin while the edge information is normal, capture exposure is one possibility; if edge information is also weak, processing or film handling may be involved. A scan preview alone cannot identify the cause.
Evidence: Manufacturer guidance
Write a useful record
For a recreated analogue setup, log the stock, metering method, camera exposure setting and processing reference. Change only one variable in a controlled test. The result is a traceable exposure index for that setup, not a new universal speed rating for the film.
Sources & further reading
Sources for the specific facts and product descriptions above. Our worked examples, interpretations and practical suggestions are original editorial guidance.
- Eastman Kodak, Basic Photographic Sensitometry WorkbookManufacturer technical workbook
- ILFORD PHOTO — common film processing problemsManufacturer guidance