Rare intermediate format between 16 mm and 35 mm — used almost exclusively for aerial shots or specialized documentary work.
You will rarely encounter 17.5 mm — and when you do, it's usually as a last resort or in very specialized niches. The format exists in the borderland between 16 mm and 35 mm, technically feasible, but an economic and logistical nightmare. Anyone dealing with it needs to understand: this is not an established standard system like 16 mm or Super 16, but a fringe format that only prevailed in situations where no other options were practical.
Historically, 17.5 mm appeared primarily in aerial photography and early scientific documentaries — where one was stuck between formats. A 35 mm camera was too heavy or too expensive for drones or aircraft operations, and 16 mm did not offer the detail resolution required for mapping or medical imaging. So, raw film was cut down or special cameras were used that precisely employed this width. You will find references to 17.5 mm mainly in archived documentaries from the 1950s to the 1970s — and then there's silence.
In modern production, the format has played virtually no role. Digital has definitively settled the issue. However, if you encounter old material in this format — during archive restoration or digitization of historical footage — you will need specialists. Standard editing tables and projectors won't work, and stock is practically depleted. The optics differ from 16 mm, and the perforation is irregular. It's like a film fossil — technically interesting, practically priceless.
Remember: 17.5 mm is a format of necessity, not choice. It embodies the compromises of earlier times before digital and standardization made all these exotic intermediate formats obsolete. Should it appear in your project, expect significant additional costs for specialized lab work — or digitize it quickly before the last duplication facilities close.