ISO↔EV Converter
Enter a metered exposure value and its reference ISO, then the ISO of the process you're actually shooting, to see the stop difference and the equivalent exposure value.
Converted exposure value
Metering for a much slower process
Exposure value (EV) combines aperture and shutter speed into a single number, but that number is only meaningful relative to a stated ISO. The relationship EV(target) = EV(reference) + log₂(target ISO / reference ISO) lets you take a reading from any ordinary meter and re-express it for a process running at a very different sensitivity.
Once you have the target EV, you still need to pick an actual aperture and shutter speed combination that produces it — and if that combination lands on an exposure time long enough to trigger reciprocity failure, pair this with the reciprocity & exposure calculator for a further correction.
Frequently Asked Questions
Why do I need to convert EV between ISOs at all?
A light meter reports an exposure value calibrated to whatever ISO you've dialed in — commonly 100 or 400 for a modern meter. Historical processes run far slower: a daguerreotype behaves like roughly ISO 1, and wet-plate collodion is typically in the low single digits. If you meter at ISO 100 and then simply use that reading on a process with an effective ISO of 1, you'll under-expose by many stops, because the reading was never calibrated for that emulsion.
What does the 'stop difference' mean?
Each doubling or halving of ISO is one stop of sensitivity. The stop difference here is log base 2 of (target ISO / reference ISO) — a negative number means the target process is slower and needs more exposure (a lower EV, i.e. a wider aperture or slower shutter), and a positive number means it's faster and needs less.
Is this the same as just using my camera's manual ISO dial?
The math is the same relationship your camera uses internally, but most digital cameras and light meters don't go anywhere near ISO 1–25, so you often can't simply dial in the process speed. This converter gives you the equivalent EV so you can work out an aperture/shutter combination by hand or via another calculator.
How accurate is a historical process's 'ISO' rating?
Treat published ISO-equivalent figures for daguerreotype, ambrotype, and tintype as rough historical estimates, not calibrated modern film-speed ratings — they vary with the specific chemistry, plate age, and conditions. Use this conversion as a starting point and refine it with your own test exposures.
Educational tool only. Historical process speeds are approximate and vary by chemistry, plate age, and conditions — treat the result as a starting point and confirm with a test exposure.