Match exposure when you change aperture or ISO

Metered f/2.8 at 1/125 s and ISO 100, but you need f/8 for depth of field. How many stops, and which shutter keeps the same exposure?

Tools used in this guide

The job

Exposure value rolls aperture, shutter, and ISO into one number. EV goes up one for each stop less light — a smaller aperture or faster shutter. Once you know a combination's EV, you can solve for any leg of the triangle.

Reading EV and trading stops

f/2.8 at 1/125 s and ISO 100 is EV 9.94. Closing down to f/8 costs 3 stops; keeping ISO fixed, the matching shutter is three stops slower — 1/16 s. Open the tool, enter all three numbers, then change the aperture and read the equivalent shutter time.

Worked example

Indoor portrait measured at f/2.8, 1/60 s, EV 7 — but you want ISO 400 fixed: entering t = 1/60 s and EV 7 shows the needed ISO is about 367, so ISO 400 at 1/60 s is almost exactly right. Going from EV 13 daylight to EV 7 indoors is a 6-stop difference.

Scope

Aperture f/0.5–f/128, shutter 1/32000 s to 3600 s, ISO 3–409600. The math is the ideal exposure equation; real cameras quantize to whole or third stops, and reciprocity failure on multi-second exposures is not modeled.