Camera Aperture / Shutter / ISO Reference
Stops of aperture, shutter speed and ISO with reciprocity notes.
Overview
The Camera Aperture, Shutter and ISO Reference lists the standard "stops" for each of the three exposure variables on a camera. It shows the full-stop sequence, plus half-stops and third-stops, with notes on which combinations balance to the same exposure and where reciprocity failure starts to bite on long film exposures.
It's a quick-reference for photographers learning manual mode, video shooters memorising shutter-angle equivalents and instructors building a one-page cheat sheet. Long-tail queries it answers include "full stop aperture sequence f-stop", "third-stop ISO values list" and "shutter speed reciprocal chart".
How it works
Each stop is a doubling or halving of light. Aperture stops follow powers of the square root of 2 (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22) because the area of a circular aperture scales as the square of its diameter. Shutter speeds form a geometric series with ratio 2 (1, 1/2, 1/4, 1/8, 1/15, 1/30 ...). ISO doubles in the same way (100, 200, 400, 800).
The reference tabulates every value, marks which sit on full stops vs intermediate stops, and shows the equivalent-exposure shifts: if you open the aperture by one stop, you must shorten the shutter by one stop or drop ISO by one stop to keep brightness constant.
Examples
f/2.8 + 1/60s + ISO 400 = f/4 + 1/30s + ISO 400
f/8 + 1/250s + ISO 400 = f/8 + 1/500s + ISO 800
Third-stop ISO ladder: 100, 125, 160, 200, 250, 320, 400
Full-stop shutter ladder: 1s, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125
FAQ
Why aren't shutter speeds clean halves?
They mostly are, but the printed values are rounded for readability. 1/15s is actually 1/16s halved, and 1/125s is actually 1/128s. The labels are conventions that go back to mechanical shutters.
What is reciprocity failure?
For exposures longer than about one second on film, the relationship between time and brightness breaks down. The film needs extra exposure beyond what the simple doubling rule predicts. Digital sensors do not suffer this.
How does shutter angle relate to shutter speed?
In video, shutter angle is a fraction of a frame. At 24 fps with a 180° shutter, the effective shutter speed is 1/48s. The reference includes the conversion for common frame rates.
Why does higher ISO mean more noise?
ISO amplifies the sensor signal after capture. Higher gain magnifies read noise and shot noise along with the image, lowering the signal-to-noise ratio.
Is there a useful equivalence between digital and film ISO?
For modern sensors, ISO speed approximates the ISO 12232 standard so a meter reading is portable. Real-world noise floor differs by sensor.