Equivalent Exposure Triangle Calculator

Method, formula and assumptions

For equal nominal brightness, new time = old time x (new f-number/old f-number)^2 x old ISO/new ISO. Aperture compensation uses twice the log2 f-number ratio; ISO change uses log2 ISO ratio.

Input definitions and supported units

• Original shutter time (seconds): Measured or assumed numerical value, from 0.000001 to 10000; must be positive.
• Original f-number: Measured or assumed numerical value, from 0.1 to 128; must be positive.
• Original ISO: Measured or assumed numerical value, from 1 to 1000000; must be positive.
• New f-number: Measured or assumed numerical value, from 0.1 to 128; must be positive.
• New ISO: Measured or assumed numerical value, from 1 to 1000000; must be positive.

Worked example with answer

1/100 second at f/4, ISO 100 changed to f/8, ISO 200 requires two aperture stops and gains one ISO stop. New shutter time is 0.02 seconds (1/50), a 2x multiplier.

How to use the results

Equal brightness does not mean equal noise, depth of field or motion blur. The separate stop outputs identify which setting creates each trade-off.

Results retain full precision internally and display up to four decimal places. Very small nonzero results use scientific notation rather than appearing as zero. Check the labels before comparing values across units.

Limitations and references

Assumes ideal f-number exposure and linear nominal ISO behavior. Lens transmission, metering, highlight headroom and actual ISO calibration vary.

The method is a self-contained arithmetic, geometric or explicitly stated mathematical model. There are no imported tariffs, empirical presets or equipment specifications; measured values and allowances must come from your own process.

Related planning tools

• Long Exposure ND Filter Stack Calculator
• Exposure Bracket Sequence Budget Calculator
• Point Light Distance Exposure Compensation Calculator

Browse all Photography & Media calculators