Photography
Equivalent exposure.
Enter an exposure that works. Then enter two of the three new settings and leave one blank. The calculator solves the blank so the picture is equally bright, and shows how many stops each setting moved.
Your numbers
Result
Fill the form and press Calculate.
Worked example
You have f/8, 1/125 s, ISO 100 and want a wider f/4 at the same ISO. f/8 to f/4 is two stops more light, so the shutter must shorten by two stops: 1/500 s. Leave ISO blank instead and enter f/4 and 1/500 to see it return ISO 100.
How it works
Exposure is proportional to t × ISO / N². Keeping that constant gives t2 = t × (N2/N)² × (ISO/ISO2), N2 = N × √(t2 × ISO2 / (t × ISO)) and ISO2 = ISO × t × N2² / (N² × t2). Stops are log base 2 of the ratio; aperture stops are 2 × log2(N2/N).
The match is for brightness only. Changing aperture also changes depth of field, changing shutter speed changes motion blur, and raising ISO raises noise. Long exposures on film can need extra time (reciprocity failure); that is not modelled.
Sources and basis (checked 2026-10-05): Wikipedia: Exposure value (exposure proportional to t/N² at fixed ISO, with ISO scaling it linearly). Arithmetic on the numbers you enter.
FAQ
What is equivalent exposure?
Different combinations of aperture, shutter speed and ISO that put the same amount of light on the sensor, so the picture is equally bright.
How many stops is f/8 to f/4?
Two stops. Each stop changes the f-number by a factor of about 1.41, so f/8 to f/5.6 to f/4 is two stops of extra light.
Which setting should I change first?
It depends on what you care about: aperture for depth of field, shutter speed for motion, ISO for noise. This tool shows the exposure maths only.
Why must I leave one box blank?
Three unknowns cannot be solved at once. Fix two new values and the third follows.
Does it handle third-stop values?
Yes. Any aperture, shutter speed or ISO is accepted; results are exact and are not rounded to a camera scale.