Aperture Calculator

Find the f-number that completes your exposure given EV, shutter speed and ISO. Updates as you type.

Inputs

Target EV

Shutter (seconds)

ISO

Press Calculate to update results as you change values.

Results

Why solve for aperture?

Sometimes shutter speed is fixed by motion—freezing a runner or showing blur in water—and ISO is capped by noise tolerance. The remaining variable is aperture. This calculator rearranges the exposure-value equation so you can read the f-number that hits your target EV under those constraints.

Aperture also controls depth of field. Once you know the required f-number for exposure, compare it with the f-number you want for sharpness using the depth of field calculator. If they conflict, you must compromise on shutter, ISO or lighting.

Formula

N = √( t × 2^(EV + log₂(ISO/100)) )

Derived from EV = log₂(N²/t) − log₂(ISO/100). Full stops land near 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22.

Example

Target EV 12, shutter 1/100 s, ISO 100 → N ≈ f/4. Opening the lens to f/2.8 at the same shutter and ISO would yield about EV 11 (one stop brighter).

FAQ

What if the result is between standard stops?

Use the nearest available click or continuous aperture ring. A third-stop difference is rarely critical outdoors; in the studio you can tweak light power instead.

Does crop factor change the f-number for exposure?

No. Exposure depends on light per unit area. Crop factor changes field of view and depth of field for a given framing, not the EV math.

Aperture priorities in real shoots

Event shooters frequently lock shutter at 1/200 s or faster to freeze people and set ISO to a maximum they accept for noise. The remaining question is which f-number still delivers enough light. Enter those constraints as EV, shutter and ISO, and read the aperture. If the result is wider than your lens allows, you must raise ISO, slow the shutter, or add light. If the result is much smaller than you need for background blur, you have exposure headroom to open up and lower ISO for cleaner files.

Macro and product work often needs f/8–f/16 for depth across a small subject. Check whether that aperture still meets the EV of your tent or strobes; if not, increase flash power or accumulate light with a longer shutter on a tripod. The calculator turns that guesswork into a single clear f-number.