Light Meter Reading Calculator

Work out light meter reading instantly with clear inputs, formula shown and shareable results.

About Light Meter Reading Calculator

The metered EV is shifted for ISO, then the aperture follows from rearranging EV = log2(N squared over t) to give N as the square root of 2^EV times the shutter time. Working from a metered EV rather than the camera's own suggestion is what lets you fix shutter speed for motion and let the aperture fall where it must.

Formulas

EV at working ISO

EV = EV at ISO 100 + log2(ISO / 100)

Aperture

f-number = sqrt(2^EV x shutter time)

Worked example

With the default values below, Light Meter Reading Calculator returns Aperture required of 8.095. Change any input above and every figure here is recalculated in your browser.

Inputs used

Metered exposure value at ISO 100accepts -10–25: 12
ISO you are shootingminimum 1: 200
Shutter speed you wantminimum 0 s: 0.008 s

Results

Aperture required: 8.095 f
Nearest marked aperture: 8.1 f
Exposure value at your ISO: 13.000 EV
Approximate scene luminance: 4,096.000 cd/m2

Step by step

1Values used
2EV at working ISO
3Aperture
4Aperture required= 8.095 f
5Nearest marked aperture= 8.1 f
6Exposure value at your ISO= 13.000 EV
7Approximate scene luminance= 4,096.000 cd/m2

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Light Meter Reading figure worked out?

EV = EV at ISO 100 + log2(ISO / 100). The metered EV is shifted for ISO, then the aperture follows from rearranging EV = log2(N squared over t) to give N as the square root of 2^EV times the shutter time.

Why does this matter?

Working from a metered EV rather than the camera's own suggestion is what lets you fix shutter speed for motion and let the aperture fall where it must.

What do I need to enter?

Enter metered exposure value at iso 100, iso you are shooting, shutter speed you want. The defaults shown are a realistic worked example — swap in your own figures to get a result you can use.