The Exposure tool — meter once, then shoot the settings you actually want

The idea: exposure value

Every combination of aperture, shutter and ISO that produces the same brightness shares one number, the exposure value (EV). Lightpath computes it from the metered settings:

EV₁₀₀ = log₂(N² ÷ t) − log₂(ISO ÷ 100)

where N is the f-number and t the shutter time in seconds. Open the aperture one stop (say f/8 to f/5.6) and you must halve the shutter time to keep the same EV. Double the ISO and you halve it again. Add a 3-stop ND filter and the shutter gets eight times longer. The tool does this arithmetic and hands you a shutter speed on the standard scale.

How to use it

1. Put the camera in aperture priority or program mode and half-press on your scene. Note what it chose — for example f/8, 1/125, ISO 100.
2. Open Tools › Exposure and enter those three under Metered exposure.
3. Under I want to shoot at, set the aperture and ISO you actually want, and the ND filter if you are using one.
4. Read Set your shutter to. The line beneath tells you whether it is hand-holdable and the scene's EV.

Example: metered f/8 · 1/125 · ISO 100 (EV 13). You want f/2.8 for the background. Three stops wider means eight times shorter: 1/1000. Now add a 6-stop ND to blur a stream at f/8: 1/125 becomes about 1/2 second.

Why not just trust the camera in manual?

You can — this is the same maths — but the tool is faster than counting stops in your head under a fading sky, and it removes the mental arithmetic of ND filters, which are labelled inconsistently (ND8, ND 0.9 and "3-stop" are all the same filter). It also flags when the result is slower than you can hand-hold, which is the moment to reach for the tripod.

Field tips

• Meter with the composition you will shoot; a bright sky in the top third changes the reading.
• Sunsets shift a stop or more in five minutes. Meter, compute, shoot, and re-meter.
• At golden hour, deliberately under-expose by a third to two-thirds of a stop to protect highlights and deepen colour; you can lift shadows later, but you cannot recover a blown sun.
• Treat the hand-holdable flag as a prompt, not a promise: stabilisation buys two to four stops over an unstabilised lens, and a tripod removes the limit entirely.

References

Primary

• Wikipedia. Exposure value. https://en.wikipedia.org/wiki/Exposure_value — the definition of EV, the EV₁₀₀ convention and the tables of equivalent exposures the tool reproduces.

Supporting

• Cambridge in Colour. Camera Exposure: Aperture, ISO & Shutter Speed. https://www.cambridgeincolour.com/tutorials/camera-exposure.htm
• Parker, F. The Ultimate Exposure Computer. https://www.fredparker.com/ultexp1.htm — the classic EV-by-scene reference; useful for sanity-checking a meter reading.
• Wikipedia. Neutral-density filter. https://en.wikipedia.org/wiki/Neutral-density_filter — the ND naming systems and their stop equivalents.

← All Lightpath guides · Support