Why do EV-to-lux tables exist, but not lux-seconds-to-EV tables?
Asked 10/9/2022
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I’m confused about the relationship between exposure value (EV), lux, and exposure time. EV is defined from aperture and shutter speed, so I expected it to relate to lux-seconds rather than just lux. Yet many charts use formulas like lux = 2.5 × 2^EV, which seems to convert EV directly to illuminance. Where does the time component go, and why are these tables usually given in lux instead of lux-seconds?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
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For that formula to work Exposure Value (EV) has to first be converted to Luminance Value (LV), which is not time based per-se.
LV is the number a vintage handheld spot meter like the Pentax V gives. On that meter the LV range is 1-19. And once the LV is known it can be divided up any way you like (Ap/SS), and compensated for (ISO). And that is shown by setting the various dials on the outside of the meter to give the appropriate/desired exposure values.
LV is the same as EV at ISO 100 and they are often confused/interchanged; but LV is not the same as EV at ISO 400... The Lux conversion requires you first convert the exposure values to LV, which effectively removes the time basis (i.e. the time result going the other direction is entirely variable).
To put it another way, EV-0 is not a number; it is a set of numbers to give a "normal" camera exposure, and they include ISO. LV and Lux are not a set of numbers... they are a singular measurement/value.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
3y ago
0
Generated from our catalog & community — verify before relying on it.
EV is not measured in lux-seconds. EV is a dimensionless logarithmic value defined from camera settings: aperture and shutter time. By itself, it does not specify an absolute brightness.
To relate EV to scene brightness, you must choose a reference sensitivity, usually ISO 100. In that case EV is often treated the same as LV (luminance value), and tables can convert that ISO-100 reference value to scene illuminance in lux using a calibration constant. That’s why you see formulas like lux = 2.5 × 2^EV.
The “seconds” don’t appear because those tables are describing scene brightness, not a specific exposure duration. Once the brightness level is known, there are many equivalent shutter speed/aperture combinations that produce the same EV.
So:
- EV = exposure-setting ratio, dimensionless
- lux = scene illuminance
- lux-seconds = accumulated exposure over time
A direct EV-to-lux conversion only works after assuming a reference ISO and calibration constant. Without that reference, EV is not an absolute measure of light, and without choosing a shutter time, it is not lux-seconds.
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