# [argyllcms] Re: Feature request (and an implied question)

• From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
• To: argyllcms@xxxxxxxxxxxxx
• Date: Mon, 26 Jul 2010 21:14:24 -0700

```On 2010 Jul 26, at 8:49 PM, Graeme Gill wrote:

> Ben Goren wrote:
>
>> spotread -a -v already shows ambient light levels in lux. Would it be
>> possible
>> to also show light levels in EV units?
>> The conversion (for ISO 100) is:
>>     EV = log2(lux / 2.5)
>>    lux = 2.5 * 2**EV
>
> As I understand it, EV (Exposure value) represents a camera shutter speed and
> aperture
> combination. For a given film speed (ISO), desired exposure (K factor) and
> light level
> there will be a resulting suggested EV, but EV itself is not a measure of the
> light level.

Hmmm...as I understand it, for any given illumination level and ISO, there is
exactly one corresponding EV (and vice-versa). One may, of course, choose to
expose the film / sensor at a different EV, but that would be akin to a
musician intentionally playing a note out of tune for dramatic effect.
Therefore, from my understanding, EV is every bit as much a measure of light
level as scale notes are a measure of audio frequency.

Also, at least in my experience, whenever ISO is omitted from EV, it's assumed
to be ISO 100 (just as musical pitches are assumed to be equal temperament from
A = 440 Hz unless otherwise specified).

> So I don't think this belongs in spotread.

> It seems pretty trivial to set up a calculation (ie. by hand or with a
> that would convert lux to EV given the ISO and the desired exposure.

Already did so, in fact. Yup, it's straightforward to do...it's just a bit
inconvenient to have to take a measurement, copy the value, switch to the

> (Of course you
> also have to take into account whether you are measuring light reflected from
> the
> subject, or illumination levels.)

That formula above is for incident / illumination levels. There's a separate
one for EV => cd/m^2 for reflected / luminance measurements. I didn't need that
for my purposes, so I didn't bother to look up the conversion, but from the
tables in the light meter manual it looks like it might just be a matter of
lopping off a zero.

Cheers,

b&
```