RE: EV measurements

["Jason Revell (in5)" <jason.revell@xxxxxxxxx>]

The EV is ineffect the same as the 'stops', so a EV difference of 2 means 2
stop's diference, but when it get';s expressed as a ratio we are looking at
the %age difference between the two readings.

So if we take the 2 stops above we are looking at half the light (1 stop)
and Half again, a quater (2 stops) so the ratio is 1:4.  So the square root
of your ratio is the same as the value in stops and the square of your stops
is your ratio.

Now why this has to be on you light meter I am not sure as I was under the
impression that we all used stops to measure the difference in light/tonal
values rather than ratio's, but I guess it's an easy feature to build in.

Jason
Conservation of Fine Art
Univ. Northumbria
 ----------

Apparently you can measure the EV (exposure value) between 2 different
light sources. This reading is given in numerical values in terms of a ratio

+/-EV1    2:1
+/-EV1.6  3:1
+/-EV2    4:1
+/-EV3    8:1

What I would like to ask is , is this ratio such as  2:1 , 3:1 meant to be
taken in terms of stops of light difference ? As in 2:1 would mean 2 stops,
3:1 would mean 3 stops ?

I doubt that it is, as the manual makes no reference linking the EV to
measurement in stops. But I'd still like to ask you folks here just in case
anyone knows ;)

I just did a test, and my meter gave me an EV of -3.5 (comparing a normally
metered reading , to one where I placed a Hoya R72 on the luminance ball)

Going by their table 3.5 should be something more than 8:1 (EV3=8:1). But I
noticed coincidentally that when I measured using F stops rather than EV

Normal metered reading       : F 16 /7
Metered with R72 on the ball : f5.6 /2

16 /7 - 5.6 /2 = 3.5 stops exactly

Is this just coincidence or is the EV value displayed supposed to indicate
the number of stops of difference in the light ? I'm more than a little
confused ;)

Yours sincerely,
Ket
Email: danzig@xxxxxxxxxxxxxx
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