Re: EV measurements

["Mark W. Johnson" <markwjohnson@xxxxxxxxxxxxx>]

>ket khan <danzig@xxxxxxxxxxxxxx> wrote:
>I've only just gotten down to reading the manual for my newly bought
>Minolta 4F lightmeter....
>and asks about EV vs stops vs light ratios

>"Jason Revell (in5)" <jason.revell@xxxxxxxxx> responded with a generally >correct answer, except for the statement about square roots and squares.

Technically, stops relate to the F-stop of the lens. Each time you open
the lens 1 stop, you double the light. The EV is similar to stops, but
accounts for exposure time, ISO setting, and stops.  In other words, if
you open the lens 1 stop, or double the exposure time, or double the ISO
rating of the film, the image will be 1 EV brighter.

The EV relates to the ratios by powers of two, not square roots. That
is:
ratio:1  =  2^EV  [2 (raised to the power) EV)]

The F-stop of a lens is important because it gives a measure of image
brightness that is independent of focal length, but results in the odd
values like 5.6 between the powers of 2 like 4 and 8.  If you square the
F-number you can compare the ratio of light between two F-stops.  In
practical use, it is much easier to think of each stop as doubling or
halving the light.  The following table does not have much practical
use, but gives a sense of how the numbers relate to each other.

F	F^2	EV(1)	2^EV	Ratio
--------------------------------------
1	1	0	  1	  1:1
1.4	2	1	  2	  2:1
2	4	2	  4	  4:1
2.8	8	3	  8	  8:1
4	16	4	 16	 16:1
5.6	32	5	 32	 32:1
8	64	6	 64	 64:1
11	128	7	128	128:1
16	256	8	256	256:1
--------------------------------------
(1) the relative EV

The dynamic range encompassed by the normal range of photography is
pretty staggering.  If you compare:
ISO 800 * F 1.4 * 1/15th second
vs
ISO 25 * F 16 * 1/1000th second
That translates to (using the term stops loosely):
5 stops of film speed,
7 stops of lens speed, and
6 stops of shutter speed
or 18 stops total.  The ratio in this case is 2^18 or a staggering
250_thousand fold difference in image exposure.  I shot some beautiful
shots of comet Hale-Bopp with ISO 800, @F1.8, for 10 to 30 seconds,
which pushes this ratio up to 26 or so stops, or a ratio of 60_million:1




Jason Revell (in5) wrote:
> 
> 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
> *
> ****
> *******
> ******************************************************
> *  To remove yourself from this list, send:          *
> *         UNSUBSCRIBE INFRARED                       *
> *       to                                           *
> *         MAJORDOMO@xxxxx                            *
> *----------------------------------------------------*
> *   For the IR-FAQ, IR-Gallery and heaps of links:   *
> *  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
> ******************************************************
*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************