When EV is used to expess the light value LV it should have a film speed attached to it i.e. EV13@ISO100. And then sometimes EV is used to express exposure differences i.e. +2EV compensation. But yes the first definition of EV is like what Ralph post said.
[QUOTES=Helen B;371427]
"The Ev represents both the shutter speed/aperture
combination and the film speed/scene brightness combination."
Ev = Tv + Av = Sv + Bv"
Can that be? I must lack a full understanding.
"Time value; Aperture value; [Film] Speed value; and Brightness value.
Tv = 0 for a shutter speed of one second.
Av = 0 for an aperture of f/1.
Sv = 0 for a film speed of ISO 3.125 arithmetic (and hence Sv = 5 for
ISO 100).
Bv = 0 for a scene brightness of 1 foot-lambert. Best, Helen"
And, EV 0 is an exposure value of f1, 1s, at ISO 100.
Sv = 5 for ISO 100? What do I not understand? Dan
Dan,
“EV 0 is an exposure value of f1, 1s, at ISO 100.”
As explained in a few different ways here, Ev does not represent all three (aperture, time and film speed) at once. So Ev 0 means one second at f/1 for any speed of film.
“Ev = Tv + Av = Sv + Bv
Can that be?”
Yes, that is it, quite simply.
Ev = Tv + Av is the formula that describes the well-known expression of Ev as being a description of a set of aperture/time combinations.
Tv + Av = Sv + Bv is what a light meter does. It combines the film speed and the scene brightness to produce a set of aperture and time values that satisfy the equation.
Brightness value, Bv, is a measurement of scene luminance that could be used in place of ‘Ev 0 at ISO 100’ for example. That is Bv –5. However, that aspect of the APEX system doesn’t seem to be very well known, and this thread is evidence of that – the general understanding is that Ev describes aperture and time, and the brightness / film speed part seems not as well known. Maybe things would be different if 100 had been chosen as the zero value for Sv. Then ‘Ev 0 at ISO 100’ would correspond to Bv 0. That would, however, move Bv 0 away from the convenient value of one foot-lambert.
Best,
Helen
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Scroll down until you get to the Exposure Factor Relationship Chart B. You will see it all laid out in one spot.
Scroll up to Chart A for the Exposure Value Chart for further clarification.
Frank,
That appears to be a different, and I think confusing, use of EV than the one described in the original German description and the later ASA standard.
Now I understand where Dan's misunderstanding arises from. I think that Fred Parker made a big, confusing mistake by changing the ASA definition of Ev to that of Bv (and he also offset it by 5, compared to the ASA system, but that's OK).
Best,
Helen
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Some of my snaps are here and here.
That appears to be a different, and I think confusing, use of EV than the one described in the original German description and the later ASA standard.
Now I understand where Dan's misunderstanding arises from. I think that Fred Parker made a big, confusing mistake by changing the ASA definition of Ev to that of Bv (and he also offset it by 5, compared to the ASA system, but that's OK).
Best,
Helen
Helen:
I am the one who is confused now.
I found Fred's site a few years ago and it helped me understand the idea of Exposure Value. I have even copied the charts, deleted un-needed parts, reduced them in size, and taped them on the back of my old cameras to use as a reference.
Now you are telling me that Fred is mistaken. Will it affect the quality of my photographs? Can the information be corrected and posted in a format as simple as Fred's? I would hate to have to do calculations in my head while out in the field taking pictures.
Fred's table is referenced by a lot of people so if something needs correcting I am all for it.
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No, the difference in the meaning of EV will not affect the quality of anyone's photographs, nor does it affect the value of the advice Fred gives about scene brightness.
It would be very easy to change his method to be entirely in line with the original APEX method. What he calls EV could be called BV, with the subtraction of five - so what he calls 'EV 0' would become 'Bv -5', for example. The text could also be re-written with the discrepancies removed.
"Can the information be corrected and posted in a format as simple as Fred's?"
Yes, easily. I'd be happy to draft suggested changes if it wouldn't upset anyone.
Best,
Helen
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Some of my snaps are here and here.
As explained in a few different ways here, Ev does not
represent all three (aperture, time and film speed) at once.
So Ev 0 means one second at f/1 for any speed of film.
Best, Helen
I may be missing something but I think I will disagree.
Let's see if I can make my case.
With out instruments there would be no EVs but there
are instruments, light meters. They are calibrated against
standards to such and such sensitivity. I think as one
post mentioned, the electronic film and associated
circuity are adjusted to an ISO equivalent of 100.
The measure of a lights intensity is expressed
in terms of F stop and shutter speed.
But the EV does not change as Ralph suggests with
changes in film speed. EV 0 = a light intensity of
F1 and 1 second at an instrument sensitivity of
ISO 100. That is the definition of EV 0 and
the condition under which it is measured.
Or, any other intensity. Dan
"EV 0 = a light intensity of
F1 and 1 second at an instrument sensitivity of
ISO 100. That is the definition of EV 0 and
the condition under which it is measured."
Dan,
Where do you get that definition from? It is incorrect. This is how Ev is defined:
Ev = Tv + Av = Sv + Bv
2^Av = N^2 (N is f-Number)
2^Tv = 1/T (T in seconds)
2^Sv = S/π (S is ASA film speed, now ISO)
2^Bv = Bl (Bl in foot-lamberts) = B/π (B in candles per square foot)
with the zero points I gave before. That is not my opinion, it is how it is described in the original ASA standards: PH-2.5 and PH-2.12.
Best,
Helen
__________________
Some of my snaps are here and here.
The Ev represents both the shutter speed/aperture combination and the film speed/scene brightness combination.
Ev = Tv + Av = Sv + Bv
Time value; Aperture value; [Film] Speed value; and Brightness value.
Tv = 0 for a shutter speed of one second.
Av = 0 for an aperture of f/1.
Sv = 0 for a film speed of ISO 3.125 arithmetic (and hence Sv = 5 for ISO 100).
Bv = 0 for a scene brightness of 1 foot-lambert.
Best,
Helen
Helen has it exactly correct. There are many references in the literature to this exact scheme (few are this concisely stated however).
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Robert B. Rhode and Floyd H. McCall, Introduction to Photography - 1st ed., (c) 1965, MacmillanCompany, pages21-28 and especially page 25. By the time the second edition was published, it appears that built in light meters were comon place and so, the need to discuss exposure minimized.
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