Exposure Value Numbers (EV)

Frank R · Oct 3, 2006

Thanks for clearing that up.

I normally shoot 100 speed film so I guess it has not been an issue.

It seems like when I switch to 400 I could make a new chart. It does not seem like it would be too much trouble to generate it in Excel and print it out.

Helen B · Oct 3, 2006

David H. Bebbington said:
One thing about APUG, it has me crawling about in the dusty recesses of the lumber room of my mind!

Notwithstanding that I am one of the world's worst mathematicians, and that the people who were charged with teaching me logarithms and such like were eventually led away crying, I appear to have discovered the following:

The equations "Ev = Tv + Av = Sv + Bv" etc. which have been much quoted on this thread appear in my Ilford Manual of Photography 6th edition and are stated to have formed part of the APEX (Additive Photographic Exposure) system, which as was mentioned elsewhere was described in the relevant ASA standard of 1960 governing film speed and exposure. Crucially, as I understand it, and "understand" may not be the right word, for the equations to work all the parameters have to be expressed as logarithms to base 2. No doubt there are appropriate tables of logs to base 2 somewhere (as opposed to the usual logs to base 10), and when dentistry without anesthetic carried out by a gorilla loses its appeal, I may well turn to reading them, but I feel the fact remains that APEX was a system which, while sound in principle, virtually no one felt was useful. The good Fred Parker may have thought it a good idea to revive the APEX system, but I can see no earthly reason, as I said before, for any else to join him!

Regards,

David

Yes David, that is the system I have been describing here, and I hope that I have given all the necessary formulae. I'm sorry to have to tell you this, but your understanding of the maths is deficient: log2 tables are not required, and never were. You can use log10 tables, but you don't have to resort to such advanced mathematics with all the cruel memories of having to sit through maths lessons when you were young. I hated them. The ability to halve and double, add and subtract will see you through, however.

Whether or not the APEX system is of use to any individual is not for me to judge. The simple theory behind it (and it is very simple) comes in handy for me now and then. The rest of the time it does no harm.

Best,
Helen

Christopher Walrath · Oct 3, 2006

And yes, David B., is right. The creativity is what we all must have as photographers to see our subject and to project our mind's image through our work. The craft, that is the technical side of things, only allow us to better get to the point of taking the picture by allowing us to bypass the rote and rite of the how tos and getting us to the task of pressing the shutter button quicker, or, should I say, more confidently. We practice more and know our equipment better so that it does not get in the way of a great photograph.

dmr · Oct 4, 2006

First of all, I want to say that this has been one of the most interesting discussions I've seen in a long time. It's really cleared up a lot of confusion, for me at least.

You often see, on the web, EV numbers attached to film speed. From what I'm gathering from this, true EV is exposure only, not related at all to film speed. (Therefore, much info on the web is either patently incorrect or based on misunderstanding, or a poor job of explaining.)

Helen B said:
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)

Ok, not to get too geeky here, but the way I see this, Av, Tv, etc., would be base 2 logarithms. Is this correct? If so, it makes a lot of sense how they are added to get the final EV numbers.

Thanks

David H. Bebbington · Oct 4, 2006

dmr said:
First of all, I want to say that this has been one of the most interesting discussions I've seen in a long time. It's really cleared up a lot of confusion, for me at least.

The possibilities for confusion are endless! When I was writing my previous posts, I double-checked every statement against a Pentax digital spotmeter, a Gossen Digiflash meter and a Rolleiflex T camera with a meter and an EV shutter. Each time, I confirmed that EV 1 = 1 sec. at f1.4 with ISO 100 film. Just now, I picked up an old Weston Master V meter from my desk, set the dials to 100 ASA and I sec. at f1.4 and hey presto! - it's showing EV 1 too BUT the meter has a larger central part to its dial which is labelled LIGHT and has numbers from 1 to 16 which are offset by 1.3 from actual EV values (which appear on this meter in a very small window), so that for example "LIGHT 1" is EV 2.3 (which is what I was talking about earlier when I said meters could be calibrated in arbitrary figures). A brilliant piece of needless confusion on the part of Weston (except that possibly it was intended for the round numbers on the "LIGHT" scale to correspond to round numbers of lux - the "1" position would be 2 lux). I'm going to lie down now.

Regards,

David

Helen B · Oct 4, 2006

dmr said:
...

Ok, not to get too geeky here, but the way I see this, Av, Tv, etc., would be base 2 logarithms. Is this correct? If so, it makes a lot of sense how they are added to get the final EV numbers.

Thanks

Yes, they are base 2 logs - though you don't need log2 tables to calculate them, of course.

David,

There's no need for confusion. The definition of Ev is clear and unequivocal. One second at f/1.4 is Ev 1 regardless of film speed. Pentax Digital Spotmeters happen to use Ev at ISO 100 for the display of brightness, which could be confusing if you didn't know how Ev was defined. They could have used Bv, which would have been simple and correct.

Best,
Helen

dmr · Oct 4, 2006

Helen B said:
Yes, they are base 2 logs - though you don't need log2 tables to calculate them, of course.

Now I'm starting to understand this!

I searched on Wikipedia and found the following formula, I thought I could cut-paste it, but it's an image, so this may look messy:

EV = log2(N^2/t)

Where N is the f-stop and t is the exposure time.

Using f16 and 1/60 and solving for EV I get 13.90689... so this passes the sanity test.

The Wiki article explains very well how this is related to (but not dependent on) film speed and luminance/illuminance.

David H. Bebbington · Oct 4, 2006

Helen B said:
David,

There's no need for confusion. ...

I'm not in fact confused, Helen, particularly after this very informative thread and your contribution to it! What I was talking about in my last post was the fact that a Weston Master V has a scale on it in large numbers which look like EV values but aren't (and also a scale in very small numbers of actual EV values). Since this meter was made (quite a while ago) manufacturers have obviously felt that using real EV numbers to calibrate their meters was more logical.

Regards,

David

dancqu · Oct 5, 2006

[QUOTES=Helen B;372920]
"The definition of Ev is clear and unequivocal."

That is true.

"One second at f/1.4 is Ev 1 regardless of film speed."

Also true.

"Pentax Digital Spotmeters happen to use Ev at ISO 100..."
Best, Helen"

Yes, ISO 100. And that is likely the sensitivity of the
meter's combined electronics. EV values are established
only at ISO 100. Other ISOs must have correction factor
applied to arrive at the true EV. Dan

Helen B · Oct 5, 2006

dancqu said:
[QUOTES=Helen B;372920]
"Pentax Digital Spotmeters happen to use Ev at ISO 100..."
Best, Helen"

Yes, ISO 100. And that is likely the sensitivity of the
meter's combined electronics. EV values are established
only at ISO 100. Other ISOs must have correction factor
applied to arrive at the true EV. Dan

Dan,

You missed off an important part of the phrase. I wrote: "Pentax Digital Spotmeters happen to use Ev at ISO 100 for the display of brightness,..."

I'm not quite sure that I see what you are trying to say. Do you now see that your earlier statement was a misunderstanding?
("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.")

Ev is not a light intensity. Bv is.

By the way, you can't measure the sensitivity of a light meter in ISO. The ISO speed measurement is based on intensity times time; and a lightmeter's sensitivity is only dependent on intensity.

Best,
Helen

dancqu · Oct 6, 2006

[QUOTES=Helen B;373305]Dan,

"You missed off an important part of the phrase. I wrote:
"Pentax Digital Spotmeters happen to use Ev at ISO 100
for the display of brightness,...""

I've a Sekonic with EV and f stop shutter speed modes. I've
a few other in camera and off camera meters and all relate
EV with ISO 100 although some do not read out in EV.
I really doubt the Pentax is the only meter that "just
happens" to use ISO 100 as the base for
indicating light intensity.

"I'm not quite sure that I see what you are trying to say. Do you
now see that your earlier statement was a misunderstanding?

("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.")

Ev is not a light intensity. Bv is."

That's too easy. Bv is a light intensity. I've only extended the
definition of a level of light to include f stop and shutter speed.
Of course related to some specific calibrated sensor such as
found in light meters.

"By the way, you can't measure the sensitivity of
a light meter in ISO." Best, Helen

They do deliver up EVs or their equivalents; f2-4sec? And at
some specific sensor output. Dan

Helen B · Oct 6, 2006

Dan,

I'm still not sure what you are trying to say. To help me, please could you state how you define or calculate Ev, and/or what you think is wrong with

Ev = Tv + Av = Bv + Sv

Thanks,
Helen

Christopher Walrath · Oct 6, 2006

EV - Exposure Value / Index
TV - Shutter Speed
AV - Aperture Size
BV - Brilliance of Subject
SV - Sensitivity of Film

Helen B · Oct 6, 2006

It's OK Christopher, I know exactly what the standard definition of EV is (which I sincerely hope would be evident from reading this thread), I just don't know what Dan's definition is.

Thanks,
Helen

BradS · Oct 6, 2006

I think that in dan's mind, Sv is always fixed at 5 (ASA100). He cannot get his head around the idea that Sv is variable. He wants Sv to be fixed instead of varying with film speed. Instead, he likes to think in terms of applying a correction factor after the fact....

In Dan's mind, the equation is:

Ev = Av + Tv = Bv + 5 + (correction factor)

where the correction factor is identically Sv - 5

I think it works out the same.

RalphLambrecht · Oct 6, 2006

Chan Tran said:
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.

Chan

I like it, but how about LV13@ISO100/21 instead?

RalphLambrecht · Oct 6, 2006

Helen B said:
It's OK Christopher, I know exactly what the standard definition of EV is (which I sincerely hope would be evident from reading this thread), I just don't know what Dan's definition is.

Thanks,
Helen

I do admire your patience!

bruce terry · Oct 7, 2006

Formula-Smormula! Scary stuff this math. Think I'll simply start calling the number at the end of the pointy-thing-in-the-window of my Pentax Analog my BV rather than my EV and keep on truckin.

Bruce

dancqu · Oct 8, 2006

bruce terry said:
...the pointy-thing-in-the-window of my Pentax Analog... Bruce

Interesting you should mention the "pointy-thing-in-the-window.
I've a Olympus OM1n with one and an OM2000 with a centered
green light. I suppose the green light was meant as an
improvement but I like the pointy-thing more so.

Are you converting your camera's f stop and shutter speed
settings to an EV value?

I'm giving some little time to formulating a quick way to do
just that; derive the EV from those two settings. Dan

bruce terry · Oct 8, 2006

... Are you converting your camera's f stop and shutter speed
settings to an EV value?

Dan - The other way around with me. I've got one of those Calumet Zone System stickers stuck on the bottom arc of the fixed dial plate adjacent to the movable outer EV (BV) ring. I usually spot read the shadow area I want to place in IV, set the outer ring equivalent of the metered BV# opposite IV on the sticker, doublecheck any other important part of the image that might fall wrong, and adjust if I must. That done, I THEN go to the inner dial for the correct speed/aperture combinations for my preferred exposure of my preferred zone (assuming film speed has been preset).

Love that big, visual, analog dial - interplays with my dense head much better than those silly digits do.

Bruce

Chan Tran · Oct 9, 2006

RalphLambrecht said:
Chan

I like it, but how about LV13@ISO100/21 instead?

I am not arguing which term we should use as I don't really know. What I meant is that when a camera manufacturer said that their AF system would work at EV-1@ISO100 I know just how bright the subject has to be in order for it to work. I don't see the term LV or even BV used often, which could be more correct.

dancqu · Oct 10, 2006

Chan Tran said:
What I meant is that when a camera manufacturer said
that their AF system would work at EV-1@ISO100 I know
just how bright the subject has to be in order for it to work.
I don't see the term LV or even BV used often, which
could be more correct.

Is that EV minus 1 or EV 1? EV is correct; the industry standard.
An EV minus 1 would be F1-2sec; a very dim light. Perhaps ISO
100 was adopted because it is a medium speed for films and
at the time of it's adoption ISO 100 made for a practical
application of the then sensor technology. Dan

Steve Smith · Oct 10, 2006

Helen B said:
You missed off an important part of the phrase. I wrote: "Pentax Digital Spotmeters happen to use Ev at ISO 100 for the display of brightness,..."

I think some of the confusion may be that if a lightmeter says e.g. EV13 @ ISO100 it is not saying that the light level is EV13 @ ISO 100 but is saying that if you are using ISO100 film, then a camera setting of EV13 is required.

My Rolleicord enables you to set an aperture/shutter combination and it shows you what the EV setting is. If you adjust the shutter speed lever, the aperture lever moves as well maintaining the original EV setting.

Steve.

Helen B · Oct 10, 2006

Steve Smith said:
I think some of the confusion may be that if a lightmeter says e.g. EV13 @ ISO100 it is not saying that the light level is EV13 @ ISO 100 but is saying that if you are using ISO100 film, then a camera setting of EV13 is required.

Steve,

Aren't those just two different ways of expressing the same thing? The only thing I'm confused about is Dan's definition of EV, though Brad's suggestion looks right.

I agree with Chan Tran - everybody understands sensitivities quoted as 'EV -1 @ ISO 100' for example because that is the standard practice in the industry, and replacing that with the equivalent 'BV -6' would not be as readily understood. When the APEX system was formulated, it wouldn't have been difficult to make BV = 0 for EV = 0 at ISO (then ASA) 100; ie SV = 0 for ISO 100 instead of SV = 0 for ISO 3.1

Best,
Helen

dancqu · Oct 13, 2006

Steve Smith said:
I think some of the confusion may be that if a lightmeter says e.g.
EV13 @ ISO100 it is not saying that the light level is EV13 @ ISO
100 but is saying that if you are using ISO100 film, then a camera
setting of EV13 is required. Steve.

Actually any EV or fraction there of at a specific ISO is a
specific level of light. That specific level of light derives from it's
ISO connection. Not very well put but, IIRC, Helen B has made
it some what more clear with her explanation of the term Bv
and Sv. Both of which are on the metering side of the
equation. I'll add, an equation with which I have no
argument save for Sv being equal to five.

Then again Ev = 0 = Av + Tv when f = 1 and sec = 1
at ISO 100. Or the level of light is f1-1sec. Again ISO
100. Dan

Exposure Value Numbers (EV)

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