Exposure Value Numbers (EV)

[Christopher Walrath]

OK. I have learned many things over the last three years and am continually learning more. But EV numbers elude me, just beyond my grasp. Here is what I need to know. And long wordy explanations are welcome and encouraged.

According to Ansel Adams' 'The Negative', f11 @ 1/60 exposure is equivalent to EV13. As are f8 @ 1/125, f16 @ 1/30, and so on. I know that, correspondingly, f11 @ 1/30 (one stop increased exposure) would relate to EV12. f16 @ 1/60 would relate to EV14. Now. For the fun part, at least for me.

Does EV13 relate ONLY to f11 @ 1/60 and other related combinations? Or does EV13 shift exposure settings depending on the film being used (ISO rating)? To which ISO rating do these standards relate;50, 100, 200, 400, 800? I thought that EV13 was to denote a specific range of camera settings in line with 11@60, regardless of ISO and you simply adjusted the EV you used +/- however many according to which ISO you used. Which begs answer again to the which ISO does the EV Index relate to.

Any and all answers welcome. Thank you.

 

[Donald Miller]

OK. I have learned many things over the last three years and am continually learning more. But EV numbers elude me, just beyond my grasp. Here is what I need to know. And long wordy explanations are welcome and encouraged.

According to Ansel Adams' 'The Negative', f11 @ 1/60 exposure is equivalent to EV13. As are f8 @ 1/125, f16 @ 1/30, and so on. I know that, correspondingly, f11 @ 1/30 (one stop increased exposure) would relate to EV12. f16 @ 1/60 would relate to EV14. Now. For the fun part, at least for me.

Does EV13 relate ONLY to f11 @ 1/60 and other related combinations? Or does EV13 shift exposure settings depending on the film being used (ISO rating)? To which ISO rating do these standards relate;50, 100, 200, 400, 800? I thought that EV13 was to denote a specific range of camera settings in line with 11@60, regardless of ISO and you simply adjusted the EV you used +/- however many according to which ISO you used. Which begs answer again to the which ISO does the EV Index relate to.

Any and all answers welcome. Thank you.

This will hopefully not be wordy (the subject doesn't merit a long explanation).

A given EV is a number that transcends all combinations of exposures of equal amounts of light passed to the film.

The EV is not affected by the ISO of the film used.

 

[Christopher Walrath]

So EV is light transmittance, not light sensitivity, right?

 

[Donald Miller]

So EV is light transmittance, not light sensitivity, right?

Correct.

 

[Anupam Basu]

For ISO 100, EV 13 is two stops below sunny 16. For ISO 200, it's three stops below and so forth.

-A

 

[Christopher Walrath]

Thank you very much for the answers. If someone would like to go into detail for those who follow who might want more definition, please, feel free.

 

[Helen B]

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

 

[RalphLambrecht]

In 1955, the term exposure value (EV) was adopted into the ISO standard. The purpose of the EV system is to combine lens aperture and shutter speed into one variable. This can simplify lightmeter readings and exposure settings on cameras. EV0 is defined as an exposure equal to 1 second at f/1. EV tables cover typical settings, and with it, a lightmeter EV reading can be translated into a variety of aperture and shutter speed combinations, while maintaining the same exposure. Each successive EV number supplies half the exposure of the previous one, following the standard increments for film speed, aperture and exposure time. This makes EV numbers an ideal candidate to communicate exposures in the Zone System, since zones are also 1 stop of exposure apart from each other.
Most lightmeters have an EV scale in one form or another. Usually, a subject reading is taken and an EV number is assigned to that reading. This EV number can be used for exposure records and an appropriate aperture/time combination can be chosen depending on the individual image requirements. Some camera brands allow for this EV number to be transferred directly to the lens. Aperture ring and shutter speed settings can then be interlocked with a cross coupling button, and different combination can be selected, while maintaining a given EV number and constant film exposure. On Hasselblad lenses, this button is appropriately referred to as the ‘reciprocity lock’.
EVs are shorthand for aperture/time combinations and, therefore, independent of film speed. However, a change in film speed may require a different aperture/time combination and, therefore, a change in EV. As an example, let’s assume that a spotmeter returned a reading of EV10 for a neutral gray card, and a moderate aperture of f/8 is chosen to optimize image quality. Looking at an EV table, we see that a shutter speed of 1/15 s would satisfy these conditions. Let’s further assume that we would be much more comfortable with a faster shutter speed of 1/60 s, but we don’t want to change the aperture. The solution is a change in film speed from ISO 100/21 to 400/27, where the faster film allows f/8 at 1/60 second. Again looking at an EV table, we see that this combination is equal to EV12. Changing the film speed setting on the meter from ISO 100/21 to 400/27 will result in a change of measured EV to maintain constant exposure.
Some meters make fixed film speed assumptions while measuring EVs. The Pentax Digital Spotmeter, for example, assumes ISO 100/21 at all times. This meter will not alter the EV reading after a film speed change, and due to its particular design, this does not cause a problem. However, it is important to know that some meters simply return a light value (LV) instead of an exposure value (EV). We can still use their exposure recommendations in form of aperture and shutter speed, but LVs are only numbers on an arbitrary scale, measuring subject brightness and must not be confused with EVs.

 

[dancqu]

[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

 

[David H. Bebbington]

A rather simpler explanation:
Deep confusion has resulted from sloppy use of terminology (mainly by camera and meter makers).

Exposure value (EV) strictly speaking has nothing to do with any light level, it is simply a number and a shorthand way of expressing the fact that going one shutter speed faster and one aperture larger (or vice-versa) gives the same exposure. EV 14, for example, is ALWAYS 1/125 @ f11 (or 1/60 @ f16, or 1/250 @ f8, etc.).

EV 1 is 1 second at f1.4. It is possible to say that the light level that requires an exposure of 1 second at f1.4 WITH ISO 100 FILM has a "Light Value" (LV) of 1. Unfortunately, the term EV is used for everything, even when LV is meant. Meters are stated to be able to read down to EV 0 (1 second at f1 with ISO 100 film, etc.).

Regards,

David

 

[Chan Tran]

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.

 

[Frank R]

Go to this link, read it, and bookmark it:

http://www.fredparker.com/ultexp1.htm

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.

 

[Helen B]

[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

 

[Helen B]

Go to this link, read it, and bookmark it:

http://www.fredparker.com/ultexp1.htm

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

 

[Frank R]

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

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.

 

[Helen B]

Frank,

"Will it affect the quality of my photographs?"

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

 

[dancqu]

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

 

[Helen B]

"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

 

[BradS]

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).

 

[BradS]

see for example:

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.

 

[David H. Bebbington]

This may lose me the respect of my peers and all right-thinking people, but I have managed to take pictures for over 50 years without any knowledge whatsoever of Fred Parker. Now that I have heard of him, and have skimmed through the so-called exposure guide on his website, he appears to me to be the most spectacular obsessive and obfuscator I have ever encountered, with an argument whose main thrust seems to be that all exposure meter makers are idiots and that the only true way to work out exposure is with his voluminous tables.

People, you do not need any of this. It will not help you take better pictures, it will not give you any genuine insight into sensitometry or image formation.

To repeat very briefly:
An exposure meter measures light intensity, It can be (and many meters historically have been) calibrated in random figures. It is, however, also possible to calibrate in EV figures, which are the shutter speed/stop combinations which a film would need if:
1) It was of ISO 100 speed
2) You had chosen to actually rate the film at this speed
3) The meter was aimed at an 18% gray area.
If 1), 2) and 3) apply, and your camera shutter is calibrated in EV, you could apply the meter reading directly to the camera and get a good exposure. If not, you need to apply a correction (actually an offset) for the actual film speed you are using and the actual zone you are metering and extract a reading in the form of f/stops and shutter speeds. Some meters have dials which do all of this (such as the Lunasix I once owned), all others allow different film speeds to be set but may require you to mentally apply zone compensation. For example, if you meter an area and want it to be zone 3 (instead of the default 18% gray or zone 5), you increase the EV number by 2 (i.e. cut the exposure by 2 stops) and transfer say EV 15 to the meter scale instead of EV 13 (as I do with my Pentax Spotmeter or my Gossen Digiflash, for example).

Let those who have a passion for sensitometry and arcane formulae go into this subject as deeply as they like - from the point of view of practical photography, I guarantee you will be none the wiser afterwards and that what I have written above and in my previous posting is ALL YOU WILL EVER NEED TO KNOW!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Regards,

David

 

[Frank R]

Frank,

"Will it affect the quality of my photographs?"

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

Helen: Thanks for clarifying that. I am glad I don't have to change anything drastically.

I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.

I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?


David: I agree somewhat with your assessment of the Parker site. Although the EV Table B shows in chart form what you stated in your previous post. People learn differently, and presenting the relationship in a graphical format may help some newcomers grasp the relationships easier.

I do see the table as a little crowded too. That is why I copy them and delete a lot of extra stuff (like 800 speed film and EV numbers below say, 12). I use it, along with my light meter, as a starting point for determining the exposure I want.

 

[BradS]

Helen: Thanks for clarifying that. I am glad I don't have to change anything drastically.

I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.

I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?

The example you give is only half of the equation. You're missing the fact that you must balance the equation...

As Helen has very plainly stated twice now, the equation is:
Ev = Tv + Av = Sv + Bv

look carefully, Your example is: 1/100s, f/16. Shutter speed and f-number. These correspond to the symbols Tv and Av respectively. If you look up (or calculate) the Tv for 1/100sec you find that it is: Tv=7, similarily, for f/16, Av=8 so, Tv + Av = 7 + 8 = 15. Now, to be useful, this must be equal to the other side of the equation...so, we require that

15 = Sv + Bv

Sv is fixed by the film speed. For ASA 100 speed film, Sv = 5. Thus, for this situation to produce something like properly exposed negative, one would need to be in light with brightness value, Bv of 10. In which case you would have:

Ev = Tv + Av = Sv + Bv
15 = 7 + 8 = 5 + 10

If your film speed were say, ASA 400 (which corresponds to Sv = 7), then you could shoot in less bright light at the same shutter speed and aperature.

to wit...

Ev = Tv + Av = Sv + Bv
15 = 7 + 8 = 7 + 8

The Sv for ASA400 film is 7.

I am sure that you will find that Fred Parker has made some simplifying assumptions in producing his "guide". I think thatyou may find that he has adjusted everything for ASA 100 film speed?

 

[Helen B]

Helen:...
I have a concern though: this morning I looked at two old cameras that have EV scales on the shutters. If I set the shutter at f16 and 1/100 sec (the sunny 16 rule), the pointer indicates an EV value of 15, just like the EV chart that I referenced shows.

I am sure your calculations are entirely accurate. But do you think another standard was adopted along the way for consumers?
...

(Brad has already answered this well, so this is just another way of saying it.)

No, it isn't another standard, it is a misuse of the existing one, perhaps with the intent of simplicity (mistaken in my view, because the use of BV is as simple*). f/16 and 1/125 is indeed EV 15, but it is EV 15 at any film speed, not just ISO 100. On FP's chart f/16 and 1/125 can be between EV 17 and 10, depending on film speed. That is incorrect. f/16 and 1/125 is EV 15. It is not any other exposure value.

What Fred Parker calls EV is actually a scene brightness value. It is numerically equal to EV at ISO 100, but it is not EV plain and simple.

If you scroll down and look at his illuminance table (which is correctly labelled 'EV at ISO 100' rather than just 'EV') you can quickly deduce that he uses an average scene reflectivity of 13.5% to generate the table.

*Kodak avoid this altogether by using letters to designate scene brightness, such as in their exposure calculator dial in the Pro. Photoguide - to which FP's table bears a strong similarity.

Best,
Helen

 

[David H. Bebbington]

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