JohnFrim Loc: Somewhere in the Great White North.

In a recent post I felt compelled to make reference to the subtle distinction that while some folks think exposure is determined by f-stop, shutter speed AND sensor sensitivity (ISO), purists tend to point out that ISO is NOT part of "exposure". A day later an article on Exposure Value System (EVS) appeared on the website PictureCorrect, so I thought I would share that with the UHH community.

No doubt some folks may take issue with a few of the statements (e.g., "An EV is therefore a convenient “system” for describing the quality of light."; I am not sure that quality is the best term for that) but I am not trying to start a controversy. I just thought this was a fairly simple description of EV or EVS for those not familiar with the concept.

The article is at http://www.picturecorrect.com/tips/the-exposure-value-system-evs/

Below is the text of the article without the nice pictures and graphics.

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The Exposure Value System (EVS)

Posted: 08 Dec 2015 10:56 PM PST

In order to understand how EV settings work, you must first be familiar with the concept of f-stop and shutter speed settings.

Aperture

Briefly, the smaller the f-stop number, the larger is the aperture size it represents. Each aperture setting is either half or twice the size of its neighbor. So f/8, for example, is half the size of f/5.6, and f/5.6 is twice the size of f/8.

Shutter Speed

Similarly, shutter speed steps are also either half or twice as fast as their neighbor. For example, 1/30 is twice as long as 1/60, and 1/60 is half as long as 1/30.

A number of different combinations of aperture size and shutter speed all produce the same degree of exposure. For example, f/5.6 at 1/60 of a second provides the same level of light exposure as f/8 at 1/30 (where the amount of light halves, and the duration of exposure doubles).

Of course, there are even more permutations: f/8 at 1/30 is also the same as f/11 at 1/15 (half the light for twice as long), or f/4 at 1/125 (four times the light for a quarter of the time), and f/2.8 at 1/250 (eight times the light for an eighth of the time), etc.

A one step change to either setting is known as a “one stop” change.

The History of Exposure Value

To simplify the process of setting alternative aperture and shutter speed combinations, a German camera shutter manufacturer called Friedrich Deckel first developed the Exposure Value (EV) concept in the 1950s. The likely impetus for this was the rise in popularity of color film, which required greater exposure accuracy than black and white film photography (modern 35mm color film started to become available in the mid 1930s).

In 1954, numerous camera (and shutter) manufacturers adopted Deckel’s Exposure Value Scale (EVS), including Hasselblad, Kodak, Konica, Olympus, Ricoh, Seikosha, and Voigtländer, to name but a few.

They introduced lenses with coupled shutters and EV scales, such that, after setting the exposure value, adjusting either the shutter speed or aperture made a corresponding adjustment in the other to maintain a constant exposure.

When camera models with built-in light meters started to emerge, some also metered against an EV scale (as opposed to an aperture or shutter speed scale), and correct exposure was accomplished by transferring the meter’s EV reading to the lens, through adjustment of lens apertures and/or shutter speed settings.

What is Exposure Value?

The Exposure Value (EV) is a numerical scale that represents a combination of a camera’s shutter speeds and f-numbers, such that all combinations yield that the same exposure have the same EV value.

Exposure Value scale steps also align with intervals on the photographic exposure scale. In other words, an increment of one step on the EV scale represents a one step (often referred to as a stop) increase in exposure, and conversely a one step decrease corresponds to a one step reduction in exposure. For this reason, some cameras had, and still have, exposure compensation features that are graded as EV steps (e.g. +/- 2 EV).

For example, if EV 9 corresponds to f/4 and 1/30 of a second, EV 8 is f/4 at 1/15 of a second, and EV 10 is f/4 at 1/60 of a second (plus any other combination of settings that produce the EV scale value).

The EV scale starts at 0, which represents a 1 second exposure at f/1.0. Lenses with an aperture that big are rare, but it’s the same as a 2 second exposure at f/1.4, or a 4 second exposure at f/2, etc.

EV 15 equates to full sunlight with distinct shadows, while EV -4 would be a scene lit by a full moon. An EV is therefore a convenient “system” for describing the quality of light.

The EV scale can thus be used as a rough guide to exposure settings in the absence of a light meter. So EV 14 is hazy sunlight with soft shadows, EV 13 is cloudy bright with no shadows, 12 is overcast, and so on (for a 100 ISO film).

While the EV scale is still in use today (and has some merits in describing lighting conditions), it fell from favor as a means of setting a camera exposure toward the end of the 1960s, when meter coupling became more common, and removed the need to manually transfer a meter reading to lens settings, or even set anything at all (with the camera doing this automatically). In effect, the importance of correct color exposure had been automated, leaving the way clear for casual photographers to concentrate on other aspects of the craft.

While the EV was (and still is) a simple method of expressing two variable settings (aperture and shutter speed) as a single number, the problem with this generalization is that it ignores some fundamental principles of photography: the f-number determines the depth of field and the shutter speed determines the amount of motion blur.

About the Author:
Film camera collection. John A Burton is a film photographer.

Rongnongno Loc: FL

To throw a monkey wrench in all that you have the ISO that adds a third dimension to this expose.

ISO does not invalidate what is being said. ISO 'merely' shift the table of exposure.

JohnFrim Loc: Somewhere in the Great White North.

Correct, Ron. You know it, I know it, hopefully others do as well. If not, there are resources one can turn to for more info.

But no doubt we will hear comments that ISO is a necessary part of the "correct exposure". That statement is also correct depending on what the term "exposure" means to the person.

Disagreements often arise because people are talking about different contexts. Sometimes they are talking apples and oranges, but at least they are talking fruit. It gets worse when the discussion is apples vs carrots (fruits vs vegetables), and hopefully it never gets to apples vs Chevrolet. But then, this is UHH, and anything goes...

corryhully Loc: liverpool uk

on the zeiss contraflex ev numbers are transferred from the meter to the ev range on the side of the lens. the aperture and shutter speed are then coupled and are moveable as one. yep iso/asa has to be set first.

ev numbers in red

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JohnFrim Loc: Somewhere in the Great White North.

Very nice piece of history. Thanks for showing it.

If I am not mistaken, the meter runs off a selenium cell, so no battery required for the meter?

corryhully Loc: liverpool uk

Yes john , no batteries required. Still working well and accurately. Probably down to the little flip door closing the meter.

charles brown Loc: Tennesse

[quote=JohnFrim]
But no doubt we will hear comments that ISO is a necessary part of the "correct exposure". That statement is also correct depending on what the term "exposure" means to the person.

Not sure why ISO would be a necessary part of the "correct exposure" Regardless of the ISO set, the relationship of shutter speed to aperture setting as described in the article would still be the same to get the correct exposure. I think that the use of Auto ISO causes some confusion inasmuch as the ISO can change from shot to shot. It sometimes puzzles me that people say they are shooting manually because they set their own shutter speed and aperture but at the same time have ISO set to auto.

BebuLamar

Because the same amount of exposure resulting in different image brightness if the sensor is set for different ISO.

JohnFrim Loc: Somewhere in the Great White North.

That is exactly the point I was alluding to. For those who understand EV they accept that, as the article points out, aperture and time are the only components that comprise "exposure". But we have all heard of the "exposure triangle" that brings ISO into the picture (ooohhh, pun there!). In that discussion the image file on the card is said to have been made with the "correct exposure" only when the ISO was set to an optimal value.

As for shooting "manual" but with Auto-ISO, I think I and others beat that to death (yet again) in a previous thread.

JohnFrim Loc: Somewhere in the Great White North.

Right. The same exposure will lead to image files of different brightness if the ISO is changed. Hence, some will consider those files as representing over or under exposure for the chosen sensitivity of the sensor. It really is an apples and oranges discussion when people are not talking about precisely the same thing. Semantics and hair splitting sometimes matter.

mallen1330 Loc: Chicago western suburbs

Methinks that is why ISO was not considered part of the calculation. When you put a new roll of film in, the first thing you do is set (or check) the ISO value on the little shutter speed wheel. From that point on, it's not something you can adjust, until you change the film.

BebuLamar

You're right but that's not the reason! Take an example. Two people sunbathing at the same location and at the same time. They both received identical exposure to sunlight but one has skin burnt and the other doesn't. That because one is more sensitive to sunlight but both received the same exposure i.e same intensity for the same length of time.

JohnFrim Loc: Somewhere in the Great White North.

ISO is not part of the calculation because it is not part of the definition of EV. Check out the explanation in Wikipedia at http://en.wikipedia.org/wiki/Exposure_value where just above the first table they explicitly state "EV corresponds simply to a combination of a shutter speed and an aperture setting, independent of any ISO setting."

Note the differences in the equation definitions between EV as a camera setting based on f-number and exposure time and H-sub-v as the photometric exposure at the sensor as described in the first two sections of the reference. The latter depends on E-sub-v and exposure time. The quantity E-sub-v is the luminance at the camera sensor plane, and that quantity is determined by both the aperture setting AND the luminance of the scene.

To further appreciate the differences in these parameters, note that exposure time is in the denominator of EV but in the numerator of H-sub-v. Thus, for a given f-stop, a longer "time" yields a lower value of EV but a higher value of H-sub-v. The source of confusion or ambiguity is because, "Common practice among photographers is nonetheless to use “exposure” to refer to camera setting as well as to photometric exposure."

This thread has gotten a bit more technical than I expected, and most photographers probably don't care about such details because they know what they mean when they say exposure or exposure value. For those who do like getting into the meat of things, it is important that you use standardized terms when arguing a point.

mallen1330 Loc: Chicago western suburbs

Right! I understand. I'm just saying that with digital, we (I), can adjust aperture, shutter speed, AND ISO for each shot in order to get the best image. With film, I only adjust the first two.

Understanding EV is important, and thanks for posting the definition and history. In practical use, with my film camera, the proper exposure -- the proper EV for a given shot -- changes depending on the speed of the film I'm using. Yes, ISO is not part of the EV calculation - true. And, was not a shot to shot variable in the early semi-automatic cameras that adjusted F stop or shutter speed with their built-in light metering. But, with digital, it is now.

I also understand that this is all beside the point of the original post. I apologize for that.

BHC Loc: Strawberry Valley, JF, USA

Please note the parenthetical phrase in the referenced article. In fact, the entire EV system is, as reported earlier, based on an exposure of 1 second at f/1.0, BUT (there is more to the original definition) at a film exposure speed of ASA 100, as shown in the second paragraph of:

http://www.dpreview.com/glossary/exposure/exposure

For a graphical representation (spreadsheet), go to:

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