Exposure understanding and definition

[Bill Ferris]

When talking about the absence of Exposure Compensation dial, the Zeiss manager explained: "If a photographer is manually controlling shutter speed and aperture, then the only way they have of further affecting exposure is ISO. So they can use the ISO dial as exposure compensation, effectively."

If you think about it carefully, this statement is absolutely correct.

It's not correct. It's wrong. But the guy who said it, is a suit. So, what do you expect?

If you set the shutter speed and the aperture, then changing the ISO will have exactly the same effect on the resulting image as if you change Exposure Compensation when in one of the auto-exposure modes.

Just think about it!

The only thing that is "wrong" is the use of the word "exposure", which is being used in one of its various meanings, but not one of the meanings accepted in scientific circles.

The error was using "affecting" rather than "compensating for." ISO has no effect on exposure.

 

[riveredger]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure.

You are saying that exposure was predicated on the film speed, but that film speed was never understood as a part of exposure. Can you reconcile that contradiction?

 

[Iliah Borg]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure.

You are saying that exposure was predicated on the film speed, but that film speed was never understood as a part of exposure. Can you reconcile that contradiction?

There is no contradiction.

 

[Michael Fryd]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure.

You are saying that exposure was predicated on the film speed, but that film speed was never understood as a part of exposure. Can you reconcile that contradiction?

Film speed is not part of the exposure, however it is common for the photographer to match the exposure to the speed of the film.

Consider two 35mm film cameras, both pointing at the same scene, form the same location, with a 50mm lens at f/8, and a shutter speed of 1/60 of a second.

One camera is loaded with ISO 100 film, the other has ISO 400 film. Both cameras get the same exposure, even though the have films with different ISO. Thus ISO is not a factor in exposure, as changing to a different ISO film, but leaving all other factors the same, does not change the exposure.

However, the above is not a typical scenario. Photographers rarely choose their exposure independent of the film speed. More likely they target an exposure that will give them a negative with good density, and that exposure varies with film speed.

Photographers generally tried to get an exposure that matched the film speed. As it was easier to change exposure than film speed, the exposure was generally targeted to the film speed.

So while the ISO may influence the photographer's choice of exposure, it is not a factor in the exposure.

 

[Ron Poelman]

"If a photographer is manually controlling shutter speed and aperture, then the only way they have of further affecting exposure is ISO. So they can use the ISO dial as exposure compensation, effectively."

Perhaps Capture or File would have been a better word than Exposure.
"Effectively" suggests no downside, which is untrue.
Until the day ISO has no effect on IQ, nothing has changed.
It would be a pretty rare shot that required Speed and Aperture to be fixed before ISO is thought about, I'm thinking.

 

[Ellis Vener]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure.

You are saying that exposure was predicated on the film speed, but that film speed was never understood as a part of exposure. Can you reconcile that contradiction?

It was a baseline. Depending on the film you might rate it at what the manufacturer suggested or slightly differently or develop it differently (“push” or more rarely “pull”) But you didn’t really think about as changeable from frame to frame on a roll of film. With digital cameras we collectively got the notion the could change the ISO setting from one frame to the next.

What we really are doing when we use any ISO beyond the base sensitivity rating is adjusting the amount of amplitutude applied to the exposure to make the photo appear brighter.

Exposure Compensation is different. In Aperture or Shutter Prioroty or I guess in Program modes, we are regulating the quantity of light used for the exposure.

 

[TeddyD]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure. The concept was density triangle, not exposure triangle; same like now it is default manufacturer rendering lightness triangle, not exposure triangle.

Going back to film cameras some had the EV on the same wheel as ISO (ASA or DIN) and some models didn't have EV at all, like Canon AV1. So I understand perfectly where the original statement comes from..

The analogy between high ISO grain in film and high ISO noise in digital is an easy one, but it is a lazy one and the wrong one.

Not entirely, You can't either do nothing usefull with RAW file if you don't process it first, same as with unprocessed film.

 

[Iliah Borg]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure. The concept was density triangle, not exposure triangle; same like now it is default manufacturer rendering lightness triangle, not exposure triangle.

Going back to film cameras some had the EV on the same wheel as ISO (ASA or DIN) and some models didn't have EV at all, like Canon AV1. So I understand perfectly where the original statement comes from..

Not sure I understand what original statement you are referring too.

The analogy between high ISO grain in film and high ISO noise in digital is an easy one, but it is a lazy one and the wrong one.

Not entirely, You can't either do nothing usefull with RAW file if you don't process it first, same as with unprocessed film.

You can't do anything with an unprocessed JPEG too. Before conversion, it's not even RGB, and it's mosaicked.

 

[Ellis Vener]

By definition there is no such thing as an unprocessed JPEG. What you described is raw data.

 

[Iliah Borg]

By definition there is no such thing as an unprocessed JPEG. What you described is raw data.

I'm describing what is inside JPEG. Converting JPEGs to RGB and filling in the missing chroma values is a non-trivial task. https://en.wikipedia.org/wiki/Chroma_subsampling#4:2:0

On a side note, JPEG can contain raw data too.

 

[Tom Axford]

On a side note, JPEG can contain raw data too.

I presume that if it contains raw data, the sensor must have produced data directly in that format (before compression, of course).

Do any such sensors exist?

 

[FingerPainter]

There are no "generations" behind "exposure triangle", it is a recent thing (appeared circa 2003...

Surely you are referring to the term, not the concept.

The concept is demonstrably wrong (absurd would be a better word). Nothing like it was ever discussed among my colleagues.

The phrase was: for normal density, given film is rated xxx, you need yy exposure (a pair of shutter speed and f-stop, for example). Film speed was never understood as a part of exposure.

You are saying that exposure was predicated on the film speed, but that film speed was never understood as a part of exposure. Can you reconcile that contradiction?

How hungry I am will dictate whether I eat a two-egg plain omelette or a three-egg cheese omelette. Yet my hunger is not part of the omelette's recipe.

 

[FingerPainter]

...

It would be a pretty rare shot that required Speed and Aperture to be fixed before ISO is thought about, I'm thinking.

Most of my shooting is that way. ISO gets set last, usually by the camera using Auto-ISO.

Exposure is what is affecting the quality, not the ISO setting. Now, of course, if one is foolish enough to needlessly set ISO manually first, to a value above base, that decision can put a limit on exposure, and hence on quality.

 

[Lee Jay]

...

It would be a pretty rare shot that required Speed and Aperture to be fixed before ISO is thought about, I'm thinking.

Most of my shooting is that way. ISO gets set last, usually by the camera using Auto-ISO.

Exposure is what is affecting the quality, not the ISO setting. Now, of course, if one is foolish enough to needlessly set ISO manually first, to a value above base, that decision can put a limit on exposure, and hence on quality.

Ditto for me.

 

[Iliah Borg]

On a side note, JPEG can contain raw data too.

I presume that if it contains raw data, the sensor must have produced data directly in that format (before compression, of course).

Do any such sensors exist?

Depending on what circuitry is present on the sensor, it can output YCbCr stream.

"Small raw" formats (Canon, Kodak, Nikon) are YCbCr JPEGs, they can be produced from "normal" RGGB raw sensor data.

You can see YCbCr data in RawDigger for Canon and Nikon small raw if you set preferences like this:

There is no exact definition for raw, raw is anything that the manufacturer declares to be raw.

--
http://www.libraw.org/

 

[Bill Ferris]

The problem as always is terminology. We describe an image that is too dark as "under-exposed" and one that is too light as "over-exposed". Some might argue, but those descriptions are correctly using the word exposure:

It is entirely possible to have an image that is too light but underexposed.

the exposure that was used for the image was either too low (dark image) or too high (light image) for the sensitivity of the recording medium.

Given that

• the ISO setting generally doesn't affect the sensitivity of the sensor,
• the user can set both the output sensitivity of the camera system and the exposure
• the lightness of the image depends on both those user-controllable factors

why must it necessarily be that an image is too bright because the the exposure was too high for the sensitivity of the camera, rather than the sensitivity of the camera system was set too high for the exposure?

Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor; it's ability or efficiency to capture light. ISO does not determine sensitivity. Nor does ISO determine amplification. It is a standard used by the camera to decide to what extent a pixel well has been filled by the number of photons counted during the shutter actuation.

If part of the goal, here, is to figure out how to talk about terms like exposure and ISO in ways that are accurate and informative, I would recommend we avoid using language that can easily be misunderstood to support misleading notions.

When you take an image at 1/500 f/4 ISO 1600 that is two stops too light, the problem isn't that the exposure is too high. The ISO is too high. So that image is not over-exposed. It is too light, or over-brightened. If we could have taken the image at 1/125 without unwanted motion blur, the image is actually underexposed.

Note that this does NOT say that sensitivity affects exposure.

So far as I am aware we do not have a generaly accepted single term that descibes the consequences of changing sensitivity at constant exposure. Words like "lightness" or "brightness" convey some of the idea, but I am not convinced either is entirely what is needed.

What do you think is missing?

Image lightness is affected by two factors: exposure and ISO setting. I'd suggest that what is missing from "too light" in this case is the information regarding which of the two factors was needlessly high. However, "too light" is better then "overexposed" in this case, because it does not point the finger at the wrong factor.

I think it's helpful to include the qualifier that ISO affects post-exposure image lightness. It affects how the exposure is rendered. This helps clarify that, while related, exposure and ISO are different. One (exposure) is determined by scene brightness, the f-stop and shutter speed. The other (ISO) is a standard; a reference used by the camera to determine if a pixel well should be rendered as full, empty or somewhere in-between. It is a standard applied by the camera in rendering an image with a certain lightness.

To simplify for the novice, I take the approach of talking about a photographer typically having a goal of making an image that is pleasingly light. At our disposal, we're have f-stop and shutter speed as exposure controls managing how much light - how much information - is captured during a shutter actuation. We also have ISO as a setting used to manage the lightness of the rendered image. ISO is applied after the exposure and can be used to compensate for exposure settings that deliver a minimal amount of light to the sensor.

 

[Tom Axford]

Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor; it's ability or efficiency to capture light. ISO does not determine sensitivity.

As far as I am aware, all the camera manufacturers refer to ISO as "ISO Sensitivity". So, presumably you would say that they have all misunderstood what ISO means and need to be re-educated!

Really, are you serious? The people who make cameras do not understand what ISO means?

 

[Iliah Borg]

Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor; it's ability or efficiency to capture light. ISO does not determine sensitivity.

As far as I am aware, all the camera manufacturers refer to ISO as "ISO Sensitivity". So, presumably you would say that they have all misunderstood what ISO means and need to be re-educated!

Really, are you serious? The people who make cameras do not understand what ISO means?

They do, that's why they authored ISO standard where they describe photographic sensitivity of a DSC (digital still camera), and not of a sensor:

"The photographic sensitivity of a DSC is a particular EI value calculated from the exposure provided at the focal plane of the DSC that produces a specified camera image signal level."

and explicitly excluded raw:

"ISO speed and ISO speed latitude values shall not be reported for raw images, however, because with raw images processing that affects the values has not been performed."

ISO 12232:2019: Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index

 

[Tom Axford]

Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor; it's ability or efficiency to capture light. ISO does not determine sensitivity.

As far as I am aware, all the camera manufacturers refer to ISO as "ISO Sensitivity". So, presumably you would say that they have all misunderstood what ISO means and need to be re-educated!

Really, are you serious? The people who make cameras do not understand what ISO means?

They do, that's why they authored ISO standard where they describe photographic sensitivity of a DSC (digital still camera), and not of a sensor:

"The photographic sensitivity of a DSC is a particular EI value calculated from the exposure provided at the focal plane of the DSC that produces a specified camera image signal level."

Precisely!

and explicitly excluded raw:

"ISO speed and ISO speed latitude values shall not be reported for raw images, however, because with raw images processing that affects the values has not been performed."

ISO 12232:2019: Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index

Again, precisely, because a raw image is only an intermediate point along the path from the light image formed by the lens and the final digital image used when determining the ISO sensitivity of the camera.

 

[Iliah Borg]

Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor; it's ability or efficiency to capture light. ISO does not determine sensitivity.

As far as I am aware, all the camera manufacturers refer to ISO as "ISO Sensitivity". So, presumably you would say that they have all misunderstood what ISO means and need to be re-educated!

Really, are you serious? The people who make cameras do not understand what ISO means?

They do, that's why they authored ISO standard where they describe photographic sensitivity of a DSC (digital still camera), and not of a sensor:

"The photographic sensitivity of a DSC is a particular EI value calculated from the exposure provided at the focal plane of the DSC that produces a specified camera image signal level."

Precisely!

and explicitly excluded raw:

"ISO speed and ISO speed latitude values shall not be reported for raw images, however, because with raw images processing that affects the values has not been performed."

ISO 12232:2019: Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index

Again, precisely, because a raw image is only an intermediate point along the path from the light image formed by the lens and the final digital image used when determining the ISO sensitivity of the camera.

Please let me quote: "Use of the term, sensitivity, tends to reinforce the mistaken notion that ISO determines the sensitivity of the sensor". And further, "ISO does not determine sensitivity". The way I read the standard, ISO determines not some general sensitivity, but qualified what are those sensitivities, emphasizing that they are for the camera output. So while the wording of "ISO does not determine sensitivity" might be imperfect, I can see what was meant to be said.

On a side note, one can argue that the standard is not about sensitivity but about responsivity. In this sense, yes, "ISO does not determine sensitivity" is a warranted statement

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http://www.libraw.org/