It is exposure and it is a triangle

[Ephemeris]

Cameras are engineered in accordance with an understanding of what exposure is and how it's determined; an understanding that has foundations in science going back to the 19th century. The following is taken from the "Manual of Photography," a technical examination of cameras and the photographic process first published in 1890 with multiple later editions having followed. This excerpt is from the 6th edition (1971):

"The exposure received by a film is governed by the strength of the light falling on it and by the time for which that light is allowed to fall. We have already seen that light falling on a surface is defined as illumination. The relation between exposure (E), illumination (I) and exposure time (t) is expressed by the equation E = I×t."

You'll note there's no mention of ISO, ASA or any other measure of the sensitivity of the medium to light in the description of exposure or among the factors directly determining exposure.

Please see the equation below. We can see, for example that t (sensor time) is inversely proportional to the REI. Therefore they are linked and mentioned.

To link this understanding with contemporary digital photography, we can reference ISO 12232:2019, which defines ISO's role communicating an exposure index to the camera:

"An exposure index (EI) is a numerical value that is inversely proportional to the exposure provided to an image sensor to obtain an image. Images obtained from a DSC [digital still camera] using a range of EI values will normally provide a range of image quality levels. 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. The EI value for a specific image captured by a DSC shall be equal to the EI reference exposure of 10 lx⋅s divided by the focal plane exposure used to capture the image,..."

Again, we see an understanding of what exposure is and how it's determined that is consistent with the definition in use since the 19th century. Modern cameras are designed, engineered and built to function in accordance with this long and well-established understanding.

The answer to your question, "An exposure of what?" can, I think, be summarized as electromagnetic energy. In most cases, it's energy from the visible light portion of the spectrum. However, I suspect an imaging device engineered to be sensitive to areas of the spectrum in the IR, UV, or radio portions of the spectrum would define exposure similarly while using the relevant energy unit.

The of what is a system question.

Of a lens, a theoretical plane behind a lens, of a camera sub-system, of a camera. I'm interested in what comes out of the camera.

By what is also a system queation. That could be a source of EM, or the EM. It depends where one draws a box.

If you seek a more technical explanation or understanding, I would encourage you to search posts by bobn2 and Iliah Borg. To narrow the list of results, you might try associating the name with ISO12232:2019 or some other topical terms.

--
Bill Ferris Photography
Flagstaff, AZ
http://www.billferris.photoshelter.com

I've quoted the ISO standard a few times in the thread but the key element is the equation below.

This is from the standard you quoted, but directly taken from https://www.visionsystech.com/, a paper from Kris Balch. He describes noise based measurements and saturation and goes on to say the former is the more common method.

 

[D Cox]

As already explained, each one fixes the optimal exposure depending on the ISO (yes, there is such a thing) for a fixed scene.

If only exposure was dependent upon ISO; it's not. An optimal exposure is an exposure that satisfies a photographer's creative goals for depth of field and rendering of movement. A pleasing image lightness is often achievable through a combination of ISO selection and image processing...either in-camera or in a photo editing app.

Again, optimal depending on the ISO. Like choose the ISO, then avoid clipping the highlights or leaving too much highlight room.

Regardless of the subjective qualifier one chooses to apply, exposure is determined by scene brightness, lens aperture, and exposure time.

Sorry, incorrect. Exposure is determined by the photographer.

Are you seriously suggesting that aperture and shutter speed do not determine the exposure?

I am suggesting that it is the photographer who choses them. They are not "god" given.

Yes he/she chooses the setting, no-one is disputing that. But the photographer is not the actual aperture and shutter speed that determine the exposure so Bill's statement is correct.

Did you notice the first part of his sentence? He said "regardless of the subjective qualifier." Well, this is incorrect. My qualifier puts the photographer first.

Are you seriously suggesting that everyone in the history of DPR forums who has posted that scene luminance, shutter speed and aperture are the 3 things that control exposure has been incorrect in their statement?

Why do you, and several people here want to steer the conversation to ISO being part of exposure or not? Nobody here says that it is.

How about the EC control ? Is that "part of exposure" ?

Don

 

[Dem Bell]

As already explained, each one fixes the optimal exposure depending on the ISO (yes, there is such a thing) for a fixed scene.

If only exposure was dependent upon ISO; it's not. An optimal exposure is an exposure that satisfies a photographer's creative goals for depth of field and rendering of movement. A pleasing image lightness is often achievable through a combination of ISO selection and image processing...either in-camera or in a photo editing app.

Again, optimal depending on the ISO. Like choose the ISO, then avoid clipping the highlights or leaving too much highlight room.

Regardless of the subjective qualifier one chooses to apply, exposure is determined by scene brightness, lens aperture, and exposure time.

Sorry, incorrect. Exposure is determined by the photographer.

Are you seriously suggesting that aperture and shutter speed do not determine the exposure?

I am suggesting that it is the photographer who choses them. They are not "god" given.

Yes he/she chooses the setting, no-one is disputing that. But the photographer is not the actual aperture and shutter speed that determine the exposure so Bill's statement is correct.

Did you notice the first part of his sentence? He said "regardless of the subjective qualifier." Well, this is incorrect. My qualifier puts the photographer first.

Are you seriously suggesting that everyone in the history of DPR forums who has posted that scene luminance, shutter speed and aperture are the 3 things that control exposure has been incorrect in their statement?

Why do you, and several people here want to steer the conversation to ISO being part of exposure or not? Nobody here says that it is.

How about the EC control ? Is that "part of exposure" ?

What does "part of exposure" even mean? Luminous exposure consists of photons hitting the CFA of the sensor. Some part of these photons will produce photoelectrons that the camera will hopefully accurately count.

Leaving the shutter open for longer will increase exposure. But shutter speed is not "part of exposure". Photons are. Light is made of packets of energy. They are parts of exposure. How we control the number of these packets reaching the sensor is "exposure management".

The EC control is the lightness control. EC=0 means that mid grey in the scene (as identified by the camera) is rendered as mid grey in the image. If the photographer wants to override this "exposure-lightness" relationship they can. The EC control and the ISO setting together define the target image lightness for the given exposure.

 

[J A C S]

The EC control is the lightness control.

No, that would be the ISO. EC changes the exposure relative to what the camera would otherwise chose.

EC=0 means that mid grey in the scene (as identified by the camera) is rendered as mid grey in the image.

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

 

[steve_z]

Bill - You’re obviously an experienced photographer and I do respect your opinions, but your quiz is more about your post production LR skills than the camera’s settings. Sure, even 100 ASA film shot as if 800 can be corrected by altering the developing process. The fact that you can’t use the photo of your nice plant SOOC contradicts the point you’re trying to make, no?

Why does every handheld light meter have an ISO setting if ISO is not part of the exposure equation?

I don't see ISO as part of the "exposure equation", using your words, but as part of the "lightness equation".

The "lightness equation" being that there is a reciprocal relationship between any 2 of aperture, shutter speed and ISO in order to maintain a constant photo lightness for a given scene.

In aperture priority, if you raise ISO by 1 stop you are instructing the camera to actually halve the exposure (not keep it constant) by setting a 1 stop faster shutter speed resulting in only half the amount of light now reaching the sensor, even though the image lightness will remain the same.

Also, you can adjust the image lightness in camera by adjusting ISO while keeping the exposure constant.

 

[steve_z]

The EC control is the lightness control.

No, that would be the ISO. EC changes the exposure relative to what the camera would otherwise chose.

Not always. In manual mode on my 90D adjusting EC leaves the exposure alone and adjusts only the image lightness by changing the ISO value.

EC=0 means that mid grey in the scene (as identified by the camera) is rendered as mid grey in the image.

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

No, not true.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray. The same would occur if I used a black sheet of paper.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

 

[Ephemeris]

The EC control is the lightness control.

No, that would be the ISO. EC changes the exposure relative to what the camera would otherwise chose.

Not always. In manual mode on my 90D adjusting EC leaves the exposure alone and adjusts only the image lightness by changing the ISO value.

EC=0 means that mid grey in the scene (as identified by the camera) is rendered as mid grey in the image.

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

No, not true.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray. The same would occur if I used a black sheet of paper.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

The metering mode affects, well the metering.





As an aside, people asking about locking exposure to a focal point.

From R5 advanced manual.

 

[steve_z]

What I posted doesn't contradict any of that.

 

[Ephemeris]

What I posted doesn't contradict any of that.

Which post is this a response to? Jacs sub thread of 21hrs ago?

 

[steve_z]

Have a look in the "In reply to" field in the post's header.

 

[Ephemeris]

Have a look in the "In reply to" field in the post's header.

Not very helpful Steve.

I see this.



And





The method of replying .make's it difficult.

Were you responding to me ?

 

[Mako2011]

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

That's not always the case. Many Nikon bodies, for example, only solve for 18% grey when in Matrix metering mode and solve for 12.3% in Spot and Center weighted metering

 

[steve_z]

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

That's not always the case. Many Nikon bodies, for example, only solve for 18% grey when in Matrix metering mode and solve for 12.3% in Spot and Center weighted metering

That's interesting and I certainly don't doubt that, but I wonder why for Spot and Center weighted metering it is 12.3% and not 18%.

Wouldn't that make the subject darker (depending on the scene in the frame outside the metered area) if metered for 12.3% than it would be if metered at 18%?

 

[Ephemeris]

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

That's not always the case. Many Nikon bodies, for example, only solve for 18% grey when in Matrix metering mode and solve for 12.3% in Spot and Center weighted metering

That's interesting and I certainly don't doubt that, but I wonder why for Spot and Center weighted metering it is 12.3% and not 18%.

Wouldn't that make the subject darker (depending on the scene in the frame outside the metered area) if metered for 12.3% than it would be if metered at 18%?

R50




R3

 

[steve_z]

So what? That is an explanation of the various metering modes but doesn't answer my query why for matrix mode Nikon use 18% gray and for spot and Center Weighted modes it used 12.3%.

I am making the assumption Nikon designers/engineers had the option to use 18% gray for all the metering modes but chose 12.3% for the "spot" modes mentioned above.

I am wondering what their thinking was for that decision.

 

[Mako2011]

Can we stop talking about medium gray? Unless you shoot in a spot metering mode, medium gray plays no role.

If I fill the frame with a white sheet of paper using evaluative/matrix metering and center the meter needle the resulting image will be very close to 18% (middle) gray.

Cameras are calibrated to output average 18% for the elements in the scene being metered.

That's not always the case. Many Nikon bodies, for example, only solve for 18% grey when in Matrix metering mode and solve for 12.3% in Spot and Center weighted metering

That's interesting and I certainly don't doubt that, but I wonder why for Spot and Center weighted metering it is 12.3% and not 18%.

Nikon uses 12.5% (may be closer to 12.4) as neutral grey because that is what the film manufacturers use to be in line with the old A.N.S.I. standards.

Matrix was a departure and I think the switch to 18% as that's what a number of grey cards were calibrated to and fit the Matrix algorithms better at the time

Wouldn't that make the subject darker (depending on the scene in the frame outside the metered area) if metered for 12.3% than it would be if metered at 18%?

Yes it can....but the practical dif is small and more noticeable Spot vs Matrix ...less with Center Weighted as CW metering still looks at the whole scene while biasing for the center

 

[Mako2011]

From Graystar:
" If you want to know the story between 18% and 12% gray read this..."
http://dougkerr.net/pumpkin/articles/Scene_Reflectance.pdf
http://dougkerr.net/pumpkin/articles/Exposure_Calibration.pdf

 

[steve_z]

Ok, thanks for the explanation. It makes sense.

 

[Ephemeris]

Cameras are engineered in accordance with an understanding of what exposure is and how it's determined; an understanding that has foundations in science going back to the 19th century. The following is taken from the "Manual of Photography," a technical examination of cameras and the photographic process first published in 1890 with multiple later editions having followed. This excerpt is from the 6th edition (1971):

"The exposure received by a film is governed by the strength of the light falling on it and by the time for which that light is allowed to fall. We have already seen that light falling on a surface is defined as illumination. The relation between exposure (E), illumination (I) and exposure time (t) is expressed by the equation E = I×t."

You'll note there's no mention of ISO, ASA or any other measure of the sensitivity of the medium to light in the description of exposure or among the factors directly determining exposure.

To link this understanding with contemporary digital photography, we can reference ISO 12232:2019, which defines ISO's role communicating an exposure index to the camera:

"An exposure index (EI) is a numerical value that is inversely proportional to the exposure provided to an image sensor to obtain an image. Images obtained from a DSC [digital still camera] using a range of EI values will normally provide a range of image quality levels. 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. The EI value for a specific image captured by a DSC shall be equal to the EI reference exposure of 10 lx⋅s divided by the focal plane exposure used to capture the image,..."

Again, we see an understanding of what exposure is and how it's determined that is consistent with the definition in use since the 19th century. Modern cameras are designed, engineered and built to function in accordance with this long and well-established understanding.

The answer to your question, "An exposure of what?" can, I think, be summarized as electromagnetic energy. In most cases, it's energy from the visible light portion of the spectrum. However, I suspect an imaging device engineered to be sensitive to areas of the spectrum in the IR, UV, or radio portions of the spectrum would define exposure similarly while using the relevant energy unit.

If you seek a more technical explanation or understanding, I would encourage you to search posts by bobn2 and Iliah Borg. To narrow the list of results, you might try associating the name with ISO12232:2019 or some other topical terms.

 

[JohnSil]

Bill - You’re obviously an experienced photographer and I do respect your opinions, but your quiz is more about your post production LR skills than the camera’s settings. Sure, even 100 ASA film shot as if 800 can be corrected by altering the developing process. The fact that you can’t use the photo of your nice plant SOOC contradicts the point you’re trying to make, no?

Why does every handheld light meter have an ISO setting if ISO is not part of the exposure equation?

I don't see ISO as part of the "exposure equation", using your words, but as part of the "lightness equation".

The "lightness equation" being that there is a reciprocal relationship between any 2 of aperture, shutter speed and ISO in order to maintain a constant photo lightness for a given scene.

Hmmmm, I don't see it as part of the "Lightness Equation" but a part of the "Darkness Equation"!!! It can't be both simultaneously or we'd have ourselves a good old fashioned paradox!!! ;-)

See, your triangle would have way too many interpretations. Maybe that's why the founding father whom of course are not near as smart as we here are, called it the Exposure Triangle!

John