Steps From Levels?

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or determine
the number of exposure steps between two pixels or areas in the image?

Wilba wrote:
Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

--
Chris Malcolm

Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if a
pixel has the value r, g, b, can I determine the RGB values for one step up
or down from that?

Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if a
pixel has the value r, g, b, can I determine the RGB values for one step up
or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

--
Chris Malcolm

Chris Malcolm wrote:
Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if a
pixel has the value r, g, b, can I determine the RGB values for one step
up
or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

You're still thinking about an original scene, which is irrelevant to what
I'm trying to work out. But the zone system clue lead me to Norman Koren
(http://www.normankoren.com/zonesystem.html), and the Matlab equation
(bottom of the page), which is pretty much what I was looking for. It uses a
gamma corrected sine curve to give the levels for 9 zones from black to
white, and the result looks fine to my eye.

I worked backwards to get a zone number for any arbitrary level, and from
there it's easy enough to work out the level for a number of zones/steps
from there.

I'm not sure whether that's any good for anything, but it was interesting to
do. Thanks for the clue. :- )

Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the answer.

If I'm looking at an image in something like Photoshop (in which I can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if a
pixel has the value r, g, b, can I determine the RGB values for one step
up
or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

You're still thinking about an original scene, which is irrelevant to what
I'm trying to work out. But the zone system clue lead me to Norman Koren
(http://www.normankoren.com/zonesystem.html), and the Matlab equation
(bottom of the page), which is pretty much what I was looking for. It uses a
gamma corrected sine curve to give the levels for 9 zones from black to
white, and the result looks fine to my eye.

I worked backwards to get a zone number for any arbitrary level, and from
there it's easy enough to work out the level for a number of zones/steps
from there.

I'm not sure whether that's any good for anything, but it was interesting to
do. Thanks for the clue. :- )

But note that what you've found applies in specific conditions, which
although conventionally standard, are changed by any kind of unusual
parameter settings in camera or image editing. So if you know nothing
about how the image was produced you don't know whether your equations
apply, although they probably roughly do for most unedited jpegs on
simple default settings straight out of the camera. I say "simple
default settings" because my camera like some others has a standard
default setting of doing some local dynamic range optimisation for the
in-camera jpeg conversion, and that makes those equations invalid if
the lighting conditions etc. call for that opimisation to make more
than insignificant adjustments.

--
Chris Malcolm

Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the
answer.

If I'm looking at an image in something like Photoshop (in which I
can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the
image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if
a
pixel has the value r, g, b, can I determine the RGB values for one
step
up
or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

You're still thinking about an original scene, which is irrelevant to
what
I'm trying to work out. But the zone system clue lead me to Norman Koren
(http://www.normankoren.com/zonesystem.html), and the Matlab equation
(bottom of the page), which is pretty much what I was looking for. It
uses a
gamma corrected sine curve to give the levels for 9 zones from black to
white, and the result looks fine to my eye.

I worked backwards to get a zone number for any arbitrary level, and from
there it's easy enough to work out the level for a number of zones/steps
from there.

I'm not sure whether that's any good for anything, but it was interesting
to
do. Thanks for the clue. :- )

But note that what you've found applies in specific conditions, which
although conventionally standard, are changed by any kind of unusual
parameter settings in camera or image editing. So if you know nothing
about how the image was produced you don't know whether your
equations apply, although they probably roughly do for most unedited
jpegs on simple default settings straight out of the camera. I say "simple
default settings" because my camera like some others has a standard
default setting of doing some local dynamic range optimisation for the
in-camera jpeg conversion, and that makes those equations invalid if
the lighting conditions etc. call for that opimisation to make more
than insignificant adjustments.

I don't see how any of that has anything to do with the "normalized screen
luminance" that the equations work with. There is no camera involved, no
JPEG conversion, no image editing, no lighting conditions, just grey pixels
with specific values.

Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the
answer.

If I'm looking at an image in something like Photoshop (in which I
can
easily read the RGB levels for any pixel), how can I measure or
determine
the number of exposure steps between two pixels or areas in the
image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the RGB
levels you're looking at. Not only are there likely to have been
several, but some of them are non linear and some are local. In other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is, if
a
pixel has the value r, g, b, can I determine the RGB values for one
step
up
or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

You're still thinking about an original scene, which is irrelevant to
what
I'm trying to work out. But the zone system clue lead me to Norman Koren
(http://www.normankoren.com/zonesystem.html), and the Matlab equation
(bottom of the page), which is pretty much what I was looking for. It
uses a
gamma corrected sine curve to give the levels for 9 zones from black to
white, and the result looks fine to my eye.

I worked backwards to get a zone number for any arbitrary level, and from
there it's easy enough to work out the level for a number of zones/steps
from there.

I'm not sure whether that's any good for anything, but it was interesting
to
do. Thanks for the clue. :- )

But note that what you've found applies in specific conditions, which
although conventionally standard, are changed by any kind of unusual
parameter settings in camera or image editing. So if you know nothing
about how the image was produced you don't know whether your
equations apply, although they probably roughly do for most unedited
jpegs on simple default settings straight out of the camera. I say "simple
default settings" because my camera like some others has a standard
default setting of doing some local dynamic range optimisation for the
in-camera jpeg conversion, and that makes those equations invalid if
the lighting conditions etc. call for that opimisation to make more
than insignificant adjustments.

I don't see how any of that has anything to do with the "normalized screen
luminance" that the equations work with. There is no camera involved, no
JPEG conversion, no image editing, no lighting conditions, just grey pixels
with specific values.

First of all let me remind you of your original question which is
still quoted at the start of this message.

"If I'm looking at an image in something like Photoshop (in which I
can easily read the RGB levels for any pixel), how can I measure or
determine the number of exposure steps between two pixels or areas in
the image?"

The "number of exposure steps" derives directly from the original
light levels in the original scene and what kind of lens etc. was used
to capture the image.

Secondly let me remind you that the equations you have found, which
deal only only with screen luminance levels, do so in a context
derived from the exposing and printing of photographs according to a
simplified version of Ansel Adams' Zone System.

Your questions, and what you say is one of the most interesting
partial answers you have found, have photography exposure values built
solidly into their foundations. They escape from explicitly dealing
with these by normalising and abstracting from them. My point is that
that normalisation and abstraction only holds provided the translations
involved are normal in the ways assumed, which in today's
sophisticated world of digital photography they often aren't.

--
Chris Malcolm

Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:
Wilba wrote:
Chris Malcolm wrote:
Wilba wrote:

Probably a dumb question to most of you, but I don't know the
answer.

If I'm looking at an image in something like Photoshop (in which I
can easily read the RGB levels for any pixel), how can I measure
or determine the number of exposure steps between two pixels
or areas in the image?

You can't without knowing the details of all the transformations
between the original luminance levels in the sensor image and the
RGB levels you're looking at. Not only are there likely to have
been
several, but some of them are non linear and some are local. In
other
words you can't.

That's kinda what I was thinking, thanks.

On second thoughts, I'm not so sure ... :- )

Forget the original. Put it this way - in the image itself as it is,
if
a pixel has the value r, g, b, can I determine the RGB values for one
step up or down from that?

Only if you know the parameters noted in my above remark. If you
usually restrict yourself to editing which keeps those parameters the
same, then you could build up a table or formula which you could apply
to all your photographs. But if like me you frequently adjust the
contrast, use local dynamic range adjustments, or even worse, fiddle
with the "curves", then the values will differ for each image, and to
a lesser extent, each part of each image.

If you want to carry out some specific experiments on a particular
image, then your easiest way of bypassing all the mathematical model
fitting would be the good old fashioned method of taking a photograph
and making careful notes at the time of specific important light
values by doing spot measurements with a light meter or the
camera. Then you can simply see how the light values have translated
into pixel luminance values, and also how those change with your
editing, camera settings, and so on.

Some of the work you're interested in may already have been done and
explained in scientifically minded tutorials on the zone exposure
system.

You're still thinking about an original scene, which is irrelevant to
what I'm trying to work out. But the zone system clue lead me to
Norman Koren (http://www.normankoren.com/zonesystem.html),
and the Matlab equation (bottom of the page), which is pretty much
what I was looking for. It uses a gamma corrected sine curve to give
the levels for 9 zones from black to white, and the result looks fine
to
my eye.

I worked backwards to get a zone number for any arbitrary level, and
from
there it's easy enough to work out the level for a number of
zones/steps
from there.

I'm not sure whether that's any good for anything, but it was
interesting
to do. Thanks for the clue. :- )

But note that what you've found applies in specific conditions, which
although conventionally standard, are changed by any kind of unusual
parameter settings in camera or image editing. So if you know nothing
about how the image was produced you don't know whether your
equations apply, although they probably roughly do for most unedited
jpegs on simple default settings straight out of the camera. I say
"simple
default settings" because my camera like some others has a standard
default setting of doing some local dynamic range optimisation for the
in-camera jpeg conversion, and that makes those equations invalid if
the lighting conditions etc. call for that opimisation to make more
than insignificant adjustments.

I don't see how any of that has anything to do with the "normalized
screen
luminance" that the equations work with. There is no camera involved, no
JPEG conversion, no image editing, no lighting conditions, just grey
pixels
with specific values.

First of all let me remind you of your original question which is
still quoted at the start of this message.

Sure.

"If I'm looking at an image in something like Photoshop (in which I
can easily read the RGB levels for any pixel), how can I measure or
determine the number of exposure steps between two pixels or areas in
the image?"

Right.

The "number of exposure steps" derives directly from the original
light levels in the original scene and what kind of lens etc. was used
to capture the image.

OK, but as I've been saying over and over, that's not what I meant. There's
no point insisting on answering a question I'm not asking. :- )

Secondly let me remind you that the equations you have found, which
deal only only with screen luminance levels, do so in a context
derived from the exposing and printing of photographs according to a
simplified version of Ansel Adams' Zone System.

Exactly! It's all about the final image. It's the apparent lightness of
tones in the final image I'm interested in, not the actual lightness of
tones in a scene. I only want to know how to determine the levels a number
of steps lighter or darker than an area in a final image (and the other way
around).

That image could be entirely created in software, as is Norman's "Zone
system chart for gamma = 2.2 (PC's, sRGB color space)"
(http://www.normankoren.com/zonesystem.html). When I look at that chart I
can see that zone 4 is one step darker than zone 5, 7 is two steps lighter
than 5, and so on, and I want to be able to determine via measurement that,
"this area is about X steps darker or lighter than that area ...", in an
image on my monitor.

Knowing what I know now I might put the question this way - If I'm looking
at an image in something like Photoshop (in which I can easily read the RGB
levels for any pixel), how can I measure or determine the number of zones*
between two pixels or areas in the image?

* See Norman Koren's "Zone system chart for gamma = 2.2 (PC's, sRGB color
space)" at http://www.normankoren.com/zonesystem.html.

Your questions, and what you say is one of the most interesting
partial answers you have found, have photography exposure values built
solidly into their foundations. They escape from explicitly dealing
with these by normalising and abstracting from them. My point is that
that normalisation and abstraction only holds provided the translations
involved are normal in the ways assumed, which in today's
sophisticated world of digital photography they often aren't.

If I'm viewing an image on a well-calibrated PC monitor with gamma = 2.2 and
sRGB colour space, what translations are involved?