ISO and actual sensitivity in DSLR's (D70, *istD, 20D, S3...)

Roland Karlsson wrote:

be your problem only - when I give up to explain it to you.

I quit. And not becasue you're right.


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Alan Browne wrote in news:d29hv5$3hg$1
@inews.gazeta.pl:

For tour own reasons you've completely misdirected the purpose of the OP.

C'mon Allan. Stop this before you go too far.

You might not believe what I say. And you might
not even like my posts. But please stay civil.


/Roland

Roland Karlsson wrote:

Alan Browne wrote in news:d29hv5$3hg$1
@inews.gazeta.pl:


For tour own reasons you've completely misdirected the purpose of the OP.


C'mon Allan. Stop this before you go too far.

Too far for what? Will you plonk me?

You might not believe what I say. And you might
not even like my posts. But please stay civil.

The purpose of the OP was related to camera ISO settings being correct
wrt to exposure. That's all. You've taken it well outside that
definition. Looking at the 18% point is the only comparative means of
checking (for reasons stated) across sensor types. Beyond there one
could look at which sensor goes further into the shaddows or higher into
the highlights but that is not the purpose of the article. I do agree
that the article mention of JPG is contentious, but doing the tests the
way I did them eliminates all variables other than color temp
(corrected) and gamma.

In my reply to Dave Martindale I stated that the gamma is of no
consequence, but on reflection it is, I believe, the reason why the 18%
grey is at 118 rather than 256/2 (128).


--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.

Alan Browne wrote in news:d29ie3$5jk$1
@inews.gazeta.pl:

I quit.

OK.

And not becasue you're right.

For some reason you believe Dave Marintdale when
he says the same thing.

IMHO - You have misunderstood how color spaces and mapping
from RAW to a certain color space works. And when I try to
explain it you just have to defend yourself instead of
listening. Bad choice IMHO. But that is how it is sometimes.

See you in some other thread, with a new blank opinion
about each other I hope.

See ya,
Roland

On 28 Mar 2005 17:50:42 GMT, Roland Karlsson
wrote:

Alan Browne wrote in news:d29fed$nlg
:

sigh As stated before all the data I posted was with all of the RAW
conversion parameters set to 0 with the exception of color temp. That
is to say no effect on the image. If I change parameters, such as
brightness, contrast, aturation, etc. you can be sure that the info
values change accordingly.

From the 7D manual: "Unlike the other image-quality modes, RAW image
data is unprocessed and requires image processing before it can be
used". Since the 'processing' I've done is _none_, there is no effect.

Secondly, due to the neutral conversion the JPG shows the same values
(close enough) at the same points in the image.

Please Alan.

Reread what I wrote. There is no 118 in the RAW data, the value
118 is computed. And it is the same computation made when
converting to JPEG. The 118 is the result of a non linear
computation.

Can you cite the non-linear aspect of this particular conversion the
Adobe RAW importer is doing?

Have you browsed the source code? (it is available on the net), and
seen the part of code that does the non-linear magic?

If no curves are used on import as Alan has said, and he made no other
exposure adjustments, why wouldn't the conversion be *roughly* linear?

Isn't that what we want and expect from a RAW importer?

--
Owamanga!
http://www.pbase.com/owamanga

On Mon, 28 Mar 2005 17:33:14 +0000 (UTC), (Dave
Martindale) wrote:

Alan Browne writes:

118/255 within Photoshop has no meaning without a coding.
When you do that coding the RAW data is gone. There might
be zillions of unlinearities before arriving att this number.

It is as pure as the driven snow as it completely ignores display and
conversion issues. It is not JPEG. It is not TIF. It is not PSD. It
is not anything other than what the sensor read at a specified color
temperature. Period.

Well, no. The sensor read some voltage, which was probably converted
to a number in a range of 0-4095 or 0-16383 or something similar. Then
Photoshop's raw converter mapped that to the non-linearly coded (gamma
corrected) value 118 out of 255. 118 isn't raw data, and it also can't
be related back to an original sensor measurement without knowing what
the Adobe raw converter does internally.

I'd suggest again that gamma correction is done for display purposes
only and won't affect the reading the info tool has for a particular
pixel. Eg, photoshop doesn't re-map the image based on gamma
correction, Adobe does this at the OS level instead.

And if the raw converter does any part of its processing adaptively,

A big IF.

depending on image content, you can never know exactly what the original
data was - even with a knowledge of internal operation of the converter.

Agreed. IF.

--
Owamanga!
http://www.pbase.com/owamanga

Alan Browne wrote:

Dave Martindale wrote:

Alan Browne writes:


118/255 within Photoshop has no meaning without a coding.
When you do that coding the RAW data is gone. There might
be zillions of unlinearities before arriving att this number.



It is as pure as the driven snow as it completely ignores display and
conversion issues. It is not JPEG. It is not TIF. It is not PSD.
It is not anything other than what the sensor read at a specified
color temperature. Period.



Well, no. The sensor read some voltage, which was probably converted
to a number in a range of 0-4095 or 0-16383 or something similar. Then


I agree to this point, I was a bit heavy in stating "the senor" above.
It should be, as stated in the 7D manual: "Unlike the other mage-quality
modes, RAW image data is unprocessed and requires image processing
before it can be used". It is such when I look at the pixel values,
except for the light source temp.

Photoshop's raw converter mapped that to the non-linearly coded (gamma
corrected) value 118 out of 255. 118 isn't raw data, and it also can't


At the RAW converter stage, I'm looking at the numbers for a given pixel
converted to the 0..255 scale, not the post conversion image/data where
gamma has been applied. There is, to put a point on it, no gamma
coefficent settable in the RAW converter and I have all settings set to 0.

Let me change the above to:

I stated that the gamma is of no consequence, but on reflection and I
believe confirmed by the below, is the reason why the 18% grey is at 118
rather than 256/2 (128).

http://www.cipa.jp/english/hyoujunka.../DC-004_EN.pdf
refers on page 6.

Cheers,
Alan



--
-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: there's no such thing as a FreeLunch.

On 28 Mar 2005 18:29:15 GMT, Roland Karlsson
wrote:

Alan Browne wrote in news:d29gr3$sve
:

If everyone else is correct about this, then I would like someone to
elaborate on the extraordinary coincidence that my values were very
close to 118 (except blue which was a little bit lower).

I hope you can bear with me - although you start to get
annoyed

If we assume that neither the camera nor the conversion
utilty finds any reason why it should do any compensations
and we also assume that the lens is without much flare and
we also assume that the non linear color space conversion
is well known (e.g. sRGB or Adobe RGB - well implemented) -
then a correctly exposed evenly lit 18% grey card shall
have a specific value in the picture.

I assume someone else have made similar meassurements like
you and found out that this value is 118. So - if you do the
same meassurements thoroughly you shall end up with 118.

But - and this is my point. Another camera or conversion
utility might end up with 132. And this value do not show
that the camera/utility has another sensitivity. It only
shows that the value is 132. To understand the sensitivity
of the acmera you must look at more values. And most important
are those near to 255 and 0.

But, if I display those two images side by side, one containing 132
and one containing 118, the 132 image will look overexposed compared
to the 118 image. Thus, an EV correction would be needed on the 132
equipment to bring it back in line.

This is the whole point of known ISO values.

In a way it seems like you want to use something like
the zone system. In the zone system, the picture is matched
into 10 zones in the print. But ... you are trying to
use only one zone.

Due to the complexity, and Alan's constant disclaimer regarding
DMax-DMin (or shoulder/toe) he isn't attempting to cover all zones,
just one: The gray card reading.

--
Owamanga!
http://www.pbase.com/owamanga

Alan Browne wrote in
:

Too far for what? Will you plonk me?

That was not what I meant. I think it would be a shame if
the arguing is going out of hands. It is not _that_ important.
We will meet in other threads. It would be a shame
if we had biased opinions on each others post.

In my reply to Dave Martindale I stated that the gamma is of no
consequence, but on reflection it is, I believe, the reason why the
18% grey is at 118 rather than 256/2 (128).

This is one of the main points in my postings.
You have to take the unlinerities and also
the biases in the conversion when evaluating the
number 118. The number 133 might be just as
good. It depends on the mapping.

Now - I undertand what you try to accomplish - and I appreciate
that. And - under some limited circumstances - it works. But
- there are weeknesses IMHO. I tried to point those out.


/Roland

On Mon, 28 Mar 2005 13:53:15 -0500, Alan Browne
wrote:

Alan Browne wrote:

I stated that the gamma is of no consequence, but on reflection and I
believe confirmed by the below, is the reason why the 18% grey is at 118
rather than 256/2 (128).

http://www.cipa.jp/english/hyoujunka.../DC-004_EN.pdf
refers on page 6.

That's the sRGB gamma characteristics, not those of your display. I
still maintain the brightness, contrast (you can even turn the damn
screen off) or system gamma settings will make a jolt of difference to
the pixel RGB value that the info tool shows you in photoshop.

You (I think) already clarified that your tests were in sRGB color
space with gamma characteristics outlined in IEC61966-2-1.

--
Owamanga!
http://www.pbase.com/owamanga