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  #211  
Old August 28th 05, 10:13 PM
Bart van der Wolf
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wrote in message
news
In message
coh.net,
(Philip Homburg) wrote:

SNIP
it might be useful to start with more common ones, such as D50.


I've never heard that terminology before. Is that sunlight?


http://www.brucelindbloom.com/CIESpe...alculator.html
graphically shows the spectral contribution of several "Reference
illuminants" as per CIE.
D50 is somewhat similar to a light source of 5000 Kelvin color
temperature, a bit warmer than 'average' daylight, the native color
temperature of calibration target references.

Bart

  #212  
Old August 29th 05, 02:38 AM
Alan Browne
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wrote:

In message ,
Alan Browne wrote:


Can you find the "natural" (neutral gain) ISO of the sensor?



Define it, and I will find it.


Simply put, that region where the sensor needs only unity gain (analog
and digital).

For example:
http://www.dpreview.com/reviews/koni...lumi-graph.gif

....has the 7D noise lowest at ISO 200. This suggests that the unity
gain ISO is somewhere in the region of ISO 200 (though not exactly there).

Cheers,
Alan


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  #213  
Old August 29th 05, 03:01 AM
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In message ,
Alan Browne wrote:

wrote:


In message ,
Alan Browne wrote:


Can you find the "natural" (neutral gain) ISO of the sensor?


Define it, and I will find it.


Simply put, that region where the sensor needs only unity gain (analog
and digital).


I don't think any ISO uses unity gain. There's nothing magic about
unity, anyway. Still has to pass through silicone junctions.

For example:
http://www.dpreview.com/reviews/koni...lumi-graph.gif

...has the 7D noise lowest at ISO 200. This suggests that the unity
gain ISO is somewhere in the region of ISO 200 (though not exactly there).


My guess is that it was a mistake, or an anomaly of the in-camera
RAW-JPEG conversion. I wouldn't read too much into DPReview charts.

Take a blackframe at each ISO, and look at the histogram of the RAW
data. How wide are the bell curves? How does it look when converted to
8-bit space, with the mean offset to 128? How does a grey card,
externally metered, register level-wise above the middle of the noise
curve for that ISO? These are the things that are relevant, IMO;
comparing JPEGsfrom cameras that shoot RAW, based on the camera's
metering quirks, is useless and misleading.

--


John P Sheehy

  #215  
Old August 29th 05, 07:18 AM
Stacey
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Jeremy Nixon wrote:


You'd have to decide
what method to use to call something "ISO sensitivity", and it may not
be the same one the manufacturer used.


Yea I guess no one remembers "calibrating" B&W film to their "system" of
camera/meter/sensor? Nothing has changed, lots of people still haven't
leaned the basics of photography, most of it's trial and error till you
calibrate your own workflow.

--

Stacey
  #216  
Old August 29th 05, 09:37 AM
Philip Homburg
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In article ,
Jeremy Nixon wrote:
The camera makers don't tell us exactly how they are measuring ISO
sensitivity, and there are (at least) two different acceptable ways to
do it; one using highlights and one using shadows. You'd have to decide
what method to use to call something "ISO sensitivity", and it may not
be the same one the manufacturer used.


Well, any dSLR that has an ISO 100 when measured using the shadows method
will not be a big hit. So we can safely assume that for low ISO the
highlights (saturation based) method is used.


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency
  #217  
Old August 29th 05, 09:45 AM
Philip Homburg
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In article , wrote:
I just tried it now with my 550EX flash, in manual mode, based on the
guide number; f/22 at 8.23 feet from flash to target with 105mm zoom.
The green channel predominates here, with blue at about 80% of the green
RAW values, and red at about 50%. The average green value in the white
square is about 1520. The blackpoint of the 10D at ISO 100 is about
126, so that leaves us 1494 RAW values for the 90% reflectance square
That would put 100% at 1660 above blackpoint. The 10D clips at ISO 100
anywhere from 3997 to 4005 or so (depending on pixel column), so the
maximum above blackpoint is about 3875. 1660/3875 = 0.43, or ISO 43, if
the standard is 100% reflectance. Now, I shot this in a white hallway
28" wide from 8.2 feet, so there is some extra light bouncing off the
walls, so the actual value is actually lower, although the practical
value in a narrow white hallway is about right.


First of all, if you want to demonstrate a 1 stop headroom, you simply
overexpose a stepwedge by one stop and compare it to a normally exposed
stepwedge.

Playing the numbers game doesn't say much.

I don't think that playing with flashes and relying on the guide number is
a good way to get repeatable results.


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency
  #218  
Old August 30th 05, 03:06 AM
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In message coh.net,
(Philip Homburg) wrote:

In article , wrote:
I just tried it now with my 550EX flash, in manual mode, based on the
guide number; f/22 at 8.23 feet from flash to target with 105mm zoom.
The green channel predominates here, with blue at about 80% of the green
RAW values, and red at about 50%. The average green value in the white
square is about 1520. The blackpoint of the 10D at ISO 100 is about
126, so that leaves us 1494 RAW values for the 90% reflectance square
That would put 100% at 1660 above blackpoint. The 10D clips at ISO 100
anywhere from 3997 to 4005 or so (depending on pixel column), so the
maximum above blackpoint is about 3875. 1660/3875 = 0.43, or ISO 43, if
the standard is 100% reflectance. Now, I shot this in a white hallway
28" wide from 8.2 feet, so there is some extra light bouncing off the
walls, so the actual value is actually lower, although the practical
value in a narrow white hallway is about right.


First of all, if you want to demonstrate a 1 stop headroom, you simply
overexpose a stepwedge by one stop and compare it to a normally exposed
stepwedge.


Based on what reference? A stop more of what? The camera's metering?
An incident meter? One of the grey squares?

Playing the numbers game doesn't say much.


RAW data *is* numbers.

I don't think that playing with flashes and relying on the guide number is
a good way to get repeatable results.


I suppose it's just a coincidence that my Sekonic meter, sunny f/16, and
the 550EX in manual mode all give very similar results?

If you think a camera's metering is more accurate than the GN of a
speedlight flash, you put a lot of faith in the wrong things, IMO.
--


John P Sheehy

  #219  
Old August 30th 05, 06:44 AM
Philip Homburg
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In article , wrote:
In message coh.net,
If you think a camera's metering is more accurate than the GN of a
speedlight flash, you put a lot of faith in the wrong things, IMO.


No, I think that a standalone light meter is designed to be accurate (around
D50).


--
That was it. Done. The faulty Monk was turned out into the desert where it
could believe what it liked, including the idea that it had been hard done
by. It was allowed to keep its horse, since horses were so cheap to make.
-- Douglas Adams in Dirk Gently's Holistic Detective Agency
  #220  
Old August 30th 05, 10:11 PM
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In message coh.net,
(Philip Homburg) wrote:

In article , wrote:
In message coh.net,
If you think a camera's metering is more accurate than the GN of a
speedlight flash, you put a lot of faith in the wrong things, IMO.


No, I think that a standalone light meter is designed to be accurate (around
D50).


And just how much do you think the flash, or shade, would affect the
reading? No frequency should vary more than about 1/3 stop between
"D50", and the meter isn't even narrow-band by any measure, just
slightly weighted.

What maximum RAW channel value do you think I would get on the 10D if I
had a D50 light source and an accurate meter calibrated to it, for 100%
reflectance?

Under no lighting situation that I've ever used my 10D with external
metering (incident or grey card), has 100% reflectance come anywhere
near the RAW maximum.

I state with 100% confidence that you can set your external meter to ISO
50 with the 10D set to ISO 50 (or 20D set to ISO 64), if you use
absolute exposure, and your brightest highlights are matte-reflective
only. If you need to capture detail in glare, or specular highlights,
then you might not get away with it (although the red and blue channels
will still have headroom to record greyscale highlights). Reflectance
of matte whites is usually 90% or less, and you may be able to push the
exposure more in some lighting.


Remember, the reputation that some DSLRs for blowing out highlights come
foremost from JPEGs that clip away RAW highlights, and even when the RAW
data itself is clipped, it is because the camera's AE increased the
exposure by a few stops because of weighted averages from the metering
(like a sliver of sky surrounded by shady city buildings on both sides
of the street). Incident metering rarely blows highlights and often
leaves headroom unused.
--


John P Sheehy

 




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