D70 Photoshop and gray cards

If you were to shoot a gray card and then bring it up in Photoshop,
using the info pallet what you be the values of a correctly exposed
gray card be?

No Name

If your monitor is not calibrated you can come up with anything but using a
"moderately calibrated" monitor the grey card will be roughly 160R, 160G,
160B. Some people think it should be 128 because that is the "middle" of the
grey scale -- they are wrong.

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"No Name" <> wrote in message
news:...
> If you were to shoot a gray card and then bring it up in Photoshop,
> using the info pallet what you be the values of a correctly exposed
> gray card be?

Tony Spadaro

"No Name" <> wrote in message
news:...
> If you were to shoot a gray card and then bring it up in Photoshop,
> using the info pallet what you be the values of a correctly exposed
> gray card be?

There is an (understandable) confusion about this subject, it pops up
regularly, and it
finds its roots in the different concepts of mid-gray.

Some speak of mid-gray as halfway between black and white, so 127.5 on a
scale from 0 to
255. However, that is NOT the mid-gray of a gray card, because that should
reflect something
like 18% of the light incident to the card at a certain angle(!).
Perceptually that is very roughly the same as the reflection of an average
scene.

In addition, assuming you are viewing a gamma corrected file like a JPEG
from you camera,
most digicam files have an adjusted gamma of about 1/2.2 to provide a
compensation for the
native display (CRT) gamma (it also allows to encode with a bit more
accuracy, but that's a
different story).

If we put that in a formula, it would approximately look like this:
255*0.18^(1/2.2) = 117 (or 30058 in a scaled 16-bit file, and 15029 in a
15-bit Photoshop CS file).

However, that assumes the ISO speed setting of your camera to be determined
in a way that
18% reflection is recorded in the Raw camera data at 18% of its 12-bit ADC
range, which it
is not necessarily the case. If it were, the formula would become
((2^12)-1)/16*0.18^(1/2.2)
= 117.

The International Organization for Standardization has determined a standard
(12232) for
determining an ISO speed value for digicams, in such a way that the number
approximately
compares to film ISO speeds
(http://www.iso.ch/iso/en/CatalogueDe...CSNUMBER=21020
&ICS1=37&ICS2=40&ICS3=10).
However, it is determined on a decision that is based on the determination
of two types of
optimal exposure. One exposure level considers the maximum exposure before
the sensor
saturates (more exposure will not result in higher voltage), and the other
exposure level is
based on an acceptable Signal-to-Noise level.

So, to make a long story short, you could find a value of 117, but no
guarantee can be
given, also since (handheld) exposure meters can be ISO calibrated for a
12.5% average
refection instead of 18%. That would increase exposure and lead to somewhat
higher RGB
values.

The confusion probably stems from the approximately similar values of 50%
brightness and the 18% average scene reflection AND gamma correction
values, but they have an unrelated basis.

Bart

Bart van der Wolf

That depends on the colorspace you are using (it could be anywhere from
45 to 153 on commonly used colorspaces - but tends to be closer to 100
to 12.

It should show a LAB L* value close to 50% with a* and b* close to
zero, regardless of colorspace (just assuming the colorspace is close
to correct for the camera/scanner).

Chris



In article <>, No Name
<> wrote:

> If you were to shoot a gray card and then bring it up in Photoshop,
> using the info pallet what you be the values of a correctly exposed
> gray card be?

Chris Cox