Grey card for accurate colours

I have a grey card which according to the manufacturer reflects all
light wavelengths by the same amount.

If I place it next to an object I want to photography and then during
photo processing adjust the white balance so that the RGB values on the
grey card are equal, will I see the true colours of the object?

Example: I take a photo of a statue or a block of marble, but due to the
illumination (could be artificial light, outdoor overcast or sunny for
instance) I cannot know what the true colours of the statue or the
marble are. But if I have a reference object of which I know the colours
it should be possible to get an accurate colour management.

Of course in this procedure I make sure that the grey card is
illuminated by the same light as the object I'm photographing.
--

Alfred Molon
------------------------------
Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site

Alfred Molon wrote:
I have a grey card which according to the manufacturer reflects all
light wavelengths by the same amount.

If I place it next to an object I want to photography and then during
photo processing adjust the white balance so that the RGB values on the
grey card are equal, will I see the true colours of the object?

Example: I take a photo of a statue or a block of marble, but due to the
illumination (could be artificial light, outdoor overcast or sunny for
instance) I cannot know what the true colours of the statue or the
marble are. But if I have a reference object of which I know the colours
it should be possible to get an accurate colour management.

Of course in this procedure I make sure that the grey card is
illuminated by the same light as the object I'm photographing.


If you light a grey card with yellow light, what colour will it be?

True that you can use a grey card to determine a 18% neutral balance
between white and black but before any of this... You must first have
the colour of white established.

This means you need both white and 18% grey.

A more reliable method and one that will also establish the true colour
of white is to use a white disc over the lens (whitebal is one brand)
and take a reference shot to calibrate the camera.

You will probably need to do this several times in a morning or
afternoon but it is very reliable. I bought one from Hong Kong via Ebay
for a few dollars.

"Alfred Molon" wrote in message
...

I have a grey card which according to the manufacturer reflects all
light wavelengths by the same amount.

If I place it next to an object I want to photography and then during
photo processing adjust the white balance so that the RGB values on the
grey card are equal, will I see the true colours of the object?


In theory, yes. In practice, no necessarily. You can also fill the frame
with the card and use custom white balance in camera too.

I have never got on well with using grey cards to set white balance. Maybe
not a streamlined workflow, but I prefer to shoot in RAW and adjust if
necessary in post process. You will find that sometimes you want a colour
cast to a subject (for example, an indoor shot of a person using ambient
light would not look right if there wasn't an orange colour cast).

On Sun, 7 Sep 2008 11:04:27 +0200, Alfred Molon
wrote:

I have a grey card which according to the manufacturer reflects all
light wavelengths by the same amount.

If I place it next to an object I want to photography and then during
photo processing adjust the white balance so that the RGB values on the
grey card are equal, will I see the true colours of the object?

Example: I take a photo of a statue or a block of marble, but due to the
illumination (could be artificial light, outdoor overcast or sunny for
instance) I cannot know what the true colours of the statue or the
marble are. But if I have a reference object of which I know the colours
it should be possible to get an accurate colour management.

Of course in this procedure I make sure that the grey card is
illuminated by the same light as the object I'm photographing.

Someone may correct me here, but I'm under the impression that using
the gray card, or any other device to set the white balance, makes the
image appear *as the object appears*. In other words, you don't get
the "true color" of the object, you get the true color of the object
as it appears under the lighting conditions at the time the photograph
is taken.

The object, if placed in different lighting conditions, may appear to
be different in color. If the object was then photographed, after
having the white balance set for the new conditions, the image would
then correctly represent the different conditions.

Two photographs taken of the statue - when the statue was illuminated
by different lighting conditions in each - would be different in
color, but each would be correct.
--
Tony Cooper - Orlando, Florida

tony cooper wrote:

Someone may correct me here, but I'm under the impression that using
the gray card, or any other device to set the white balance, makes the
image appear *as the object appears*. In other words, you don't get
the "true color" of the object, you get the true color of the object
as it appears under the lighting conditions at the time the photograph
is taken.

It's the opposite - the purpose of white balance is to compensate for
off-white lighting. It applies a color cast which is supposed to make
the object the same color it would have been had it been under white
light. If you wanted to accurately capture the colors as they actually
were at the time of the shot, you would calibrate the camera at the
factory and then never touch it.

Alfred Molon wrote:
I have a grey card which according to the manufacturer reflects all
light wavelengths by the same amount.

If I place it next to an object I want to photography and then during
photo processing adjust the white balance so that the RGB values on the
grey card are equal, will I see the true colours of the object?

Example: I take a photo of a statue or a block of marble, but due to the
illumination (could be artificial light, outdoor overcast or sunny for
instance) I cannot know what the true colours of the statue or the
marble are. But if I have a reference object of which I know the colours
it should be possible to get an accurate colour management.

Of course in this procedure I make sure that the grey card is
illuminated by the same light as the object I'm photographing.

Grey cards are meant as an exposure reference; not as a color reference.

Set your camera to the temperature of the light source and you'll be
much closer.

Flash: 5500K
Incandescent: 2800K
Fluoresecent: 4000K (fudge)
Sunny winter day: 8000K (+/- 2000).

etc.

If you shoot raw, you can fix this during raw import.



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On 9/7/08 9:12 AM, in article
,
" wrote:

On Sep 7, 7:32*am, Shon Kei Picture company
wrote:
..


If you light a grey card with yellow light, what colour will it be?

True that you can use a grey card to determine a 18% neutral balance
between white and black but before any of this... You must first have
the colour of white established.

This means you need both white and 18% grey.

A more reliable method and one that will also establish the true colour
of white is to use a white disc over the lens (whitebal is one brand)
and take a reference shot to calibrate the camera.

You will probably need to do this several times in a morning or
afternoon but it is very reliable. I bought one from Hong Kong via Ebay
for a few dollars.- Hide quoted text -

- Show quoted text -

Frankly using a gray card works just fine nearly all the time.
It worked for me for almost 40 years. For exposure.

For color balance, use the other side (white).

In article ,
Alan Browne wrote:

If you shoot raw, you can fix this during raw import.

Forgive me if this is an elementary question...

I've just started using Aperture (I've got version 2.1.1). I don't
understand all the different flavors of color manipulation that are
available.

Under White Balance, there's sliders for Temp and Tint. They both change
the colors, but I'm at a loss to understand exactly what they're doing
that's different.

Under Enhance/Tint, there's three color wheels labeled Black, Gray, and
White. Under Levels, I can pick the Red, Green, and Blue channels and
three different (unlabeled) sliders around for each.

And then lastly, under Color, for each of red, yellow, green, cyan, blue,
and magenta, I've got four sliders labeled Hue, Saturation, Luminance, and
Range.

That's, if I count correctly, 31 different degrees of freedom for adjusting
colors in one way or another. How do I sort out this madness into
something that makes sense. How do I look at an image and say, "Oh, the
XXX is too high".

In article , Shon Kei
Picture company says...

If you light a grey card with yellow light, what colour will it be?

In practice the light does never consist of just one colour - it's
always distributed over a certain range of colours. I'm talking about
outdoor daylight or artificial light illumination.
Then it's up to you to decide if you want those white walls to look
yellow in the image or not.
Daylight on a sunny day is blueish

True that you can use a grey card to determine a 18% neutral balance
between white and black but before any of this... You must first have
the colour of white established.

This means you need both white and 18% grey.

I don't follow you here. I just want to know how to process the images
to remove the colour cast caused by the non-white illumination. For that
a grey card should be sufficient.

For instance, I used a grey card to measure the colour of the sand in
Merzouga:
http://www.molon.de/galleries/Morocco/Merzouga/Sand/

The sand is indeed orange.

Here I was curious to know what colour the stones of the pyramids in
Gizeh have:
http://www.molon.de/galleries/Egypt/...img.php?pic=10

They are some kind of orange-pink.
--

Alfred Molon
------------------------------
Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site

In article , Roy Smith says...

Under White Balance, there's sliders for Temp and Tint. They both change
the colors, but I'm at a loss to understand exactly what they're doing
that's different.

Temp adjusts the blue/yellow balance, tint the red/green balance.
--

Alfred Molon
------------------------------
Olympus 50X0, 8080, E3X0, E4X0, E5X0 and E3 forum at
http://tech.groups.yahoo.com/group/MyOlympus/
http://myolympus.org/ photo sharing site