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Gray card re-visited

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Shay Stephens said:
Looks like the gray card is too bright, the white paper behind the
ink is probably showing through too much. My estimate is that the
card is between .3EV and .7EV too bright.
Click to expand...
Indeed, the histogram of both was slightly to the right of the center, which proves that my photos are somewhat overexposed. I used Manual exposure without a tripod, so maybe my reading was not takes carefully enough.
Shay Stephens said:
Interstingly, the indoor
photo shows the gray portion nearly 128,128,128 but since the gray
is too bright it drags the white area down with it. If you print
another one, try to darken the output of the printer, but keep the
gray card image you are printing at 128,128,128.
Click to expand...
I'm not sure I understand this.

What I can do is to do again the printing with higher quality. This make the printed area denser. I'll try that tomorrow.

Interestingly, when I compare side-to-side the indoor gray with the photoshop 128,128,128 - they are very close to each other.

I thank you, Yehuda
 
Also I think you may be confusing what the 18% refers to, it does not refer to the grayscale scale but instead the amount of light that is reflected from the card. In the case of 18% that means that the eye sees a middle gray color that is 50% on the grayscale. Both use % but the numbers mean different things.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
 
That's true but 18% reflectance in dim light compared to 18% reflectance in bright sunlight makes the shade

of the card appear different. If you wish to match the shade of the grey card with a printed grey, how else can you do it but by comparing them side by side, in the same light?
Shay Stephens said:
Also I think you may be confusing what the 18% refers to, it does
not refer to the grayscale scale but instead the amount of light
that is reflected from the card. In the case of 18% that means
that the eye sees a middle gray color that is 50% on the grayscale.
Both use % but the numbers mean different things.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
Click to expand...
 
It would appear different to the eye, but if metered off of and properly exposed it would look the same (or nearly so) as a gray card in normal light when photographed. The right hand portion of the comparison shows my gray card behind a focus target I took a photo of, the shade of the gray card in the background looks the same as the middle part of the comparison that was painted at 128,128,128. The left hand side of the comparison showing a painted 46,46,46 section is much darker than the photographed gray card.



You could still use a gray card printed using 46,46,46 you would just have to determine how far from 0EV it reads at, just as you have to if you printed a gray card using 128,128,128.

The comparison of any printed gray card will of course have to be compared and calibrated, ideally against a know 18% gray card, but it can be done by comparison of the exposure derived by using it. One can use the histogram display when analyzing the photo to determine if the photo is under or overexposed, then adjusting the EV value accordingly until a standard EV compensation value is derived. I have been able to even meter off of my hand since I know how bright it is in comparison to a gray card, but with exposure to sun my hand changes reflectance with time, so the gray card is much more stable as an exposure standard. I suspect a printed gray card would be less stable over time as well.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
 
Taking that the graycard's color is 128,128,128 might it serve for screen color calibration? Open PhotoShop, make a rectangle filled with 128,128,128 and change the screen settings until it matches the graycard.
Am I correct in my assumption?

Yehuda
 
Yehuda Katz said:
TurboTed said:
I don't know the phototechnical background of an 18% gray card.

Assuming 18% gray is actually the same as 82% of white, then the
value would be 210 for RGB for 210 for the percent brightness in
Photoshop.

This gives a very light gray result which I don't know is even
close to what is considered a "gray card"
Click to expand...
I think that a better first approximation would be to put a real
graycard next to the monitor and compare it with several "grays".
Click to expand...
I don't trust my monitor, it can be so different than real life. Plus, I just can't seem to get over comparing light produced by a monitor as opposed to light reflected by a card.

The other thing you could do is print out contact sheet of gray squares and see which one matches the real thing.
Yehuda Katz said:
In the meantime I've found a graycard at Ritz in our area, and I'll
be able to carry out those experiments by myself.

Thank you, Yehuda
Click to expand...
 
I think the confusion stems from thinking that the camera adjusts EV to see the grey card at 50% and then translating that to mean the grey card should be printed at 128-128-128 (which is also 50% on the grey scale).

Both grey scale and reflected light use scales that correlate. Imagine a neutral color card calibrated to 100% reflectance; it would appear very white to the human eye. A card calibrated for 0% reflectance would appear as black. The same holds for grey scale measurements. A grey scale of 100 appears white and 0 appears black.

In one of your posts you stated:

"In the case of 18% that means that the eye sees a middle gray color that is 50% on the grayscale. Both use % but the numbers mean different things."

I think you meant to say the "camera" sees a middle gray color that is 50% on the grayscale (after metered off an 18% grey card). The eye sees a grey card at different grey scales depending on the amount of light reflected.

The eye seeing the grey card at 50% has little relevance when you consider that a grey card can appear (to the human eye) nearly black in very low light and light grey in extremely bright light.
Shay Stephens said:
It would appear different to the eye, but if metered off of and
properly exposed it would look the same (or nearly so) as a gray
card in normal light when photographed. The right hand portion of
the comparison shows my gray card behind a focus target I took a
photo of, the shade of the gray card in the background looks the
same as the middle part of the comparison that was painted at
128,128,128. The left hand side of the comparison showing a
painted 46,46,46 section is much darker than the photographed gray
card.



You could still use a gray card printed using 46,46,46 you would
just have to determine how far from 0EV it reads at, just as you
have to if you printed a gray card using 128,128,128.

The comparison of any printed gray card will of course have to be
compared and calibrated, ideally against a know 18% gray card, but
it can be done by comparison of the exposure derived by using it.
One can use the histogram display when analyzing the photo to
determine if the photo is under or overexposed, then adjusting the
EV value accordingly until a standard EV compensation value is
derived. I have been able to even meter off of my hand since I
know how bright it is in comparison to a gray card, but with
exposure to sun my hand changes reflectance with time, so the gray
card is much more stable as an exposure standard. I suspect a
printed gray card would be less stable over time as well.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
Click to expand...
 
Another way to think of it is "2.5 f-stops" instead of "18%."

When your reflected meter reading is 2.5 f-stops lower than the incident reading, that's your 18% gray. You could test your card by metering with your camera: the card should read 2.5 f-stops (or EV units in manual) darker compared to metering by aiming the camera directly at the light source.

(If you're being really technical and checking my math, yeah, 2.5 f-stops actually comes to 100 / 2 ^ 2.5 = 17.677% gray, but the 2.5 f-stops is the average incident/reflected difference that the 18% card approximates.)

-jeremy

--
Jeremy Birn
http://www.3dRender.com/
 
Ok, but we are talking about cameras here. What my eye sees has no bearing on what a camera exposure will look like. If the lighting is so dark that a gray card looks very dark gray, a the photo when properly exposed is still going to be 128,128,128 when eyedroppered on a computer screen.

I am a little lost as to what your point or argument is right now. My original point was that, when a gray card is properly exposed and photographed and then analyzed on a computer, the eyedropper results will show the color to be 128,128,128 on the RGB scale regardless of the monitor calibration, the brightness of the light when the photo was taken, or what the eye sees when looking at that card in the light it was photographed in.

Yehuda wanted to know what color to print a gray card using the RGB scale. And so my answer was RGB=128,128,128.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
 
I think you're confusing due to the 18% gray value translating into 50% gray in the RGB scale.
This is complicated, may be someone might clarify it better but simply put,

photographic films (reflecting) response to brighness (and the corresponding perception of human eye) varies in a logarithmic scale. In a logarithmic scale the middle gray corresponds to 18% reflectance value. However, the CRT is linearly calibrated and therefore the middle gray (and corresponding human perception) takes 50% reflectance value.
18% gray in reflecting paper = 50% gray in a CRT (rgb = 128)

However,
Jim Walker said:
Both grey scale and reflected light use scales that correlate.
Imagine a neutral color card calibrated to 100% reflectance; it
would appear very white to the human eye. A card calibrated for 0%
reflectance would appear as black. The same holds for grey scale
measurements. A grey scale of 100 appears white and 0 appears black.

In one of your posts you stated:

"In the case of 18% that means that the eye sees a middle gray
color that is 50% on the grayscale. Both use % but the numbers mean
different things."

I think you meant to say the "camera" sees a middle gray color that
is 50% on the grayscale (after metered off an 18% grey card). The
eye sees a grey card at different grey scales depending on the
amount of light reflected.

The eye seeing the grey card at 50% has little relevance when you
consider that a grey card can appear (to the human eye) nearly
black in very low light and light grey in extremely bright light.
Shay Stephens said:
It would appear different to the eye, but if metered off of and
properly exposed it would look the same (or nearly so) as a gray
card in normal light when photographed. The right hand portion of
the comparison shows my gray card behind a focus target I took a
photo of, the shade of the gray card in the background looks the
same as the middle part of the comparison that was painted at
128,128,128. The left hand side of the comparison showing a
painted 46,46,46 section is much darker than the photographed gray
card.



You could still use a gray card printed using 46,46,46 you would
just have to determine how far from 0EV it reads at, just as you
have to if you printed a gray card using 128,128,128.

The comparison of any printed gray card will of course have to be
compared and calibrated, ideally against a know 18% gray card, but
it can be done by comparison of the exposure derived by using it.
One can use the histogram display when analyzing the photo to
determine if the photo is under or overexposed, then adjusting the
EV value accordingly until a standard EV compensation value is
derived. I have been able to even meter off of my hand since I
know how bright it is in comparison to a gray card, but with
exposure to sun my hand changes reflectance with time, so the gray
card is much more stable as an exposure standard. I suspect a
printed gray card would be less stable over time as well.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
Click to expand...
Click to expand...
 
Instead of printing a gray card why don't you use a white paper and open up 2-2.5 stops?
Else, take a reading from your palm and open up 1 stop.
I think you can buy kodak gray cards from amazon.com
Yehuda Katz said:
Hi friends,

I want to print by myself a gray card. Can you give me rhe R-G-B
value of 18% gray?

Thank you, Yehuda
Click to expand...
 
Shay Stephens said:
Ok, but we are talking about cameras here. What my eye sees has no
bearing on what a camera exposure will look like.
Click to expand...
Ah, but that's the problem! Actually we're trying to match the properties of a physical grey card so that Yehuda can print out a grey card. No cameras are needed for that. You could just as well go to Home Depot and have them color match flat grey paint to match a grey card.

All you have done is set your camera exposure to an 18% grey card, which (as you stated) makes the camera look at it at 50%. Thus your photo matches 128-128-128 grey (which also just happens to agree with 50% grey scale)! If you printed it out it would still be 50%
Shay Stephens said:
If the lighting
is so dark that a gray card looks very dark gray, a the photo when
properly exposed is still going to be 128,128,128 when eyedroppered
on a computer screen.

I am a little lost as to what your point or argument is right now.
My original point was that, when a gray card is properly exposed
and photographed and then analyzed on a computer, the eyedropper
results will show the color to be 128,128,128 on the RGB scale
regardless of the monitor calibration, the brightness of the light
when the photo was taken, or what the eye sees when looking at that
card in the light it was photographed in.

Yehuda wanted to know what color to print a gray card using the RGB
scale. And so my answer was RGB=128,128,128.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
Click to expand...
 
Oh Good Yehuda! I'm glad you found it. I ended up going to my camera store here and picking up that "Kodak Gray Cards." It contains two thick 8 x 10 18% gray cards with white on the other side. It was 14.95 out here.

Thank you for bringing up the subject. It's one more thing I need to learn.

I need to work on my Monitor calibration next. I have a Dell Trinitron 19" monitor that seems to be pretty close to what the true colors are. I just sent one back to them that was too dark and had live wavey lines going through it. It would have worn out my eyes.

I have this one set on 91.3khz and 1280 x 1024 resolution. It can up up to 1600 x 1024, but that seems a bit small for my eyes.

D.
Yehuda Katz said:
DD,

Thank you. I found it finally at a Ritz shop here.

Yehuda
Click to expand...
--
DeeDee G.
http://www.pbase.com/deedee_g/root
 
DeeDee G. said:
Oh Good Yehuda! I'm glad you found it. I ended up going to my
camera store here and picking up that "Kodak Gray Cards." It
contains two thick 8 x 10 18% gray cards with white on the other
side. It was 14.95 out here.
Click to expand...
Same here. Tomorrow I'll get one for myself.
DeeDee G. said:
Thank you for bringing up the subject. It's one more thing I need
to learn.

I need to work on my Monitor calibration next. I have a Dell
Trinitron 19" monitor that seems to be pretty close to what the
true colors are. I just sent one back to them that was too dark
and had live wavey lines going through it. It would have worn out
my eyes.
Click to expand...
I too had once wavy lines. After a hint from a friend, I removed the screen a few inches away from the elecric outlet, and gone the streaks.

Thank you very much for your mail.

Yehuda
 
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