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Expo Disc Review

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gary ng said:
Can I achieve the same result using say a ND filter ?
Click to expand...
gary...not the same thing. Expo Disc is used to achieve perfect color via setting a custom White Balance.

J.S.
Member said:
jan sanders said:
For perfect color on my D60 and D-30 I use an Expo Disc, end of story.
Much discussed here as well as WB issues. Now there is a review of
the Expo Disc available, you can find it here:

http://www.digitalfocus.net/sections/reviews/ColorManagement/Expodisc/Expodisc_1.htm

at the Digital Focus site.
Enjoy!

Jan Sanders
Los Angeles
Click to expand...
Click to expand...
 
jan sanders said:
For perfect color on my D60 and D-30 I use an Expo Disc, end of story.
Much discussed here as well as WB issues. Now there is a review of
the Expo Disc available, you can find it here:

http://www.digitalfocus.net/sections/reviews/ColorManagement/Expodisc/Expodisc_1.htm

at the Digital Focus site.
Enjoy!

Jan Sanders
Los Angeles
Click to expand...
I just tried an old Expo disk I had. The colors under some very difficult light are the best I've seen, extremely accurate, dead on colors. Highly recommended!

Troy
 
Jan,

I confused you. My understanding of the Expo Disc and custom white balance is that it is basically a very 'thick' filter of a 18% gray. So say if the scene has a light source that end up with an overall yellow bias, what the camera sensor see would be a uniform yellow. If this picture is used to for white balance correction(custom white balance), the camera just subtract this amount of yellow from the actual picture taken.

So in theory, I can use a very dark ND filter(may be multiple of it) to simulate the 18% gray.
jan sanders said:
gary ng said:
Can I achieve the same result using say a ND filter ?
Click to expand...
gary...not the same thing. Expo Disc is used to achieve perfect
color via setting a custom White Balance.
Click to expand...
 
A ND filter doesn't remove color like the expo-disc does. In fact, the ND filter is supposed to be neutral. Preserving the exact colors, just reducing the total amount of light.

The expo-disc is supposed to turn all colors into 18% gray.
gary ng said:
I confused you. My understanding of the Expo Disc and custom white
balance is that it is basically a very 'thick' filter of a 18%
gray. So say if the scene has a light source that end up with an
overall yellow bias, what the camera sensor see would be a uniform
yellow. If this picture is used to for white balance
correction(custom white balance), the camera just subtract this
amount of yellow from the actual picture taken.

So in theory, I can use a very dark ND filter(may be multiple of
it) to simulate the 18% gray.
Click to expand...
--
The Unofficial Photographer of The Wilkinsons
http://thewilkinsons.crosswinds.net
Photography -- just another word for compromise
 
Hum, that is the point puzzling me. If the expo-disc is supposed to turn all colors into 18% grey, how can it be used for setting custom white balance ?

My understanding of the custom white balance from the description of using a white/gray card is that, the camera expect to see the color cast from the surrounding light and only by measuring this cast based on the assumption that the object(white card) that it is shooting at is color neutral, it can detect the color cast and make subsequent adjustment to photo taking in the same lighting environment.
DavidP said:
The expo-disc is supposed to turn all colors into 18% gray.
gary ng said:
I confused you. My understanding of the Expo Disc and custom white
balance is that it is basically a very 'thick' filter of a 18%
gray. So say if the scene has a light source that end up with an
overall yellow bias, what the camera sensor see would be a uniform
yellow. If this picture is used to for white balance
correction(custom white balance), the camera just subtract this
amount of yellow from the actual picture taken.

So in theory, I can use a very dark ND filter(may be multiple of
it) to simulate the 18% gray.
Click to expand...
--
The Unofficial Photographer of The Wilkinsons
http://thewilkinsons.crosswinds.net
Photography -- just another word for compromise
Click to expand...
 
Let me try to understand how the Expo-Disk works. My sense of it is that both Gary & David P have a piece of it, but Gary is essentially more correct.
gary ng said:
My understanding of the custom white balance from the description
of using a white/gray card is that, the camera expect to see the
color cast from the surrounding light and only by measuring this
cast based on the assumption that the object(white card) that it is
shooting at is color neutral, it can detect the color cast and make
subsequent adjustment to photo taking in the same lighting
environment.
Click to expand...
Member said:
DavidP said:
A ND filter doesn't remove color like the expo-disc does. In fact,
the ND filter is supposed to be neutral. Preserving the exact
colors, just reducing the total amount of light.

The expo-disc is supposed to turn all colors into 18% gray.
Click to expand...
Click to expand...
The way the camera knows if something is white is by looking for, in RGB, 256,256,256. If I shoot a Kodak white card in a given situation, and use that image as the source for my custom white balance, it will be different from 256,256,256, depending on the light reflecting off the card. The camera will look at the image, determine how far the recorded R, G, & B values are from 256 (or from 128, if I used a 50%Gray card), and produce a profile that will compensate for these differences.

Expo-Disk saves you the incovenience of finding something neutral (gray or white) in the scene to make the reading off. First, by reducing the levels of light reaching the sensor to 18% of the average level reflected by your subject, it sets up the correct exposure in the camera for the average as if it were 18% reflective.

Then, it has to allow some of the light in the scene change the value from what the Expo-disk would see if it were shooting into the sun at midday. Therefor, it does not remove all color, or the expo-disk wouldn't be giving the camera the information it needs to adjust for the color cast.

Putting it in simpler terms, the camera asks the expo-disk "what does white look like after 82% of the light is removed from the scene and the remaining light (the color mix reflected off the subject) passes through the expo-disk?" Then the camera has to say "That ain't white...we gotta add some of this and take away some of that..."

Somebody tell me if I got it right?

--
Walter K
 
In the review that Jay linked to, it was stated that a 72mm expo-disk will work with a 77mm (diameter) lens, by holding it in front of the lens. The assumption was made that it is only the center of the lens that is involved, possibly taking into consideration the crop factor.

However, doesn't all the light passing through the lens still influence the reading? If that is true, then the WB set through the smaller expo-disk might be close, but not necessarily accurate?
--
Walter K
 
This disk is just a diffuser that mixes together all the
light coming in to "average it", and an 18% ND filter to
darken it.

Custom white balance requires the ability to measure the
"color balance" of the light hitting the subject. Similar
to exposure metering, this can be done via "incident"
or "reflective" metering, and "reflective" metering requires
an assumption of the reflective attributes of the subject.
If the light is slightly yellow, and you measure it by
seeing how it reflects off a blue subject, a reflective meter
is going to think the light source is bluer than it is.
Measuring color balance reflectively requires a "neutral"
subject, like a white card. So if you shoot a frame of a
white card, and the reflected light is yellow, now you know
the color of the light source, and you can "subtract" that
color from all subsequent shots of your real subject. If you
can do incident metering of color balance, though, then you
don't have to worry about providing a subject with neutral
reflective qualities.

If you place an expodisc on the camera, it diffuses the
incoming light and averages together the light from various
sources. So you put the camera in front of the subject in
"incident light meter" position, and shoot the incoming light,
and you'll get an averaged reading of the various light sources
in the correct proportions. This removes the color of the subject
from the equation, so it removes the requirement of having a
perfectly neutral subject to shoot. This is how the expodisc
would give most reliable results, I'd think.

Now, this review tells you to always point the camera at the
subject. If you do this, you're once again dependent on the
reflective properties of the subject, if you're taking a
close-up of a purple dinosaur, your expodisc shot is going
to have a purple tint, which the camera will think comes
from the light source. The expodisc still helps here
by diffusing the light that hits the filter, which gives you
an "average" over a much greater area than just the field of
view of the lens. So if you're shooting a purple dinosaur with
a telephoto lens, you can hope that the stuff surrounding the
purple dinosaur has a variety of colors, enough to help balance
out the average color of the scene, as the expodisc will pull
in light from all directions in front of the camera.

The review also suggests that its okay to hold up a smaller
filter in front of a larger lens, since most cameras only meter
the center of the frame. This logic isn't quite valid, as the image
in the center of the frame is derived from light that strikes the
entire surface of the lens, so this technique will leak in some
light around the edge of the filter, which just gives you slightly
less effective "averaging".
 
It looks to me as though the Expo Disc is basically a diffuser plus a neutral density filter that works like the white plastic domes used on light meters to take ambient readings. It works by averaging all the light from the scene so the camera's meter and white balance circuitry sees the average scene brightness and color.

A couple of problems I see with the Expo Disc:

1) It has no way of knowing what field of view you are photographing so it could be inaccurate if you are using a telephoto lens, for example, and the little piece of the overall scene you are photographing is much darker or lighter than the surrounding area. This is a situation where careful spot metering would work better.

2) Another problem is that it can't really read the color temperature of the light illuminating the scene, just the average scene color. Granted this gets warmer or cooler as the light changes, but in a forest, for example, it might think the light is green when it is really the scene that is green and the light is yellow.
WalterK said:
Somebody tell me if I got it right?

--
Walter K
Click to expand...
--
Jonathan Sachs
Digital Light & Color
 
Without getting into too much detail, read the series of articles on Color Management on the site. This explains it better. In a quick answer:

Consider White as 0% grey (0% R, 0% G, 0%B). The 18% grey is also (18% R, 18% G, 18% B). To your camera this makes no difference as the only noticable difference to your sensor is the luminance value. Since you are not metering in this case, you are taking a WB reading, any shift is noted as a change in the color temperature.

As for metering, your camera assumes that all scenes should average out to 18% grey (it is built into the assumptions of your in camera meter). The ExpoDisc then mates this to a proper reading of the scene at 18% grey - providing perfect metering of the scene as well.

Check out the articles.

Peter Sills
Digital Focus
gary ng said:
My understanding of the custom white balance from the description
of using a white/gray card is that, the camera expect to see the
color cast from the surrounding light and only by measuring this
cast based on the assumption that the object(white card) that it is
shooting at is color neutral, it can detect the color cast and make
subsequent adjustment to photo taking in the same lighting
environment.
DavidP said:
The expo-disc is supposed to turn all colors into 18% gray.
gary ng said:
I confused you. My understanding of the Expo Disc and custom white
balance is that it is basically a very 'thick' filter of a 18%
gray. So say if the scene has a light source that end up with an
overall yellow bias, what the camera sensor see would be a uniform
yellow. If this picture is used to for white balance
correction(custom white balance), the camera just subtract this
amount of yellow from the actual picture taken.

So in theory, I can use a very dark ND filter(may be multiple of
it) to simulate the 18% gray.
Click to expand...
--
The Unofficial Photographer of The Wilkinsons
http://thewilkinsons.crosswinds.net
Photography -- just another word for compromise
Click to expand...
Click to expand...
--
Peter Sills
Digital Focus
http://www.digitalfocus.net
 
jan sanders said:
For perfect color on my D60 and D-30 I use an Expo Disc, end of story.
Click to expand...
Unfortunately, it's not the end of my story.

I have the disk and doing some exposure test/practice with the 72mm Expo Disk held infront of the 77mm opening of a Canon 28-70mm f/2.8L I came up with confusing exposure results. Oh! And yes, I did read the directions, several times:-)

The sky/Sun exposure test came up spot on with a meter reading of 1/125th @ f/16, the ISO setting was 100. But decent histogram exposures are 2 1/2 stops off the metered results, underexposed. When the ED exposure is used, the image is almost black.

I don't have enough practice with the Expo Disk to make a pronouncement. What I've come up with so far, is that in reflective light situations, I take two reading, one of the object being photographed without the ED and one with the ED held in front of the lens. I then average the two readings with the +2/-2 under/over exposure compensation thumb wheel and get excellent exposure results and I then use the Histogram to finalize my exposure choice. Doing it this way may take four to six images but the evenness of the exposure makes the effort worth it. The extra images, I just erase them before I take anymore images and the numbering stays consecutive.

Waiting for some more time on a sunny day so I can do some more practice as I think the Expo Disk will be an invaluable exposure tool, once I learn how to use it:-)
 
gary ng said:
Hum, that is the point puzzling me. If the expo-disc is supposed to
turn all colors into 18% grey, how can it be used for setting
custom white balance ?
Click to expand...
It's doesn't turn all colors into 18% grey, it allows 18% of what ever light is coming though it to reach the metering sensor. The color may be red, yellow, purple but the amount of colored light will be 18%. Then from there, the light sensor, in the camera, works with the color shift that is being presented to the sensor to get a decent white balance.
 
I hold it in front of the lens & shoot an image pointed straight at the light source(IE: straight in to the sun).

This is not really what the directions say, but it usually works great.

-John
 
WalterK said:
However, doesn't all the light passing through the lens still
influence the reading? If that is true, then the WB set through the
smaller expo-disk might be close, but not necessarily accurate?
Click to expand...
Not in reality. If you read your instruction manual, it will tell you that you only need to fill the central part of the view finder up with your grey or white card to get an accurate reading.

The amount of light that's leaking around the edges of the Expo Disk, 72mm Vs 77mm, is but 2 1/2mm or the thickness of a dime/nickel. Not a sufficient amount of light to skew the readings.
 
Thomas Gardner said:
I have the disk and doing some exposure test/practice with the 72mm
Expo Disk held infront of the 77mm opening of a Canon 28-70mm
f/2.8L I came up with confusing exposure results. Oh! And yes, I
did read the directions, several times:-)

The sky/Sun exposure test came up spot on with a meter reading of
1/125th @ f/16, the ISO setting was 100. But decent histogram
exposures are 2 1/2 stops off the metered results, underexposed.
When the ED exposure is used, the image is almost black.
Click to expand...
While I believe the Expo Disc was originally meant to help set exposure, I don't think that's what most folks are using it for today. I think you might have better luck setting your exposure in the usual manner and then use the Expo Disc to set your custom white balance.

I purchased the Expo Disc quite some time ago, and while useful on occasion, I'm afraid it normally just sits in the bottom of my bag most of the time. I've done extensive testing with it...followed Jan's recommended methodology exactly...but in the end, it just doesn't seem to be consistant enough for me to be able to trust it. I know others feel differently about it and think it does just fine in most all situations. Bottome line is...you just have to experiment and see what works best for you.
 
The Expodisc is extremely well made and I'm sure calibrated to a fine degree of accuracy.

But it simply turns your camera into an incident meter and thus you have to use it as an incident meter and measure the light source, not the subject.
And as far as WB, it acts like a ND filter....exactly.

Too bad it is not understood well.

The review is weak in several points.
hm said:
This disk is just a diffuser that mixes together all the
light coming in to "average it", and an 18% ND filter to
darken it.

Custom white balance requires the ability to measure the
"color balance" of the light hitting the subject. Similar
to exposure metering, this can be done via "incident"
or "reflective" metering, and "reflective" metering requires
an assumption of the reflective attributes of the subject.
If the light is slightly yellow, and you measure it by
seeing how it reflects off a blue subject, a reflective meter
is going to think the light source is bluer than it is.
Measuring color balance reflectively requires a "neutral"
subject, like a white card. So if you shoot a frame of a
white card, and the reflected light is yellow, now you know
the color of the light source, and you can "subtract" that
color from all subsequent shots of your real subject. If you
can do incident metering of color balance, though, then you
don't have to worry about providing a subject with neutral
reflective qualities.

If you place an expodisc on the camera, it diffuses the
incoming light and averages together the light from various
sources. So you put the camera in front of the subject in
"incident light meter" position, and shoot the incoming light,
and you'll get an averaged reading of the various light sources
in the correct proportions. This removes the color of the subject
from the equation, so it removes the requirement of having a
perfectly neutral subject to shoot. This is how the expodisc
would give most reliable results, I'd think.

Now, this review tells you to always point the camera at the
subject. If you do this, you're once again dependent on the
reflective properties of the subject, if you're taking a
close-up of a purple dinosaur, your expodisc shot is going
to have a purple tint, which the camera will think comes
from the light source. The expodisc still helps here
by diffusing the light that hits the filter, which gives you
an "average" over a much greater area than just the field of
view of the lens. So if you're shooting a purple dinosaur with
a telephoto lens, you can hope that the stuff surrounding the
purple dinosaur has a variety of colors, enough to help balance
out the average color of the scene, as the expodisc will pull
in light from all directions in front of the camera.

The review also suggests that its okay to hold up a smaller
filter in front of a larger lens, since most cameras only meter
the center of the frame. This logic isn't quite valid, as the image
in the center of the frame is derived from light that strikes the
entire surface of the lens, so this technique will leak in some
light around the edge of the filter, which just gives you slightly
less effective "averaging".
Click to expand...
 
Taz said:
Thomas Gardner wrote:
I purchased the Expo Disc quite some time ago, and while useful on
occasion, I'm afraid it normally just sits in the bottom of my bag
most of the time. I've done extensive testing with it...followed
Jan's recommended methodology exactly...but in the end, it just
doesn't seem to be consistant enough for me to be able to trust it.
I know others feel differently about it and think it does just fine
in most all situations. Bottome line is...you just have to
experiment and see what works best for you.
Click to expand...
I was kinda setting my hopes on the ED helping me with my subject choices and my composition:-)

I agree with your sumation that you have to practice with the disk and let your successes be your guide.

Practice, practice, practice. Sort of like the Martial Arts; you might have to do it a thousand times to get it right just once and then you'll have to do it only five hundred times to get it right the second time:-)

I have a lot of hopes for the Expo Disk and I hope I'm not disappointed:-)
 
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