Flat spectral response means/implies that it adds nothing to nor takes anything away from the signal received. So, in the case if light, if the light recieved is a bit cool, the light passed will be a bit cool. It is this cooler light that is used by the camera to generate the CWB.
Consider the alternative ... the ExpoDisc (or any other tool) receives light with a cool cast, "normalizes" it and passes this result to the camera. Where does the camera receive any benefit?
One can, in fact, test this. Set your camera to any WB setting other AWB. Mount the ExpoDisc on the lens and take a shot of incandescent light source. Now, without making any changes to the WB setting, take your camera outside and take a shot of the light from a clear blue sky. Final step, examine these two shots side by side on your computer (the nice thing here is that you don't even need a calibrated monitor for this step) and compare the colors of the two frames. You will note that the frame from the incandescent source is quite a bit warmer than that of the clear blue sky source.
I agree, the filter can make no distinction between light from any given source - there's no brains in the tool.
To get any idea of the "true" color of the light incident on the subject, you need to sample the light that is incident on the subject.
Light reflected by the subject will be altered by the fact that nothing (except a true neutral) will reflect light without altering it's color - that's how we see the color of an object. Without additional information, sampling the light reflected from a subject tells you nothing about the light source. That's how a gray card works (either in setting the camera CWB or in post) - you know this subject is neutral and can use other tools (the camera or the appropriate picker in, for example, ACR) to determine in what ways the incident light varies from "normal" light and, thus, make appropriate adjustments to the color cast in the photo.
This last part I completely agree with, assuming the surface reflecting the light is, indeed, neutral.
Thanks, Scott.
You know you are preaching to the converted!:D Just that in my last post words sometimes got in the way.
Let me try again. WB is commonly described as making the camera read a neutral target with a colour cast as neutral. So, if the grey card is reflecting coloured light, say blue, the camera makes that blue cast neutral target its neutral. Everything in its field will be shifted away from blue.
I was a bit more successful with:
"[FONT=Helvetica, Arial, sans-serif]The filter is showing the camera what neutral light looks like in that particular lighting situation."
[/FONT]As before, and as we already agree, it doesn't matter that the filter is getting incident blue light or blue light reflected from a neutral target.
So, you can see we are in total agreement that the light reflecting off a grey target, or passing through the filter, must be interpreted by the camera as being neutral, that is, the camera normalises it.
However, I have one concern, and that is unless you are in a studio with exactly only one light source, in which case the reading of that incident light is the reading of colour cast, unless that, then you are in a situation of having light from multiple sources, and multiple reflections, each probably with its own colour contribution to the scene. Considering that the filter stuck on the end of a lens is going to be pretty directional in sampling the light, just as the lens itself is, which direction, which incident light source, which reflected light, are you going to turn your filter to? On the other hand, a neutral grey target is reflecting the total incident light, from all sources, it is gathering light from all sources to feed into your directional lens cum filter, and this more comprehensive reflected light, it seems to me, is going to give a more accurate measure of the color shift affecting the subject of the photograph.
You know you are preaching to the converted!:D Just that in my last post words sometimes got in the way.
Let me try again. WB is commonly described as making the camera read a neutral target with a colour cast as neutral. So, if the grey card is reflecting coloured light, say blue, the camera makes that blue cast neutral target its neutral. Everything in its field will be shifted away from blue.
I was a bit more successful with:
"[FONT=Helvetica, Arial, sans-serif]The filter is showing the camera what neutral light looks like in that particular lighting situation."
[/FONT]As before, and as we already agree, it doesn't matter that the filter is getting incident blue light or blue light reflected from a neutral target.
So, you can see we are in total agreement that the light reflecting off a grey target, or passing through the filter, must be interpreted by the camera as being neutral, that is, the camera normalises it.
However, I have one concern, and that is unless you are in a studio with exactly only one light source, in which case the reading of that incident light is the reading of colour cast, unless that, then you are in a situation of having light from multiple sources, and multiple reflections, each probably with its own colour contribution to the scene. Considering that the filter stuck on the end of a lens is going to be pretty directional in sampling the light, just as the lens itself is, which direction, which incident light source, which reflected light, are you going to turn your filter to? On the other hand, a neutral grey target is reflecting the total incident light, from all sources, it is gathering light from all sources to feed into your directional lens cum filter, and this more comprehensive reflected light, it seems to me, is going to give a more accurate measure of the color shift affecting the subject of the photograph.
Neil
Studio setting .... one would hope (at least I would insist) that the strobes are all pretty much the same temperature and that any reflected light is either being reflected from neutral colored surfaces and/or the reflected light is of a strength insignificant enough that it doesn't contribute in any meaningful way to the exposure.
And, yes, there's going to be slight variations in the color of the light delivered by multiple strobes - based on duration of the pulse of light each strobe delivers, the amount of power of each strobe, and a whole host of other factors. But, for the most part, "matched" studio strobes are going to be pretty much the same color. Even "unmatched" strobes are going to be close enough in color (or they should be) that the differences are going to be a non-issue ... remember, photography (as an artform) is is not rocket science where we have to get the data precise to five decimal places.
But, in more random situations, one very well could have light from multiple sources .... none of which are controlled by the photographer. In this case, as in most cases, one does the best they can -- you point your lens at the strongest light source (as perceived from the point of view of the subject) as it is probably going to contribute most strongly to any color issues.
And, I agree - in a situation with multiple light sources, a good and known neutral target will do you better as the light reflected from it will be a functional average of the light incident upon that target .... which is why I have two gray targets in my kit (in case I loose one) and I don't even really know where I would find my ExpoDisc.
Comments
Thanks, Scott.
You know you are preaching to the converted!:D Just that in my last post words sometimes got in the way.
Let me try again. WB is commonly described as making the camera read a neutral target with a colour cast as neutral. So, if the grey card is reflecting coloured light, say blue, the camera makes that blue cast neutral target its neutral. Everything in its field will be shifted away from blue.
I was a bit more successful with:
"[FONT=Helvetica, Arial, sans-serif]The filter is showing the camera what neutral light looks like in that particular lighting situation."
[/FONT]As before, and as we already agree, it doesn't matter that the filter is getting incident blue light or blue light reflected from a neutral target.
So, you can see we are in total agreement that the light reflecting off a grey target, or passing through the filter, must be interpreted by the camera as being neutral, that is, the camera normalises it.
However, I have one concern, and that is unless you are in a studio with exactly only one light source, in which case the reading of that incident light is the reading of colour cast, unless that, then you are in a situation of having light from multiple sources, and multiple reflections, each probably with its own colour contribution to the scene. Considering that the filter stuck on the end of a lens is going to be pretty directional in sampling the light, just as the lens itself is, which direction, which incident light source, which reflected light, are you going to turn your filter to? On the other hand, a neutral grey target is reflecting the total incident light, from all sources, it is gathering light from all sources to feed into your directional lens cum filter, and this more comprehensive reflected light, it seems to me, is going to give a more accurate measure of the color shift affecting the subject of the photograph.
Neil
http://www.behance.net/brosepix
And, yes, there's going to be slight variations in the color of the light delivered by multiple strobes - based on duration of the pulse of light each strobe delivers, the amount of power of each strobe, and a whole host of other factors. But, for the most part, "matched" studio strobes are going to be pretty much the same color. Even "unmatched" strobes are going to be close enough in color (or they should be) that the differences are going to be a non-issue ... remember, photography (as an artform) is is not rocket science where we have to get the data precise to five decimal places.
But, in more random situations, one very well could have light from multiple sources .... none of which are controlled by the photographer. In this case, as in most cases, one does the best they can -- you point your lens at the strongest light source (as perceived from the point of view of the subject) as it is probably going to contribute most strongly to any color issues.
And, I agree - in a situation with multiple light sources, a good and known neutral target will do you better as the light reflected from it will be a functional average of the light incident upon that target .... which is why I have two gray targets in my kit (in case I loose one) and I don't even really know where I would find my ExpoDisc.
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I think it's this touching, personal confession which really clinches the argument!!:D
Neil
http://www.behance.net/brosepix