Expanding dynamic range

[Dennis]

Whatever the WB and whatever the tempaerature of the light source, the sensor cannot capture equal (maximum) values in all 3 channels unless equal (maximum)values of each primary color are passing through the lens.

It doesn't matter, what's entering the front lens, but what's hitting the sensor. You take care, that the light entering the front lens should represent a neutral grey tone, eg. by focusing to a grey card. The light, which reaches the sensor, should have equal amounts of all three primaries, ideally. If the light entering the front lens has another composition (color cast), you correct it using a CC filter. So the reflected light from a grey card, which has some unbalanced spectrum due to 'not ideal' light source, will be balanced by the CC filter. There is a difference between an unbalanced light caused by the color of the object or caused by color of the light itself. The latter, you try to eliminate with the CC Filters. The first is, what you actually want to capture.

If I'm photographing a room, for example, where eveything in it is a shade of purple (another extreme example to illustrate my point) there's no chance the sensor will captures equal values in the green channel. In this situation you would need to use a green filter to suppress the red and blue light passing through the lens, take a shot (with green filter on) of a white  (or grey) card which is illuminated by the light source rather than the reflection from the purple surfaces in the room, in order to get your custom WB.

In this case, you should avoid reading a grey card, which would be illuminated by the reflected light of your purple room. You should measure the light source directly, instead.

Dennis.

[Ray]

It doesn't matter, what's entering the front lens, but what's hitting the sensor.

Dennis,
First let me say I am not speaking from a wealth of experience creating custom white balances. I'm really just trying to understand the principle and what you are saying simply doesn't make sense to me.

Whatever hits the sensor must have passed through the lens, except internally generated noise from heat sources etc. Whether or not the WB is done properly; whether or not you've picked up too much reflected light from strongly colored surfaces in the scene, the WB process (as I understand it, and correct me if I'm wrong) is an information tag which basically says, 'this heavily green biased part of the image, which the spot meter has focussed on, is supposed to be white. The green channel must be reduced accordingly throughout the entire image during conversion.'

The white balance is thus an automatic 'levels adjustment' set of instructions. If you get it wrong, you then have to manually adjust levels in PS, or use the grey balancing eyedropper, whatever.

You take care, that the light entering the front lens should represent a neutral grey tone, eg. by focusing to a grey card.

I don't think so. You take care that the light before it enters the lens should represent the temperature of the light source. Adding any color filter to the lens, for whatever purpose, effectively changes the temperature of the light source. But that is the whole purpose of adding a filter, isn't it. You want to change the temperature of the light source in such a way that the light, after passing through the filter and lens, will either (1) contain at least some saturated levels in each channel for the purpose of increasing DR, or (2) already contain a neutral balance in respect of a white or grey surface (in the case of your suggestion of a blue filter with a tungsten light source).

I don't see why we should be making a distinction between adding a magenta filter to correct for the internal imbalance of sensor sensitivity as opposed to adding a different filter which corrects for both imbalances, internal and external, unless of course we get ourselves into a situation where we are outside of the range of the camera's custom WB adjustments, in which case we are stuffed.

I think I have a red filter somewhere. Perhaps it's time for me to experiment with creating some custom white balances under differernt conditions using a red filter. I might afterwards have to confess I'm totally wrong and confused   .

[Ray]

You take care that the light before it enters the lens should represent the temperature of the light source. Adding any color filter to the lens, for whatever purpose, effectively changes the temperature of the light source.

Perhaps I should add clarification here. From the camera's perspective, that is from the WB algorithm's perspective, it makes not one whit of difference whether the white card is tinged with blue light as a result of the light source having a high temperature, or whether the light, as reflected from the white card, has a high temperature because it has passed through a blue filter on the lens, on it's way to the sensor. The WB system is not aware of the reason for this excessively high temperature. All it knows is, that which is supposed to be white (the white card) is in fact slightly blue, and it makes levels adjustments accordingly by reducing the blue channel in the entire image. Is this not right?

Well, I certainly hope all this confusion has not given you a high temperature. I've felt like taking a few asprins whilst trying to sort through this issue   .

[allan67]

Hello,

That is a very interesting discussion.
I've come up with an idea   that might be not very practical but is it valid from theoretical point of view?
Assuming the shots in daylight
1. We take a shot with ETTR exposure - this will give green chanel at full well in linear RAW
2. Take a second shot with +1EV - this will push green to overexpose, but get blue at full well in linear RAW
3. Take a third shot with +2EV - overexposing green and blue but getting red at full well in RAW.
4. Process the three shots placing the corresponding chanels a the right with white balance tool (or exposure).
5. Combine GREEN, BLUE and RED chanels from the three shots into one image.

Will this work?  

Allan

[Dennis]

Whatever hits the sensor must have passed through the lens,

Yes, you are right. I meant it in a different way. Obviously the combination of tints of the CFA and the wave length dependant sensitivity of the imagier leads to an unbalanced channel situation, even if you sent 'balanced' light (whatever that means) through the lens. The balance is up to your light source. A grey card lit with D50 light will give a different reading, than if lit with D65 light. There is supposed to be one temperature, say eg. 5,800 K, where the reflected light of a grey card produces exactly evenly filled color channels. If you are variing the light temperature, the distribution will change in one or another direction.

So the concept here is, you first have to analyse using a grey card, what the current lighting does to the three primary channels. Then you can compensate a color cast using a CC filter, and thus manage to get evenly filled channels. This will allow to do ETTR to all channels perfectly, and not - as shown - to bring only one channel to the right, and waste some headroom with the other two channles. Okay, if the hassle is worth the effect, is another question. With daylight, an advantage of 2/3 f stop was reported. Well, that's not that much. But maybe this technique may help to avoid blowing the green channel - don't know. But with tungsten light, this filtering definately makes sense, since here the difference between the channels is way bigger.

The white balance is thus an automatic 'levels adjustment' set of instructions. If you get it wrong, you then have to manually adjust levels in PS, or use the grey balancing eyedropper, whatever.

Yes. But it's preferrable, not to have to do this by a large amount, so it's better, if the light is already balanced before it hits the sensor (eg via using a CC filter). And please be aware, I am talking NOT about object colors, but the light color (grey card reading).

You take care that the light before it enters the lens should represent the temperature of the light source.

Yes, that's what I said. That's, what you'd use a grey card for or some white target.

I don't see why we should be making a distinction between adding a magenta filter to correct for the internal imbalance of sensor sensitivity

This is your misunderstanding: There is no 'internal imbalance'. There is only an imbalance between the light and the sensor. The above mentioned magenta filter wokrs only with this specific light temperature. It's exactly the same as with film. Think about. There is no such thing as 'external imbalance'. There are only two factors:

 - the spectral sensitivity of the media (film or digital device)
 - the spectral distribution of the light

To match these two, you need one filter. Taht's all.

All it knows is, that which is supposed to be white (the white card) is in fact slightly blue, and it makes levels adjustments accordingly by reducing the blue channel in the entire image. Is this not right?

That's perfectly right. But it's better to balance the light, than to tweek the channels via WB.

Take a picture under 'heavy' tungsten light, like a candle or very dimm light bulb, the camera set to JPEG and daylight. Then add a blue filter and take another shot. Try to WB both images and compare. You'll end up with massive problems with the first image. Of course, using Raw the problem is way less significant, but it's not gone.

Dennis.

[Dennis]

Will this work?

It should. Go and evaluate it. But be aware, that it's not always possible to make three shots. The same problem like with HDR scenes: Can you bracket and use DRI technique or should you use a gradient filter?

Dennis.

[bjanes]

Hello,

That is a very interesting discussion.
I've come up with an idea   that might be not very practical but is it valid from theoretical point of view?
Assuming the shots in daylight
1. We take a shot with ETTR exposure - this will give green chanel at full well in linear RAW
2. Take a second shot with +1EV - this will push green to overexpose, but get blue at full well in linear RAW
3. Take a third shot with +2EV - overexposing green and blue but getting red at full well in RAW.
4. Process the three shots placing the corresponding chanels a the right with white balance tool (or exposure).
5. Combine GREEN, BLUE and RED chanels from the three shots into one image.

Will this work?  

Allan
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Allan,

I do not see why your method would not work if anyone wanted to take the trouble to carry it out. However, your exposure factors are off a bit. Let us use a Nikon D200 for the exercise, since channel multipliers for white balance are readily available for this camera.

[a href=\"http://www.pochtar.com/NikonWhiteBalanceCoeffs.htm]http://www.pochtar.com/NikonWhiteBalanceCoeffs.htm[/url]

In the experiment, the green channel is already balanced. For the red, we have to multiply the exposure by 1.83. In terms if f-stops this is Log base 2 of 1.83 or 0.87 stops. For the blue the multiplier is 1.36 or 0.44 stops.

If you are using Photoshop with ACR and are bracketing exposures, it might make more sense to bracket more widely and use HDR. You would get far more dynamic range with HDR than the use of filters.

Bill Janes

[Ray]

This is your misunderstanding: There is no 'internal imbalance'. There is only an imbalance between the light and the sensor. The above mentioned magenta filter wokrs only with this specific light temperature.

Dennis,
Are you sure about this? On the previous page, there's an explanation from John Sheehy, who seems to be heavily into such technical matters, which implies there's a sensitivity imbalance amongst the 3 channels. Refer to the following quote.

The way I understand it: the sensor, without any filters over it, is most sensitive to red and NIR. The NIR filter cuts NIR and a lot of reds, too, making the red weakest. Blue is already weaker than green because of the sensor's preference for the red end of the spectrum, and the dyes used in the CFA filters have some influence, as well.

These ratios are pretty universal amongst RGB CFA cameras. Green is considered a reference, blue is generally corrected about 0.95 to 1.5, and red is generally corrected about 1.6 to 2.25 (for daylight).

Since D65 is such a commonly used standard, one wonders why Canon does not include a magetna-ish CC filter, perhaps immediately under the sensor's protective covering, especially since their recent models have such excellent performance at high ISOs, or better still, why do they not simply increase the density of the blue and green filters in the filter array so that the sensor is calibrated more closely to D65?

I think I understand the implication of the magenta filter. Because the D65 WB 'tag' will always try to increase the red channel by a factor of, say 1.92 in respect of the green channel, we can never expose the red channel fully to the right because the application of the WB levels adjustment (during conversion of RAW image) will then blow the channel. If we create the conditions for a custom white balance that doesn't apply this automatic increase of levels in the red channel, by using a magenta-ish color correcting filter when creating the custom WB, then we are home and dry. We are then in a situation of being able to increase exposure by up to a full stop or so, apparently in all situations, resulting in a less noisy red channel and perhaps a slightly less noisy blue channel, depending on the camera model.

But I still think that any increase in DR will be dependent on the composition of the shadows. Shadows which consist of lots of magenta will benefit most and shadows which are predominantly green and cyanish will benefit the least in respect of any increase in DR, the reason being, if you suppress the green and (maybe) blue channels, the additional exposure simply brings the green and (maybe) blue channel back to the same level they would have been without use of a filter. One is relying mostly on a reduction of noise in the red channel to get any real increase in dynamic range. Since there's always going to be at least some red in the shadows, I shall have to concede that use of a red/magenta filter is going to be of some benefit to DR in most high dynamic range scenes.

However, I doubt that the benefits would be worth the hassle for me. If there is a way of extending the principle by using the camera's WB adjustments to correct levels after using stronger filters that match the general color composition of the scene, I think that could result in substantial increases in DR.

[John Sheehy]

5. Combine GREEN, BLUE and RED chanels from the three shots into one image.

Will this work?  

Allan

It will work, but you have to do it RAW to really get it right, as demosaicing and RGB color conversion cross-contaminate colors.  Then, you have to get this data back into a converter.  I have been thinking about this, and I know how to do it, but I haven't getten around to it yet, because I have to do some programming and I don't program all the time, so I get a bit rusty, and time is limited.

There is also the issue of registration; you need for the camera not to move between shots, otherwise you will be getting a blurred, under-sampled image.  Would work best with super-wide angle lenses on sturdy tripods.

[John Sheehy]

However, I doubt that the benefits would be worth the hassle for me. If there is a way of extending the principle by using the camera's WB adjustments to correct levels after using stronger filters that match the general color composition of the scene, I think that could result in substantial increases in DR.

That would be better yet.  The idea of the reddish magenta filter is that it brings the sensitivity of the camera, as a system, to a natural daylight WB.  That is ideal if you want full color at the highest nearly-neutral highlights (like pastels in the sun, or subtle color differences on white clouds), and as deep as possible into the shadows.  If your highlight is something else, and you want full color and DR, then the optimal filtration might be something else.

The bottom line is that you will get the best RAW capture when you can expose each of the 3 RAW color channels to the right, independently.

[Dennis]

Dennis,
Are you sure about this? On the previous page, there's an explanation from John Sheehy, who seems to be heavily into such technical matters, which implies there's a sensitivity imbalance amongst the 3 channels. Refer to the following quote.

Okay, Ray, you are right here. Given a perfect balanced light source with an absolute evenly distributed spectrum, you can record such an absolute 'internal' imbalance. You always can divide the total imbalance into two parts: One 'internal' imbalance between the sensor and a standard (like the above perfect light or D65), and the 'external' imbalance between the given light and the chosen standard. So you had to filter the whole thing in two steps: First calibrate the sensor to the standard, second match the light to the standard. This might be not very effective. It's much easier to see the imbalance as one 'piece'. So the only thing, which matters is, how records the sensor the actual light?

[bjanes]

That would be better yet.  The idea of the reddish magenta filter is that it brings the sensitivity of the camera, as a system, to a natural daylight WB.  That is ideal if you want full color at the highest nearly-neutral highlights (like pastels in the sun, or subtle color differences on white clouds), and as deep as possible into the shadows.  If your highlight is something else, and you want full color and DR, then the optimal filtration might be something else.

The bottom line is that you will get the best RAW capture when you can expose each of the 3 RAW color channels to the right, independently.
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What John says makes sense, but it is hard enough to expose to the right when using normal color balance and no filters. The histograms and blinking highlights often underestimate how far to the right the exposure is.

If you are shooting in daylight, using filters and a custom white balance and photographing a full spectrum subject with neutral (gray-white) tones that must be neutral in the image, then daylight white balance is best. The magenta filter is only an approximation. Consider the this image of the color checker. The raw histogram of the straight shot without filters is shown:

[attachment=596:attachment]

If you add magenta filtration to bring the green down to the level of the red channel, the blue is a bit too strong and some yellow filtration is needed to bring the blue into balance. I calculate that CC26M and CC13Y would be needed for full correction with +0.87 EV additional exposure (for the Nikon D200). Most would just settle for approximate balancing with a magenta filter.

For other situations, such as Ray's sunset, one should expose all the channels to the right as John suggests, but selection of the proper filters would be no simple matter. The camera color histograms would not be helpful unless the camera white balance is set such that the channel multipliers are all 1.0 (UniWB for Nikons). This is a lot of bother for a very modest improvement in dynamic range and I predict that very few photographers would take the trouble. Personally, I would use the lowest possible ISO and skip the filters for the sunset.

The magenta filter is a reasonable compromise for full range daylight illuminated subjects.