[Leica] Query re B&W photography in digital

[henningw at archiphoto.com (Henning Wulff)]

At 1:09 PM -0500 7/17/07, Lottermoser George wrote:
>"haze penetration" is not something that I've ever filtered for. I 
>always felt that if there's haze there's haze - not much one can do 
>about other than use it to aesthetic advantage. I, of course, may be 
>wrong about that. My experience with lens filters and digital are 
>mostly using: opaque IR, polarizing, red, green, yellow and orange 
>(for their various effect on sky and foliage) with the intention of 
>producing B&W images.
>
>In terms of software value:color manipulation - again I don't think 
>in terms of "haze." But rather tone control; as in one can turn a 
>red apple into very light tones, etc.
>
>Regards,
>George Lottermoser
>george@imagist.com
>

The 'haze penetration' effect of a red filter would still be there, 
except you can't control the final spectral cutoff point, ie, if you 
want the effect of a 25, you can probably get exactly that by 
excluding the other colours, but you can't get the effect of a 29 or 
70, as you can't separate the spectral response of the shorter 
wavelengths from those of the longer ones, like a deeper red filter 
would.

If you want the effect of a primary filter, either the blue 47B, 
green 58 and red 25, cutting out the other colours should work well, 
but getting a filter effect which only uses part of the spectrum 
which each of these filters passes is basically not possible.

If you are simulating a 25, noise would not be an issue.


Also as part of this topic of sensor spectral response:

If you want to get good colour response under extreme lighting 
conditions, ie, in candle light, you will have a lot of trouble 
getting a neutral colour balance, and you might wind up with serious 
chrominance as well as luminance noise even before the colour balance 
is solved.

If you use a dark colour correction filter (or 'conversion' filter as 
it's called when it gets to these densities) such as an 80A (about 
-132 decamired value), you will suppress a lot of that red 
information, while allowing the blue information to come through 
relatively unaffected, so that the values that get to the sensor are 
closer to each other. Then, when you boost the blues and reduce the 
reds in Photoshop, you start out closer and you don't have to boost 
the blue value by 4 or 6 stops along with the blue noise, which is 
often the worst that the sensor produces. You will wind up with a lot 
cleaner files. Also, because there isn't as great a difference 
between what the red and the blue channels see, it's less likely that 
parts of the red channel will be clipped, so your colour balancing 
has more of a chance.

This all supposes that there is some information in each colour 
channel. If you have essentially monochromatic sources like sodium 
vapour, or even severely restricted sources like very high efficiency 
flourescents, you can't colour balance because there are not 
continuous spectra and there is no information in some wavelengths.

-- 
    *            Henning J. Wulff
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