Archived posting to the Leica Users Group, 2007/07/18
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi George, Ted and Henning, George, I have news for you -- if you are using a polarizer (color) or yellow, orange or red (B&W) filter to darken the sky, then you are also filtering for haze reduction! That's why distant details seem so much clearer with such filters. Henning, you're the only one who seemed to know what I meant by reducing haze by filtering out blue! You'd think more here would have read Ansel Adam's The Negative! I hadn't thought about not being able to duplicate in post processing such filters that pass or cut a narrow band, so I should look at some of the filters I use, particularly the "minus blue," which is what I am primarily using for scenics lately. For those who want a mini-lesson, most of what you see as atmospheric haze is not the particulate matter itself (i.e., dust, smoke, water vapor, etc.), but the glare from light reflecting off it. The reflection is both primarily blue light and polarized, so by using a polarizing filter (for color) or a filter which reduces blue, such as a red, orange or deep yellow (for B&W), you can greatly reduce what you perceive as atmospheric haze. This is a very real effect, not some "half-assed" thing like calling a UV filter a Haze filter, which I presume Ted thought I was talking about. I asked the query because I was wondering whether this has to be done by filtering before the light hits the sensor, or can be done by manipulating curves in post-processing. Tom On Jul 17, 2007, at 11:59 AM, Henning Wulff wrote: > 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 > /|\ Wulff Photography & Design > /###\ mailto:henningw@archiphoto.com > |[ ]| http://www.archiphoto.com > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information