The Filters We Need With Digital...

Started Oct 5, 2008 | Discussions thread
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Joseph S Wisniewski
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The Filters We Need With Digital...
Oct 5, 2008

Today, I encountered three different dpReview threads with statements along the lines of "you don't need filters with digital", or "the only filter you need is a polarizer, the rest can be done in post processing. So, for the opposing point of view, some excerpts from an article I'm writing on filters and digital. The dpReview 6000 character limit is forcing me to break it into sections in an unnatural way that interferes with the flow of the material. My apologies for that...

The “Big Six” filters

That’s what I call a small selection of filters that are useful to most photographers, and that you cannot adequately replace with digital techniques...

One - The Polarizer

This is so obvious I shouldn’t need to get into "why" you need it, but I’m me, and that means I’m going to talk about it anyway. Any time light moves from air to a transparent substance, some of the light doesn’t penetrate into the transparent substance and is reflected away. Whether the light hits water on a lake, the natural oil on human skin, clear cellulose and wax on plant leaves and flowers, lacquer on a car or a guitar, or glass on a building, there are reflections. The reflections are “white light”; they “fill in” the color and reduce saturation. They reduce the detail visible under the clear substance. The blue of the sky is also polarized, and a polarizer can deepen the blue, and keep it from blowing out and rendering your sky a cloudless white or a drab gray.

You can fight this with post processing, but you won’t win. When you boost saturation, you fix the things that were "robbed" of contrast, but you also oversaturate the things that weren't suffering from contrast robbing reflections. And you can’t replace the lost detail.

Two - The 80A "Color Balancing" Filter

Most cameras have sensors that are "daylight balanced". They have nice, balanced red, green, and blue channel responses in neutral colored scenes lit by daylight. They achieve their incandescent white balance by amplifying the blue channel over two full stops relative to the red channel. That adds a great deal of noise to the blue channel, so you see some pretty ugly shadows. It also makes it very easy to blow the red channel, especially when shooting red dominated subjects (human skin, cosmetics, and fall colors near dusk and dawn when the light is warm).

Using an 80A will often let you get an interior architecture image in a single shot that would have taken multiple shots and HDR to do otherwise. It also makes it much easier to shoot incandescent or candlelit scenes without blowing the red channel.

Three – The Neodymium Enhancing Filter

Sensor manufacturers (like film manufacturers before them) spend a lot of effort trying to get the red, green, and blue filters in the camera to do a tolerable job of seeing colors the same way a human eye sees them. Normally, this is a "good thing", it reduces an annoying phenomena we techno-geeks call a "failure in observer metamerism", where colors that look identical to one "observer" (a human) look different to another observer (a camera, for example). The neodymium filter disrupts the nice "mimic the eye" characteristics of a sensor, and causes large-scale failures of observer metamerism. Neodymium (sometimes called “didymium”) does it in a way that is very pleasing in a landscape or fall color photograph: browns that would be identical in the picture (or to the eye) suddenly separate, with one turning red, another going yellow.

Again, this cannot be done in post processing, because without the filter, the camera sees all those browns as identical in hue. There's no way Photoshop can know to turn one brown into red, while another, apparently identical brown should be colored yellow, and a third identical brown should be left as "brown". Same thing happens in other colors, seemingly identical oranges separate into deeper oranges, reds, ambers, etc.

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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

http://www.swissarmyfork.com

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