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08-19-2012, 03:58 PM   #1
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Optical filtration????

As an old film hand, I have always used optical filters, either to correct or enhance light for color images, or in the case of black and white, to sharpen images or bring out one element of the image or another (flesh tones, darker skies, etc). I have set up my new K-5 to work with my old pentax lenses (yes I have good meters -- Sekonic Delux II and Minolta Spotmeter F). So the question is, what happens with optical filters on a DSLR? I know I could play around and find out, but thought to ask and see if anyone has persional experience. If shooting in black and white mode, do the old blue, green, yellow, orange and red filters do the same thing as with b&w film, or do they just mess things up? They will certainly still filter out the same frequencies, so for example, an orange or red filter should slice out a lot of blue light. But what does the DSLR brain do about that? How about color images? Will the old filters still warm, or correct florescent light, etc?

Cheers!

J

08-19-2012, 04:30 PM   #2
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The short answer is, yes sort of. Your color filters will work as before except that if the camera is set to auto white balance the camera will then 'correct' back to what it thinks the right color should be so the net result is as if nothing was done except you have the IQ degradation of the filter. I suppose it would work if you set the WB manually.

In general all of the filter stuff you used to do with glass is now done via software on the computer. Take the best shot you can, compose correctly and nail the exposure, then adjust to suit in software.

Also, it is best to do black & white conversions on the computer, you can do it in camera but the results are often not optimal. Remember the camera is not shooting B&W, it always shoots in color, and if you are in B&W mode does a conversion. Much better results if you shoot in RAW and do the conversion in software. You can easily increase or decrease any particular color channel, just like you used to do with filters.

I started in film as well, though I switched to digital many years ago. The basics do carry over, but you should consider what software you want to use and start developing those skills.
08-19-2012, 04:38 PM   #3
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QuoteOriginally posted by jatrax Quote
the camera is not shooting B&W, it always shoots in color
The camera always shoots in B&W, and "guesses" the colour value of the pixel from the Bayer Mosaic using a software algorithm.
08-19-2012, 07:53 PM   #4
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Thanks for the responses. I am still a bit confused, though. I thought the camera used actual internal filters to tell the pixels whether which of the three colors they are.

Joe

08-19-2012, 08:17 PM   #5
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Each pixel of the sensor is behind a color filter, the output is an array of pixel values, each indicating a raw intensity of one of the three filter colors. Both the computer firmware and software use an algorithm to interpolate the color levels of all colour components of each pixel. It may be that in an in camera B/W conversion, the camera simply does not bother with the algorithm and just constructs a gray scale image using the recorded luminance values. With appropriate software you should be able to do the same with RAW images as well.
Obviously, the best Black & White images are going to be recorded by a sensor with no Bayer filter at all, hence the Leica Monochrom.
08-19-2012, 09:15 PM   #6
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Other than using polarizers to remove reflections and Neutral Density filters to get longer exposures (and in some cases graduated filters to balance the sky exposure) you can pretty much do everything else in software assuming that you shoot RAW and you have a good exposure with no clipped histograms. In some very extreme situations, when you cannot get a good exposure because some particular color is way too strong or if you have mixed color temperature lights, then an optical filter may help.

The camera cannot shoot in B/W mode. It will always record RGB values defined by the color pattern on the sensor and then it will convert them to B/W in software. You can always do a far superior job by getting the original data (as in the RAW image) and using more powerful software on a computer with total control of the process and its parameters. You can simulate the effects of color filters in B/W photography and also apply color tinting to simulate different papers and chemistry.
08-20-2012, 05:44 AM   #7
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Demp10 this makes good sense. Not disputing anything, but just sort of thinking out loud through the keyboard:

The biggest use of red filters with film was to cut out blue scatter light and thereby sharpen the image for shots meant to record fine detail rather than be artistic. It was especially effective for cutting through haze. The blue scatter is a physical fact. So I guess what you and others are saying is that with DSLRs, one cannot remove the blue scatter before it reaches the lens and therefore cannot use the haze-cutting properties of red filters, because the computer cannot deal with information it does not have and a hazy picture is a hazy picture. Is this correct?
08-20-2012, 03:12 PM   #8
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Joe, you can always use a digital approach to emulate a color filter. The concept of white balance is based on that.

By adjusting certain color channels you can remove color casts in any image. If you have a hazy bluish image you can set the color balance to "cloudy" or "sky' and get a nicer warmer hue automatically. You can do even better if you manually tweak the white balance. Beyond that, by manipulating the cyan channel you can further cut down on haze. Add some contrast and clarity (micro contrast) and the image has been transformed in ways that are not possible with a simple filter during capture.

Having said that, there may be extreme situations when a physical optical filter may help, especially if you are looking for special effects. For day-to-day photography, software will give you far more options.

08-20-2012, 05:51 PM   #9
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QuoteOriginally posted by Joe Sullivan Quote
So I guess what you and others are saying is that with DSLRs, one cannot remove the blue scatter before it reaches the lens and therefore cannot use the haze-cutting properties of red filters, because the computer cannot deal with information it does not have and a hazy picture is a hazy picture. Is this correct?
I don't think that's quite right.

A few people upthread have mentioned the Bayer pattern -- each pixel on the sensor has a red, green, or blue filter over it, and the actual color image is reconstructed from that.

With your haze example, the blue/green components will be filtered out by your red screw-on filter, and pretty much equivalently by the red filters on the Bayer mask.

If you use the screw-on filter, you'll have an image reconstructed from red pixels and a whole bunch of blue and green pixels that don't receive much light and therefore don't contribute anything (because those colors were filtered out at the lens).

If you use software filtering and throw away the blue-green content, you'll have basically the same thing -- the red pixels will contribute to the final image, and the blue-green ones (where the haze is problematic) won't.

So you should be able to get rid of the haze pretty effectively in either case.

The screw-on red filter gives you the advantage of being able to see in the viewfinder what you're going to get on the sensor. Maybe you'll be able to get better critical focus without the haze in the way, with either manual or automatic focus.

In software, you can decide later if the virtual red filter is what you want, or if you want to do something different, or even keep the full color image. That's a big advantage for me.

A lot of people are comfortable these days (and I'm one of them) relying entirely on software color filtering. It's 99% as good as a screw-on color filter, if you're shooting in raw.

Unfortunately, there's no software substitute for a polarizer, so that's one to keep in your bag.

I think I'm saying basically the same thing selar and demp10 did, but I wanted to shed a little more light (ahem) on your haze question.
08-21-2012, 04:25 AM   #10
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To add to Quicksand: any physical element in front of the lens is likely to add in distortion due to reflectance on the filter without any benefit to the image quality.
08-21-2012, 06:09 AM   #11
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Thanks to everyone. I have been a serious photographer for decades -- and am used to thinking of film characteristics. Now thanks to you I am starting to think of sensor charcteristics and how they differ.

Timd -- thanks for your comment, too. As it happens, I am well aware of the optical effects of filters. It is just that with film there are times when filters are needed, especially with B&W film where one must compensate for the differences in the sensitivity of the film to the various frequencies of ight, or do other things. I still shoot film -- mostly in medium format, but will now try to begin to think in electronic terms when using a DSLR. It is quite different. Another big difference is the ability to change the ISO number in order to modify sensitivity. With film. the ISO is a given factor and one must manage with aperature and shutter. With a DSLR, within broad limits, aperature and shutter seem to be more used to manage depth of field and the effect of action, without so much concern for sensitivity. It is a fascinating new paradyme.

Joe
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