filters -v- photoshop (part 2?)

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<p >The Digital Darkroom forum has an interesting discussion on the use of filters –v- Photoshop (<a href="http://www.photo.net/digital-darkroom-forum/00SmYN">http://www.photo.net/digital-darkroom-forum/00SmYN</a>). I have a related question but since this discussion seems to have run its course I thought it best to start this as a new thread.</p>

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<p >I understood from the first discussion that there may at times be some value in using a lens mounted colour correction filter for digital photography. For example, when working with artificial light with a strong colour cast.</p>

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<p >I was wondering if there is a consensus on the use of use of lens mounted filters in B&W digital photography.</p>

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<p >On a number of occasions I have found that my B&W conversions in Photoshop have resulted in very high noise levels in the sky. My solution in the past has been to blur the sky to smooth out the noise. However the previous discussion might imply that a better solution is to apply a yellow or red filter to the lens.</p>

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<p >My thinking goes something like this…</p>

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<p >In simple terms, the application of a red filter during a B&W conversion in Photoshop involves the attenuation of the blue channel and, to maintain luminosity, the amplification of the red channel. Now the ‘blue’ sky, whilst mostly captured in the blue channel of the image, appears also as a relatively low level signal in the red channel. This red signal therefore has a relatively low signal to noise ratio. My theory then is that the amplification of this signal during the B&W conversion is the primary source of the noise.</p>

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<p >If my thinking is sound (maybe a big if) then the application of a red filter during the capture process maybe a better option. As I understand it, to maintain correct exposure one would need to compensate for the reduction in effective EV due to the filter. The result, compared to the unfiltered image, would be to shift the red channel to the right and to shift the blue channel a little to the left. As this is being done at the time of capture (and so long as the red channel does not clip) it would seem that this may ultimately result in a B&W conversion that has a better SNR in the red channel.</p>

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<p >What do you think? Have I understood correctly? Is there any benefit to using lens mounted filters for B&W digital photography?</p>

<p >Regards,</p>

<p >Mike</p>

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<p>I don't think it will play out quite this way. In either case, you drop the luminosity levels in the red channel and any variations in the sky will be described in terms of a lower value luminosity levels. If anything, the post-processing approach gives you more control since you can regulate "how red" your post-processing filter should be, and you can even apply it at different levels to different portions of the image. In addition, you could use masks to simply alter the values in the sky via curves and thereby not diminish whatever data there may be in the red/green channels.</p>

<p>Dan</p>

<p>Interesting question.</p>

<p>Why not put a red filtre on your lens and try it out? If you don't have a filtre already, they are not expensive to buy (even new) and you don't need to get new or best quality filtres for a test like this.</p>

<p>When I want B&W, I still shoot film, but, I may try red and yellow filtres with my DSLR+lens myself just to see how it works out.</p>

<p>Cheers! Jay</p>

<p>I admit to not carrying colored filters around any more but I still pack a half dozen split Neutral Densities as I have not found any PS masking/blending technique that doesn't tend toward posterization as compared to using the Split NDs at capture. </p>

<p>I admit i didtn read any of this long questions..because the only real filter you migth need in front of your lens is a Polarizer. All the other one can be simulate and easily add / remove in need.</p>

<p>You migth also use a ND grad filter for some specific need like to protect the highlight of a scene etc..but then again, 2 exposure can also do the job. (Robert,trust me it work ; )</p>

<p>So other than a Polarizer, theres no need for anything else.</p>

<p>And you are better to shoot in color and convert it to BW later than using the BW in camera setting or use a filter for that.</p>

<p>Thanks guys for your posts.</p>

<p>Jay, I don't have a red filter but I do have an old yellow filter from my film days, so I'll try some experiments with that.</p>

<p>G Dan, ok so now I really have some doubts that I know what I'm talking about. Why do you say that the luminosity in the red channel will go down? I thought that the red filter will reduce the blue and green channels more than the red and as one would need to increase the exposure to compensate then the net result would be a slight to moderate increase in the red channel not a reduction. </p>

<p>Cheers,</p>

<p>Mike </p>