Vs: [Digital BW] Filters and their split channels "equivalents"
2004-04-18 by Ukko Heikkinen
Hello Steve Let's start here: http://www.tiffen.com/Filter_&_Lens_Brochure/BFILT_30_31.htm Ukko Heikkinen
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----- Original Message ----- From: Steve Kale <stevekale@...> To: <DigitalBlackandWhiteThePrint@yahoogroups.com> Sent: Sunday, April 18, 2004 9:10 PM Subject: [Digital BW] Filters and their split channels "equivalents" Hi all Without sparking a debate about whether one must capture with B&W film (to be pure) can I ask a few questions regarding the use of colour filters in B&W photography and their (loose?) equivalents in raising or lowering the opacity of a red, blue or green layer when using split channels for B&W conversion of a colour capture? [Prior to moving to the digital domain I did not do much B&W photography because I was always very disappointed with the proof sheets I got back from the lab. Although I agree that a lot of this was likely due to the poor development of film by the lab (and I have taken on the comments from this group earlier re developing my own film etc) I am still enjoying the B&W printing of images originally captured with colour fim (mostly Velvia or Provia).] I was surprised the other day when I saw a poster of an image by Ansel Adam of (I believe) Mt McKinley (?). The skies really were a deep deep black. Also I just read yesterday the small section in his Book 1 The Camera re the use of filters, particularly the reference to using a No. 25 (A) Red filter for strong contrast in landscape photography. And so I turned back to one of my favourite images taken in the Grand Teton National Park and how I had previously ranked and weighted the Red, Green and Blue (and Luminosity) layers when converting to B&W. Specifically, I found that to approach the massive punch I saw in the Adams print I had to use the Red layer as my base at 100%, with only slight if any use of the Green and no use of the Blue layer. (Adjusting the Luminosity layer from LAB seemed similar to adjusting Green.) So I am now trying to understand filters a bit better. As I understand it a Red filter will allow red light to pass easily but will block (and hence darken on B&W film) its complementary colour. Is the "complementary colour" both blue and green? Hence, is stripping out the blue and green channels the loose equivalent of using a strong red filter? What about a deep yellow filter No. 12? Mix green and blue and one gets yellow right? What is the complementary colour of yellow? Sorry, I am sure this stuff is very basic - my knowledge of colour theory extends only to the very limited stuff I can remember from primary school. If any colour in the visible spectrum can be made up from a mix of Red, Green and Blue (isn't this the basis of RGB colour space?) then any colour filter can be simulated by altering the weighting of red, green and blue from the original capture? (ignoring for now effects from non-visible parts of the spectrum eg infra-red) In traditional B&W photography the workspace is a film/filter selection process (assuming correct exposure etc). I am trying to understand what happens to the RGB light mix hitting the film plane when a filter is used and I guess how this alteration might be simulated in a workspace like Adobe RGB where rather than limiting particular parts of the spectrum hitting the film plane we limit them afterwards in a virtual world. (I understand that all films have different sensitivities to the various parts of the spectrum and that by shooting in colour (or for that matter any) film I immediately limit my future options to the "space" able to be captured by that film. But it seems to me that one advantage of colour capture is that the range of adjustments that can be made post capture is significantly greater than those that can be made with B&W film. With B&W film all the decisions regarding filtering the RGB spectrum must be made prior to pressing the shutter. If you block the blue at capture it is gone forever and you could never produce an image with an overweight blue composite from that capture.) Any help or references to useful (digestable) reading material would be greatly appreciated. Regards Steve Please visit the Group Homepage to check the Files, and other resources as they are often being updated. http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint If you wish to receive no emails or just a daily digest, or you wish to unsubscribe, please edit your Membership preferences by visiting this same page. 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