Monochrome Conversions: Using a Yellow filter with a color camera

Discussion in 'Leica M Camera Forum' started by Brian, Mar 22, 2015.

  1. Brian

    Brian Super Moderator Subscribing Member

    Apr 3, 2013
    The question came up on another forum regarding the use of color filters with color cameras when you know in advance that you will doing a conversion. Looking at the spectral response of the Red, Green, and Blue channels of the sensors used in the M8, M9, and many other cameras: the answer is Y48. The Medium Yellow cuts out about 1/2 of the spectrum, cuts out "traditional color-separation Blue", cuts out about 1/2 of "color separation Green". The Medium Yellow filter is also known as a "cloud filter", ir darkens the blue in the sky and brings out clouds. So why not just do all this in software and kill the Blue channel? The color dye used in Mosaic filters of camera sensors are not true separation filters, but are sensitive way beyond the traditional cutoffs. The Blue channel response extends almost out to Red. By using the Yellow filter, you get all of the Blue response that extends past 480nm. With the Leica M8, that portion is almost as efficient as the Red channel. You get a much more balanced image to start with.

    Most software will attempt to demosaic and process the image not knowing that you used a yellow filter. I guess you can fight it, means post-processing. I ended up writing a very simple subroutine that does a simple nearest neighbor demosaic to get the three channels, and then add them. I used Photoshop RAW to open the image. I am learning how to write a DNG file.

    The results, Leica M8 with 28mm Elmarit, and medium Yellow filter.

    LR output of the DNG file,

    16899732845_c9ebcdcf29_o. L1015730 by fiftyonepointsix, on Flickr

    FORTRAN Output file:

    16691969547_562b7512bc_o. M1015730 by fiftyonepointsix, on Flickr

    16279310333_baa3193070_o. M1015732 by fiftyonepointsix, on Flickr

    And it's called a CLOUD filter for a reason!

    16873406786_9a9cdcb928_o. M1015737 by fiftyonepointsix, on Flickr
    Last edited: Mar 22, 2015
  2. Brian

    Brian Super Moderator Subscribing Member

    Apr 3, 2013
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