Monday, 12 September 2011

Colours into tones in black & white

Exercise - 4 photographs

This is the final exercise before the colour assignment in part 3 of the course and looks at the effect colour filters have on black and white image processing. Coloured filters are a creative tool that photographers have been using for many years with film cameras. These days it's much easier to get the same effect in Photoshop post capture. 

This exercise involves taking a straight forward image containing the colours red, green, yellow, blue and neutral grey. The image is converted to black and white and the effect on each colour is compared when a coloured filter is applied. My understanding is that a coloured filter will let it's own colour light through but block it's complementary colour light. E.g. a red filter will allow red light to pass through and therefore appear very light but will block it's complementary green light which will therefore appear very dark in a monochrome image. The strength of the colour filter will also have an effect on the image e.g. a yellow filter will have a stronger effect than a light yellow filter.

My only previous experience of using this technique is when taking landscape images in black and white and applying a red filter effect post capture in Photoshop to darken the sky as in the image below.

Tay Road Bridge Footpath
Filters can also be used in portrait photography to adjust skin tones.

These first two images show the before and after in a straight forward conversion to black and white with no colour filters applied.

These next two images have a yellow and blue filter applied respectively.

Yellow Filter
  • allows yellow light to pass through so it appears very light in tone
  • blocks blue light so it appears a much darker tone
Looking at the results above it would also appear to allow some red light to pass through as this is also lighter than the default image and has a slight darkening effect on green light.

According to the course notes a coloured filter will lighten any object of the same colour and darken the tone of the others. However the image using the yellow filter definitely shows the red object lighter than the default image.

Blue Filter
  • allows blue light to pass through lightening the tone
  • blocks yellow light so it appears much darker in tone
 In the image above the blue filter has little effect on the green area but the red area also appears darker.

The next two images show the effect of the green and red filters respectively.

 Green Filter
  • allows green light to pass through lightening the area
  • blocks red light making it appear darker
Surprisingly the most noticeable and strongest effect on this image appears to be with the yellow when it should be with the red. The yellow is much lighter than the red. The green area appears only slightly lighter than the default image.

Red Filter
  • lightens red in tone
  • blocks green light making it appear darker
Both the blue and yellow areas have also been quite strongly affected in this case.

As the results are not as I anticipated or suggested by the course notes I'm wondering if the initial default conversion in Photoshop has applied some changes in the colour range that have altered the expected outcome. There is no doubt the the original colours are red, green, yellow and blue. My understanding is that the only area that should have remained unchanged throughout the series is the mid grey colour which is unaffected by the coloured filters and that all the other colour changes are predictable.

Regardless of the results here I now have a better understanding of the effects created by the use of coloured filters and how they can be used in black and white photography. They are a useful tool that can be used to suppress or highlight areas of an image when converting to monochrome.


  1. is it going with the third assignment?? looked a bit desperate.........I hope it is going better now.



  2. Going slowly. Started reading Part 4 light to give myself a break. Got the first few images done for "colour" and will be updating blog with these soon. Going to have to move on whether I'm happy with the pictures or not as I'm getting further and further behind.