Project 4: your camera's colour performance

For this project I investigated the colour performance of my camera. Before completion I thought it achieved fairly accurate results and was intrigued to see if this assumption was correct. I had to take a photograph of the colour chart provided and compare it to the digital file on the OCA disk. The photograph was taken in daylight, white balance corrected and on auto metering.

When opening the photograph and comparing it to the digital file OCA_colourchart_as looks it looked awful!


Chart pre colour cast adjustment

Using the colour picker I measured and noted the values of first 5 squares comparing the two charts

Chart	1	2	3
OCA Chart	17,17,17 H 0,S 0, B 7%	52,52,52 H 0,S 0, B 20%	97,97,97 H 0,S 0, B 38%
400D Chart	1,3,1 H 120, S 67, B 1%	57,48,50 H 347, S 16, B 22%	144,140,140 H 0, S 3, B 56%
	4	5
OCA Chart	161,161,161 H 0,S 0, B 63%	243,243,243 H 0,S 0, B 95%
400D Chart	216,217,222 H 230, S 3, B 87%	249,251.251 H 180, S 1, B 98%

Due to the colour cast it was recommended that Enhance was used to correct it. As I am using CS4 the menus and techniques are slightly different, I had to refer to the Adobe help website to find out how to do this.

adjusting cast using curves

Chart after cast adjustment

To the eye it looks much better although there is still a difference. Measuring was tricky as the colour was not uniform and varied across the coloured squares.

Chart	1	2	3
OCA Chart	17,17,17 H 0,S 0, B 7%	52,52,52 H 0,S 0, B 20%	97,97,97 H 0,S 0, B 38%
400D Chart	0,1,0 H 120, S 100, B 1%	49,52,53 H 195, S 8, B 21%	134,134,138 H 240, S 3, B 54%
	4	5
OCA Chart	161,161,161 H 0,S 0, B 63%	243,243,243 H 0,S 0, B 95%
400D Chart	213,4, 85 H 230, S 3, B 87%	254,255,255 H 180, S 1, B 100%

The blacks were registering fairly close to the RGB levels of 0,0,0, that you would expect and the white close to 255,255,255 even at 134,134,138 the grey was not too far off 128,128,128. Not sure what was happening with the Hue,Saturation and Brightness readings though? Maybe I needed to zoom in a little closer due to the uneven tones of the boxes? I think completing this exercise has left me a little confused as to how to interpret the results. I have examined two colour boxes to see if I could figure out the numbers from there....but there didn't seem to be a correlation? There was not more red in them all or more blue....the green box indicated my camera had more R less G and less B which suggested a red cast but then the blue box measured less on all 3? My head hurts :o/ What I have concluded is that adjusting the mid-tones/BW points is something I should be doing as a matter of course even if I have set a custom white balance. I may not be able to fully understand the numbers I am seeing but there was certainly a colour cast to correct. The Saturation and brightness seem to be close but the hues are definately higher.