Colour and Exposure Accuracy in Digital Photography

Monday, November 28, 2011

Colour and Exposure Accuracy in Digital Photography

     One of Heidi and I's friends came over today for a short visit. Peggy-Anne and her Husband Glenn are architects, and Peggy-Anne has become a super artist. They say that one should change your profession 5 x during your lifetime. I have trouble with 1! Heidi, Peggy-Anne, and myself were talking and  Peggy-Anne said that she's arranged with a company to sell her beautiful artwork. Peggy-Anne   started to talk about accuracy in reproducing colour, and the next thing I knew, I was talking to her about about colour charts and gray scales.  Of course, if she started to talk architectural language, I would have trouble understanding. So I got one of those "Eureka moments", and I therefore said, that I'd simply write an instalment about this on my blog.

    I also received a new group of nice toys from someone in Maine (Larry Jacques) whom I'm trying to coax into writing an instalment. He also restores toys, and his batch of toys were used today for thios instalment. Thanks Larry!

    In the BDE (Before Digital Era), you had to have certain films to match the light that you were working with. They only made daylight and tungsten films. You used daylight for outdoors, and tungsten for most warm-light (lightbulbs) photos. There were also a range of colour filters to get accurate colour.  I don't want to go too much into detail, but here's an example when having used slide film. Unlike negative film where you could "correct colour" in the darkroom or at your favourite photo store or lab, slides recorded the image and you couldn't do any changing. Daylight slide film was fine for shooting between 10:00 - 4:00 P.M. on a bright sunny day. However on an overcast (cloudy day), you'd end up with blue-coloured slides. What you had to do is place a warming filter over the lens which was reddish in colour.

  Today in the DE (Digital era), you simply choose the white-balance on your camera to match the light source that you are photographing under. There are also special  colour charts and gray-shaded charts to use for accuracy when you need it. You can use Photoshop to adjust colour based on Photoshop readings of the colour or gray charts.

  One of the more popular and successful companies is X-Rite, which produces the Macbeth line of these special calibration charts.

 Here is the ColorChecker Brohcure and Instructions

 # 19 is white and is used to check for colour accuracy and calibration.

 Here are the chart values

Here are the exact values for this # 19 white tone

To explain colour in photography, you need to know that white is made up of equal colour values  or quantities of  RGB (Red, Green, Blue). The "opposite colours" are CMY (Cyan Magenta and Yellow).

In a perfect setting ,the values would be equal. However, if your lightbulb is older, or you're in a green-walled room, the ideal world is gone right away!

An ideal set of values for white are 

R 243

G 243

B 242 

(* I don't know why B is 1 # less)

In a green walled room, your numbers might be:

R 200

G 250

B 200

There is more green hence the higher number.

You find these numerical values in Photoshop, which I'll explain below.

Before you adjust the colour, you need to determine accurate exposure.

Above are a range of underexposed and overexposed test photos.

I should have placed the colour chart in there, and then read the

# 22 gray value in Photoshop.

Gray is used as a reference as it has no colour in it).

Here's the X-Rite ColorChecker, with only 3 exposures.

Here's the X-Rite White-Balance Card with only 3 exposures.

Here's the X-Rite Gray Scale Balance Card

All 3 can be used together, or you can purchase just one.

Any of them will give you numerical values to change (if needed) in Photoshop or any other image-editing software to obtain more accurate values and colour.

The item above is called an "eyedropper".

It is used to measure colour in a photo in Photoshop

In the upper right corner of Photoshop is the colour centre.

looking at the numbers and comparing them to the X-Rite values,

shows you that in my photo,

I have more red and magenta than blue.

If I remove quantities of these, then the photo won't be so "reddish".

 I changed the values elsewhere in Photoshop, and the above is what I got.

Notice that the background is not as red as before.

You can go to the X-rite Photo site to get more information and assistance.

I used the eyedropper to get a reading of the gray shade.

Here are the gray values.

The 3 values are certainly almost equal, so the grey will appear grey.

Here's the values for white and they certainly are within what they should be.

My problem is that my fabric that I use to photography my toys is reddish.

Even though the X-Rite reads accurately when I corrected my numbers in Photoshop, the fabric is

not a pure grey.

So I'll have to live with that or get another fabric.

I'll be coming back to expand on this instalment, but for now, let me review.

1. There are special "targets" that can be added to a photo in order to  get accurate colour.

2. These targets have specific numerical values.

3. These values can be read in most image-editing software programs.

4. White values are high as they reflect more light to the camera (250-250-250).

5. A mid-grey value is best for determining accurate exposure. Its' value is 160-160-160.

I got these numbers in the image above called "chart values).

6.  A black value is the lowest of all and reads 52-52-52.

7. If your numbers are "off", then you know that the true photo colour is inaccurate.

If it read, lets' say 270-250-250 for "white,then you'd know there is too much red in the photo.

You would have to subtract 20 units to get the accurate colour and numerical values.

To Be Continued........