Colour Adjustment - Part 3

In Part 1 we looked at the structure of colour and colour perception. Part 2 looked at the numerical controls available typically in Photoshop. In Part 3 we look at the initial exposure of an image followed by a logical progression towards the optimum final output.

Exposure

The exposure of a digital image is quite critical if really accurate colours are to be obtained. Conventional photographers will have a grasp of the relative differences in exposure latitude between transparency and negative emulsions and the fact that it is smaller for a transparency. Digital exposures are about as critical as those for transparency, although there are a number of post-shooting fixes that may be applied. Exposure is more critical on the exposure side by a factor of around 2 or 3. We conducted an experiment in which we shot a Macbeth Chart under constant lighting, but bracketed the exposure in 1/3 stop increments. We then analysed the results for colour error and plotted the data to obtain the graph shown. Note that the error rises more sharply on the over-exposure side. Given that best practice requires colour errors on the order of 4 to 7 Lab points the exposure latitude (for this quality value) is 1/3 of a stop on the over exposure side and 2/3 of a stop on the under exposure side. These results are quite surprising and in practice we obtain more latitude by tweaking our lightness values and hoping that we have not skewed our colour values. Nevertheless the measurements do not lie - it's a tough old world out there!

In addition to the actual exposure value (governed by f-stop, shutter speed and ISO rating), digital photographers must also concern themselves with the colour temperature of their light source. In these days of fast digital adjustment of colour we tend to forget that we used to buy big batches of transparency film, test it with different colour correction (CC) filters and then bung it in the fridge and hope it did not run out too fast! Does anybody still do that out there? Minor variations in lighting, when using negative film, were dealt with at the enlarging stage by CC filtration of the colour head. In digital photography the image will appear on screen "as shot" and if your White Balance is not correct then it will show, usually as an image that is too blue or too yellow. Quality digital cameras have the ability to Auto White Balance, when the chip data is interpreted by the exposure system and corrected on the fly. This is vulnerable to subject failure in the same way as automatic exposure but, like the latter, it works OK a lot of the time. The equivalent of taking an incident light reading to avoid subject failure in conventional photography is to use a grey (or white) card and do a Custom White Balance. As we shall see later this can be dramatically better than letting the camera do the job on its own. As with conventional exposure value adjustment, compromise is sometimes needed in which some parts of an image are sacrificed in favour of more important parts - typified perhaps by letting the groom’s black trousers block up in favour of retaining detail in the brocade of the Bride's dress.

In summary then, avoid over exposure and keep an eye on your White Balance. Note that Kodak Software has the ability to reinterpret the RAW image data and "recover" up to 2 stops of exposure error. The demonstration we have seen of this feature was impressive - it is accomplished by the RAW data having sufficient dynamic range to cover 12 stops, more than is used to make a print for output.

Assuming that the best exposure has been made and you have arrived home from the shoot with no opportunity to influence the RAW data it is time to examine your images in detail.

Look Before you Leap!

The first port of call back at base is the Levels Dialogue. Click Ctrl-L and check out the shape of the Histogram. Assuming that the data are roughly centred in the middle of the Histogram you can relax and start to make an examination in more detail. Recognise that, confronted with several hundred images after a reception reportage style shoot, you may not have time to spend refining every last pixel at the initial stages.

The first part of a serious examination of the image should be to look carefully over the whole image, taking note of the following:

1. Ask yourself why you are correcting the image and what you hope to achieve from doing so. If it is woefully out ask if you have options to re-shoot.

2. Is the image sharp? Look at different magnifications and be aware that Photoshop shows jagged edges at magnifications other than 25,50, 100, 200% etc. In between values such as 33.33% and 66.66% etc are not sharp on the screen!

3. Is the image too light or too dark overall? Would it benefit from selective dodging and burning?

4. Is the image saturation OK. Look for exaggerated greens in the grass and over-red faces on people? Particularly when judging greens use View>Proof Setup>Custom and select the profile you intend to print with.

5. Look to see if any part of the image would benefit from selective desaturation to recede it into the image, leaving the main subject more forward. Such measures can be very effective.

6. Turn on the gamut warning and look for any outrageous portions of the image.

7. Re-look at the levels but this time click on the Alt key and the sliders so that you can observe the primary and secondary colours that are clipped (see call out box).

8. If there are faces in the image, turn on the Info Palette and check over the values in accord with the table shown. If your values are significantly different from those listed be worried!

9. Finally check over the image to determine if any repairs or cloning retouch is required to deal with dust, blemishes or distracting highlights or backgrounds. Look also for repair to subject's dress Wonky ties, bra straps, smudged lipstick etc are trivial repairs in a digital image.

Now you are ready to start correcting your image.

The Road to Perfection

All Photoshop adjustments should be made with a weather eye on the numerical values. By all means carry out adjustments using eye-balling on the screen but make sure your screen is calibrated, unvarying and that you are not too tired, hung over, hypo-glycaemic etc! Remember that the Info Palette shows the start value and anticipated finish value of colour either side of the back slash.

Levels - the first port of call

Open the Levels Palette. You should know the RGB values that your printer is capable of delivering detail for. If for any reason there is no detail in your prints of any RGB values less than 20 points then you have to assess if anything important lies in that tone zone by Alt-dragging the slider to 20 points and looking at the clipping display. If you are not sure of your RGB values and their relationship to print details make a pure grey scale and print it out (see the box). Check the highlight end of the scale as well.

You may drag the sliders in-board. This will increase the contrast in the image. The centre slider (gamma) may be used to effect overall changes in the image density. For simple corrections these might be the only changes that you need to make.

If you click OK and then reopen the levels you may see that your Histogram has become "gappy". This may or may not be a problem depending on the quality of the image data. The effect is certainly helped if you adjust your levels in 16-bit mode if that is an option open to you at either the scanning or camera shooting stage. Be aware though that there are differing opinions about the benefits of 16-bit and 8 -bit working. You may have to drop to 8-bit at a later stage anyway as some filter functions are not available in 16-bit.

Light Skin

Photosop BIT DEPTH The Bit Depth of an image defines how many variations of colour are allowed. Photoshop has a bit depth of 8 per channel, allowing 256 shade variants in each of Red, Green and Blue pixels. The value of 256 comes from 28 and as there are 3 channels in RGB this gives 224 or 16million shades. 16 bit gives 216 variations of shade in each channel or 65,536. The combined variations of 248 is billions of colours! This gives more precision in adjusting colours, as there are finer variations to play with. Photoshop registers all files of more than 8- bit as 16-bit so a 12 bit scan will come up as 16 bit. Files at “16-bit” are normally about twice the size in mega bytes and therefore require twice the hard drive space and more RAM in Photoshop. Be aware that there is a body of opinion out there who say that it makes little practical difference to image quality.

Making a Pure Grey Scale

1. Make a New Image and set the parameters to 20cm wide, 3cm high, the mode RGB and the resolution 200ppi.

2. Hit the letter "d" to default the ForeGround and Background colours to RGB0 and RGB255.

3. Click on the Rectangular Marquee and ensure that the Feather is set to 0 pixels.

4. Click on the Gradient Tool (could be under the Bucket Tool - beware!) and ensure that the gradient is set to Linear and Black to White.

5. Drag a rectangular selection over most but not all of the image area. Then select the Gradient Tool.

6. Position the cursor over the left hand vertical of the marching ants, depress the shift key and drag the cursor along (in a straight line) to the right hand end of the marching ants - then let go of the mouse button - a gradient from Black to White appears.

7. Click on Image>Adjust>Posterise then type 21 steps. The gradient is divided up into discrete steps.

8. Print this image at the top of a sheet then examine it to determine which steps are differentiated. Go back to your on screen image, set the Eye Dropper Tool to 5x5 Average and measure the image RGB values off. Note them down; they will, for example, give you the RGB values at which your shadows will block up or your highlights will clog.

Pure Gray

Real World Adjustments

Downstream from a levels adjustment there are as many options as there are dialogue boxes and palettes. A number of them never seem to feature in the armoury of retouching professionals. These include Variations, Brightness/Contrast and to some extent the “Auto” series of Levels, Colour and Contrast. There is an element of snobbery to some of this as well as a tendency to gravitate toward the technique that you are most familiar with. All that needs to be said is that if the simple technique works for you stick with it and leave the time you save for the more difficult situations where you absolutely have to employ all the sophisticated techniques that are available. More difficult images may respond better to adjustments in CMYK, Lab or to selective colour adjustments; you may for example identify that a problem is solely due to Saturation in which case all you need attend to is the saturation in the HSB Palette.

Identifying the Enemy

The first operation is to identify the cause of the problem. Often you are left with a dissatisfied feeling about a print but with no clear idea of what the problem is. For example a shot may be dull but is this because of a lack of contrast, lack of saturation or too much of an unwanted colour?

The first figure shows your editor shot under incorrect White balance conditions. The shots to the right show the effect of reducing saturation but there’s still something wrong? Cranking the saturation right up, as in the right hand figure, reveals part of the problem - the skin tone contains almost pure red and needs some yellow.

The second shot shows the same lighting but a Macbeth Chart has been added as an accurate point of reference. By placing the dropper over the Macbeth Light Flesh tone (left hand column second up from the bottom) the parameters in the table were measured. These establish that the colour is too red (16° instead of 24°); too light (75% brightness instead of 71%) and too saturated (30% instead of 27%).

Using an Adjustment Layer for Hue Saturation Brightness, the tones can be brought into line and the table below shows that the eyeballed judgement almost got it right. At the same time the skin hue of your Editor moved from 1° to 12° i.e. closer to the reading we obtained by putting him under the spectrophotometer! Also, all the values in the Macbeth Chart moved closer to the correct values.

How good are we?

L A B One interesting test was to determine how accurately your Editor could adjust images on a calibrated screen with and without aids. In the first test I was able to match the image on screen and my own skin tones (in a mirror) to within 1.8 Delta E of each other. Without any points of reference I was able to get two independent images to within 7.8 Delta E of each other. Matching two images side by side on screen I was able to get the images to within1.4 delta E of each other. To put these values in perspective, the original print was 16.4 Delta E out, 4 is top class proofing quality and typical unprofiled inkjet printers can make averages of between 9 and 15 delta E points. The experiments confirm what is well known - that the eye is a very good comparator but no quite so good without points of reference. The subject of the round trip colour accuracy is something we are going to revisit later in this series.

Levels

Above: The main image is badly lacking in dynamic range. Note that the histogram does not extend to either end of the tone scale. Dragging the input sliders inwards from the edges improves the image as shown by the smaller inset.

The SWPP 2008 Convention was an outstanding success,
we have 174 days to get ready for the 2009 convention - which starts on January 14, 2009

Photo Quote: Photography can never grow up if it imitates some other medium. It has to walk alone; it has to be itself. - Berenice Abbott