This version of the
Basic Tone and Colour Correction tutorial has been revised
to reflect the changes introduced with Photoshop 7. It has also been
expanded to include comprehensive discussions on the following Adjustment
tools.
- Brightness/Contrast
- Colour Balance
- Levels
- Curves
- Eyedroppers
Before starting any serious work in Photoshop it
important that we have the monitor calibrated and have Photoshop
correctly configured, see the
Photoshop CS & Colour
Management essay. It's also quite helpful to have the
desktop arranged in a way that allows both the "Channels", "Layers"
and "Histogram" palettes to be viewed at the same time. Actually
customising the desktop to ensure your ideal workflow has never been
easier.
A printer friendly Adobe Acrobat PDF version is
downloadable by clicking the pdf icon
Typical Photoshop Palette Layout - Channels and
Layers shown separately
For the purposes of this tutorial I will be using an image similar to
that shown in the screenshot above. As can be seen from the "Histogram"
screenshot below; the image isn't quite full-scale. Actually it's good
practice that we check either the "Histogram" or "Levels"
dialog immediately after opening an image as this will allow us to
determine the overall tonal characteristics of the image.
Photoshop Histogram
Basic tools and how to apply them!
Opening the Photoshop "Image > Adjustments" menu will show just
how many options there are available for correcting the tone and colour
of an image. We'll be concentrating upon those within the upper grouping.
The names of some mean that it's fairly obvious what they do, for others
it's not so clear.
It's also worth mentioning that selecting any of these basic
adjustment tools via the "Image > Adjustments" menu can have its
own set of problems. An unpleasant fact of life when processing digital
images is the degradation that occurs every time we make even the most
minor of adjustments. If your images are anything thing like mine you
will need to make a number of adjustments to tone, colour, saturation,
etc. before reaching the desired result. The combined effect of all these
adjustments will be a "Histogram" that looks like a badly
maintained picket fence. The more gaps in the Histogram the more
missing information the more banding/posterisation. This form of image
degradation is usually more obvious in large areas of smooth tone, e.g.
blue sky.
The three "Adjustment" tools bounded in green provide
automatic correction of "Levels", "Contrast" or "Colour".
This latter correction option was introduced in Photoshop 7 and is
significantly better than Auto Levels. However, it does require
careful configuration. If you're really serious about getting the best
from your images you'll not want to use any of the automatic correction
tools. That said Auto Color is always worth investigating.
Adjustment Layers
Without doubt the best way of "minimising" image degradation is
to use Photoshop "Adjustment
Layers". Almost every adjustment tool found under the "Image >
Adjustments" menu can be found in the "Adjustment Layer"
dialog and with Photoshop CS we can finally use them with
16-bit images. We can create an adjustment layer by simply selecting
"Layer > New Adjustment Layer" from the Photoshop "Layer"
menu.
One advantage of applying the adjustments using the layers approach is
that it avoids working on the actual image. By working on a clear film "overlay"
we can iteratively fine-tune the adjustment; turn it off; or even delete
it at any time without impacting upon the original image. This does not
mean that adjustment layers are non-destructive, just that they are less
so! Contrary to popular believe "Adjustment Layers" are NOT
applied in one super complex adjustment. Each "Adjustment Layer"
is applied sequentially to the image form the top of the layer stack
downwards.
1.
Brightness/Contrast
The name gives a good clue as
to what this tool does, and for this reason it seems to be the first
choice for many Photoshop novices. It's designed solely for tonal
correction, and as such has no means of correcting colour.
This tool is also the worst possible choice in terms of correcting the
brightness or contrast of an image (tone). "Brightness/Contrast"
is best described as a linear operating tool in that every pixel gets
brighter or darker. By way of the example, increasing the brightness
value by 10 units means every that pixel is increased by 10 units, not
very satisfactory at all. Looking at it from slightly different angle ALL
image data will be affected by the adjustment. The following series of
screenshots demonstrate the distribution of luminosity values after the
application of a brightness increase and a contrast increase.
Increase the Brightness
Increased Brightness
Notice how increasing brightness moved everything to the right of centre,
and how increasing contrast expands the data outwards at both ends.
Reducing brightness moves all data to the left of centre, and reducing
contrast causes the image data to be compressed. The bottom line here is,
if we can avoid using "Brightness/Contrast" then it is better to
do so.
Increase the Contrast
Increased Contrast
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