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Tone and colour
correction within Photoshop can leave many new/novice users confused, not
least because of the vast array of tools but also the less than intuitive
way in which some operate. This tutorial explains the basic tools and
should also clear up some other mysteries along the way.
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.
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Brightness/Contrast
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Colour Balance
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Levels
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Curves
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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
Again, by deselecting the "Preview" checkbox we can quickly
compare the before and after effects of the adjustments.
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Before
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After
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2.
Colour Balance
The "Colour Balance"
tool seems to be favoured by many Photoshop novices. It's relatively
intuitive and quite flexible in the adjustment choices it gives. We can
work on "midtones", "shadows" and "highlights"
independently, but there are hidden dangers in this approach.
Realistically, if we decide to use this tool to make colour corrections
it's best to stick only with "midtone" adjustments.
The screenshot shows that I have increased red in the "midtones"
in an attempt to reduce the cyan cast to the image. The difference
between the Before and After images is most obvious in the "Church
driveway".
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Before
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After
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By toggling the "Preview" On and Off will we can see a
before and after effect. As mentioned above the colour balance
tool is reasonably intuitive and so relatively easy to use. However, it's
also pretty limited in what it can achieve, so I don't think much more
time needs to be spent explaining how to use it.
3.
Levels Tool
The "Levels" option is probably the tool most Intermediate
Photoshop users choose and is certainly a lot more useful than either of
the tools described previously. As can be seen on the dialog box below it
offers a histogram, as well as two sets of sliders, and three "eyedropper"
tools. With the "Levels" tool we can also adjust image brightness,
contrast and colour, but in a much more controlled manner than the
brightness/contrast or colour balance tools.
Photoshop 7 Levels
Basic tone correction using levels
Looking first at the RGB
histogram, the horizontal axis indicates the distribution of dark to
light pixels in our image, and the vertical axis the number of pixels at
any given lightness level. From this we can determine that for the above
histogram there are no totally black (left end) pixels and only a small
number of absolute white (right end) pixels in the image.
If we begin by considering
the "Input" slider bar we find three triangles, black, grey and
white. These triangles represent the "shadow", "midtone"
(brightness/contrast or gamma) and "highlight" adjustments. Moving
the "shadow" and "highlight" slider towards the centre will
increase image contrast. Moving the "midtone" slider to the left
brightens the image, whilst moving it to the right darkens the image.
Ideally we would move the two outer triangles to just clip the edge of
the first group of pixels (see screenshot below). However, it isn't
always the case that the point at which the data begins and ends in the
levels display is giving us a true indication of the actual spread of
image data.
Photoshop 6/7 and CS has a
feature called the "Clipping Display" that enables us see the
effects of an "Input" slider adjustment. Simply hold down the "ALT/OPTION"
key while moving either the "shadow" or "highlight" slider
towards the centre will display exactly which pixels are being clipped
(see screenshots below).
Adjusting the Highlights for
"Clipping" display
Highlight Clipping display
The above screenshot exaggerates the technique slightly so that the
reader can better appreciate the impact of moving the "Highlight"
slider too far. Typically we would want to stop when the image only just
begins to appear. In the above example we can see that the blue channel
has been taken to saturation (level 0) and a small section of cloud has
been "bleached out" to white. A similar exercise for the "Shadow"
clipping display produces the following results.
Shadow Clipping display
The levels "Output"
slider bar allows us to reduce image contrast by compressing the data and
making a dark pixel less dark or bright pixel less bright. The "Output"
control can be used for targeting an image for particular print
characteristics, e.g. setting the maximum black at level 5 and minimum
highlight at level 250. This feature can be quite helpful when the user
finds that their particular printer model tends to "block" what
should be perfectly printable "shadows" or "blows" well
detailed "highlights" out to white .
Basic colour channel corrections using Levels
So using the RGB "Levels"
dialog we have managed to adjust the tone of our image. However, one of
the main advantages of levels over "Brightness/Contrast" is colour
correction. What about colour correction?
By selecting the "Channel"
popup window we can choose any one of the three RGB colour channels. The
example shown below is for an adjustment to the red channel. In this case
the "midtone" and "highlight" sliders were moved to the
left thus increasing red. Moving the "midtone" slider to the right
would be the equivalent of increasing cyan. Similarly, if we had selected
the blue channel then moving the middle slider to the left increases
blue, to the right reduces blue (increases yellow).
Typically we will want to
avoid adjusting the shadow slider, but sometimes, as in our example, it's
helpful to move the highlight sliders slightly to reduced colour casts in
highlight areas. However, like the "Colour Balance" tool discussed
earlier we need to be careful.
4. Curves Tool
Whilst it might appear complex and lack the intuitiveness of the
"Colour Balance and Levels" tools "Curves" is a lot
more powerful and infinitely more flexible than any of the adjustment
tools discussed previously.
Figure 1
Figure 1 shows the default display for "Curves" with the 4 by 4
grid representing the 1/4, 1/2 and 3/4 tones. For many Photoshop users
this configuration is perfectly adequate. For those who prefer to work
with a finer grid, which broadly mirrors the Zone System used in B&W
darkroom printing circles we can hold down the ALT/OPTION key click the
mouse anywhere on the 4 by 4 grid and a 10 by 10 grid will appear.
Repeating this procedure will cause the grid to toggle between the 4 by 4
grid and 10 by 10.
"Curves" operate in much the same way as "Levels" but
have added benefits. The following screenshot will produce the same
results as dragging the "Levels Input" sliders towards the centre
causing and overall increase in image contrast.
Figure 2
The horizontal axis represents Input values and the vertical
axis Output values. Setting the Input value at level 50 to
Output value 0 has the effect of clipping every pixel between
level 0 and 50 to level 0; thus ALL pixels in that range will become
totally black. Setting Input level 200 to Output level 255
means that all pixels in the range 200 to 255 will become white. The
following image is not a pretty sight but it should give you some idea as
to what is happening in the above example. Notice that sections of cloud
are now completely without detail. The trees towards the left edge of the
image have now become black.
Image after Curve in Figure 2
has been applied
Dragging the diagonal line up and left has the same effect on the
image as dragging the Levels Gamma slider to the left (brightens "midtones").
Whilst dragging the diagonal down and right will cause the "midtones" to
darken.
Figure 3 below is effectively the same as dragging the "Levels
Output" sliders towards the centre and will result in the overall
contrast of the image being reduced.
Figure 3
Image after Curve in Figure 3
has been applied
Adjustments such as those that follow cannot be replicated by any
other Photoshop adjustment tool, which is why "Curves" tend to be
the adjustment tool of choice for more advanced users.
Figure 4 demonstrates an adjustment known as the "S-curve".
This particular adjustment increases image contrast in the "midtones"
by compressing the "shadows" and "highlights".
Notice the two black dots on the curve; there can be up to 14 of them
on the grid. These are sometimes called lock-down points and can be used
to define the shape and steepness of the curve. We can think of a steep
curve as high contrast and a shallow curve as low contrast.
Figure 4
Image after Curve in Figure 4
has been applied
Note that no colour correction has been undertaken for the above three
images.
The final screenshot in this series is another variation of the "S-curve".
This time we are reducing "midtone" contrast whilst expanding the
"highlights" and "shadows".
Figure 5
The above screenshots all demonstrated various possibilities for "tone
control" but "Curves" is the master when it comes to colour
correction. Just as we can select individual colour channels in "Levels"
we can do the same in "Curves". There really are no limits to the
number of possibilities. We can stretch and pull the diagonal line in any
direction, fix points pretty much where we want. However, be warned that
"Curves" are like an elastic band, and just as an elastic will
eventually break "Curves" will do likewise resulting in some weird
and wonderful/awful effects.
For a more comprehensive discussion on the "Curves" tool see
Real World Photoshop 6/7/CS by Bruce Fraser
and David Blatner or Photoshop 6/7/CS Artistry by Barry Haynes and
Wendy Crumpler.
5. Eyedropper Tool
Another tool within the "Levels" and "Curves" dialog is
the "eyedropper" tool. It's works identically in both "Levels"
and "Curves". The "eyedropper" tool is actually a bit more
complex to use than those discussed previously, but is extremely useful
and so worth getting to know.
Using the "eyedroppers" for neutral pointing
The "eyedropper" tool is very useful for quickly neutralising
some colour casts. We simply choose the centre "eyedropper" tool
and then click a point in the image that we know should be neutral. This
technique is sometimes called "neutral pointing". The screenshot
below should provide some insight as the how useful "neutral pointing"
can be. However, be aware that the area you select (mouse click) should
already be fairly close to neutral otherwise major colour shifts will
occur in other areas of the image.
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Before
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After
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It is also possible to use the "Eyedropper" tool to neutralise
colour casts in "highlight" and "shadow" regions, but this
requires great care and is better left to those who fully understand the
complexities of the "Eyedropper" tool (see
Real World Photoshop 6/7 by Bruce Fraser and
David Blatner for more details of this procedure).
"Targeting" or Assigning shadow and highlight
points using the "Eyedroppers"
Earlier I described a very simple way to set the black and white
points, i.e. the "value" for the darkest and brightest pixel in
the image. When using the "Eyedropper" tool it is recommended that
this step is taken after sharpening the image rather than as part of the
normal editing procedure since applying sharpening tends to undo some of
our "targeting" efforts. It's also important that we ensure that
the pixels that we will be targeting really are neutral.
To configure the "eyedropper" tool for our preferred target
values we begin by double clicking the shadow "eyedropper" icon
and then adjusting the RGB values in the "Color Picker" dialog.
Default shadow eyedropper settings
If using one of the Epson
Photo series printers then the "Shadow" can be set for a value of
R=G=B=5 (2%) as shown below.
Shadow eyedropper set at 2%
Once set; we simply click the
OK button and return to the "Levels"
dialog. To set the "Highlight" value we repeat the above process,
but this time we choose the highlight eyedropper, typical values seem to
R=G=B between 245 and 250.
Now with our white or black eyedropper tool selected we find a point
in our image that we want to make the brightest/darkest and simply click
that point. Again the "Clipping Display" can be used to identify
the location of the darkest/lightest pixels and then we zoom in close to
areas that you think might be appropriate. If after clicking a point we
realise that it was a poor choice then we simply use the "Ctrl+Z"
(or "Command+Z" on the Mac) keys to cancel the command and try
again.
Layers Palette
Remember that I have been applying the various edits using "Adjustment
Layers" so checking the "Layers" palette we should find a new "Levels"
layer. In the example shown below we see the thumbnail of a histogram.
This layer contains all the information relating to the adjustments we
made in the "Levels" dialog.
Layers Palette
Double clicking the "Levels"
layer will reopen the levels dialog and allow further adjustments. We can
also get a "before and after" preview by simply clicking the "eye"
icon.
Some recommend that we flatten the layers once the various adjustments
have been completed. However, since an "Adjustment Layer" accounts
for only a small increase in file size I believe saving it with the image
is a more prudent approach, after all we never know what the future holds
for the image. One thing that should be understood is that we cannot
sharpen an adjustment layer. Therefore to sharpen our image prior to
printing we simply make a duplicate (Image > Duplicate) and
flatten the layers of the duplicate image.
Bringing it altogether
In preparing the final image I chose to use only a "Levels"
adjustment layer. My first step was to neutralise the overall colour cast
using the grey "eyedropper" tool. I followed this by setting the "black"
and "white" points for the image using the "Shadow/highlight
Clipping Displays" as an aid. I then used the red channel "midtone"
slider to remove the remaining colour cast and finally applied some
sharpening via the "Unsharp Mask" tool
Place of Worship - Bruckless,
Co. Donegal
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