Photoshop Curves - what are they?

There is a progressive increase in density as exposure increases and the straight line portion of the curve (B) is the most effective part of the film.

At the shoulder of the curve (C) more exposure has a reduced effect until further exposure does not increase the density on the film at all and we get a horizontal line on the graph.

This type of curve is similar to the response of the eye and brain to light.
Our vision is most effective in the mid-areas and not so good in the extreme highlights and shadows.

The left graph would represent a film with higher contrast. The right graph would represent a film of lower contrast but a film that would be able to reproduce more subtle tones. The gradient of the straight line portion of the curve (B) is calculated and given a gamma value, such as G 0.6. Gamma is not the whole story but basically the higher the gamma, the higher the contrast.

A black and white film will have one curve while colour films will have three and it is the aim of film manufacturers to make all three curves exactly the same.

With film, shadows and highlights have lower contrast within their areas than the midtones; that is why the curve is not a straight line.
It is curves like these we will be adjusting in Photo shop to make colour corrections and tonal changes.

To open the Photoshop Curves dialogue box in Photoshop, in the top menu bar, go to:
Image > Adjustments > Curves.

This one really hits you in the face when you first see it.
We'll start off by simplifying it a little to make it less intimidating. Expand the box to its maximum by clicking on Curve Display Options if required.

Uncheck the boxes for
Channel Overlays,
Histogram
and Intersection Line.

Digital chips react to light in a uniform manner and do not suffer from the reciprocity failure of film.
If light is doubled (increased by one stop), the response of the pixels is doubled.
If light is halved (reduced by one stop), the response of the pixels is halved.
This happens over the whole range,
producing a straight line curve of 45 degrees.

As a result, a digital photograph will contain more detail in the shadows and highlights than most films can contain.

The RGB and CMY information in this chart is used by Photoshop Curves.
If much of this is new to you, it would be useful to print out this image and stick it on the side of your monitor.
It is image 01.jpg in the Samples-Curves folder.

• White light is made up of the rainbow colours of red, orange, yellow, green, blue, indigo and violet. This provides the three primary colours of Red, Green and Blue. When red, green and blue lights are laid over each other, they produce white light.

• Transparency or positive film uses the primary colours of red, green and blue, also known as additive colours, and when mixed they can create millions of colours.

• Negative films use the opposite colours of cyan, magenta and yellow, also referred to as secondary or subtractive colours. (This is light we are talking about here - painters have a different set of primaries.) Even though RGB can produce such a vibrate range of colours, the problem is it does not give the best results when it comes to printing with coloured inks. Here it becomes necessary to use cyan, magenta and yellow inks.

• We need to understand how all the colours and the two modes are connected.

• Red light and green light produce yellow light.

• Green light and blue light produce cyan light.

• Red light and blue light produce magenta light.

• Starting with white light, taking out green leaves red and blue; where the two overlap they make magenta - the opposite of green is magenta.

• Starting with white light, taking out blue leaves red and green; where they overlap creates yellow - yellow is the opposite of blue.

• Looking at the CMY trio, using only yellow and magenta creates red where they blend together - cyan is left out and red is the opposite of cyan.
  The bar diagram shows that the two sets of colours are opposites. When we start doing colour correction shortly, this should all fall into place.

• When cyan, magenta and yellow are projected on to our white background, because they are subtractive colours, they take their own colour out of the white light. Take out all three colours and we are left with no light and hence black. This can be seen in the black triangle in the centre of the cyan, magenta and yellow trio. When printing on white paper, we are in effect taking out light.

• But we always talk about CMY and Black. The reason is that when cyan, magenta and yellow inks are mixed together, they create a black but it remains somewhat dirty looking. To use 100% each of CMY would be expensive on ink and would create drying problems.
  A separate black ink produces richer colours. In additions, a 100K black ink combined with 50C or perhaps 50C,50M and 25Y will give a richer black.

Channels
The separate colours are called channels; so RGB mode has three channels and CMYK mode has four channels. This means that a 30Mb file in RGB will be a 40Mb file in CMYK.
A considerable amount of Photoshop work can be done on the individual channels and some of this will be covered in other tutorials for Photoshop.....



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