Using ' Curves' in Luminosity Blend Mode

Clive R. Haynes FRPS

Most people tend to make tonal adjustments in 'Curves' that affect both colour and luminosity content simultaneously.

When adjusting and modifying the shape of the curve in 'Normal Blend' (the default mode) we affect the composite RGB content of the image. For small changes, the alterations to colour, although present, are not usually significant. However when making more dramatic changes to the tonality of an image by lightening, darkening or increasing the contrast range, changes to the colour content become more pronounced and frequently undesirable.

It's often better practice to apply tonal changes to the underlying black & white (Luminosity) content of the image when we need to alter contrast. And to achieve this it's preferable to use an 'Adjustment Layer'.

The Adjustment Layer' route is the preferred option, as not only is it 'non-destructive' but also it has a useful 'mask', which is invaluable for the selective application of the adjustment needed.

Creating a 'Curves' Adjustment Layer
At the base of the Layers palette, click on the half black / half white circle icon. From the 'drop-down' list that appears, choose 'Curves'.


The new Adjustment Layer will appear together with the 'Curves' dialogue box. For this exercise I've set the 'Curve' to an 'S' shape to increase contrast' as below.

To illustrate the value of working with 'Luminosity Blend' we'll first look at the example image below, with the 'Curve' adjusted using 'Normal Blend Mode'. Notice that the contrast has increased and colour values become more intense - in this instance this is undesirable.


Above: the Opening Image
Above: Curves Applied in 'Normal' Blend Mode

The next example shows the effect of choosing 'Luminosity Blend Mode' with the identical adjustment to the shape of the Curve.

To change the 'Blend Mode', click on the drop-down arrow to the right of the 'Normal' box (at the top lh of the Layers Palette) and from the drop-down (blend) menu, choose 'Luminosity'.

The alteration to the contrast has become more subtle as it now emphasises the change of underlying tone rather than colour.

Above: Curves Applied
in 'Luminosity' Blend Mode
Above: Curves Selectively Applied
in 'Luminosity' Blend Mode
In this example, above, the 'Curves' adjustment has been initially applied to the whole area of the image. However, by using the 'Paintbrush' tool and with black, as the foreground colour, areas of the 'change' have been selectively removed.
When 'painting' on the 'Layer Mask' associated with the 'Adjustment Layer'. White will apply the change and black will undo it, judiciously swapping between the two will produce a pleasing result.
By clicking on the 'Curves icon' on the Adjustment Layer, the 'Curves' dialogue box will re-open and further tweaks and changes can be made - to be able to 're-visit' is one of the advantages of using an 'Adjustment Layer'.
The decision whether or not to apply 'Curves' in 'Luminosity Mode' will depend upon the image and the outcome that you pre-visualise. For some images, the 'boost' given to the colour content is appropriate whilst for others an increase in underlying contrast is all that is required. Experiment as to which answers your needs best by swapping between 'Normal' and 'Luminosity'.

One final note: Working in 'Luminosity Blend Mode' is equally applicable for the adjustment of 'Levels' too

For more information about using 'Curves' and 'Adjustment Layers', click on the link below

Adjustment Layers

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