Matching the images which you bring together in a composite is very important. Colours, colour temperature, brightness and saturation need to match, otherwise the images look wrong, and look like they have just been plonked together, rather than belonging together in a believable way.
Always match the subject to the background, as the background sets the scene for the lighting, which should also be correct on the subject. (Ref).
Check Layers are temporary layers you can use to match the characteristics of two images, such as colour, saturation and luminosity, so that the two images blend together seamlessly.
A Solarise Check Layer shows up every tiny imperfection in an image by magnifying detail. (Ref).
--- Matching Brightness
Matching the brightness (luminosity) of the foreground to that of the background is the first thing to do. This can be done by eye with an Adjustment Level such as Curves or Levels. Using a Luminosity Check Layer will show how close you have achieved a balance.
Luminosity (Brightness) Check Layer
A Luminosity Check Layer shows up inconsistencies in Luminosity (Brightness) between different parts of the image.
To create one, make a Solid Colour Adjustment Layer using any colour with Saturation of 0 and change the Blend Mode to Colour. (Ref, Ref).
This removed all colour information, so that you see the image in black and white, and can then judge if the brightness levels match.
However, it is important to do the black and white conversion correctly, so that it gives the same effect as the eye sees, which is a perceptual conversion, rather than an absolute conversion which is just according to colour saturation.
This check layer is also useful when you use Dodging and Burning to match the lighting of the scent and apply contouring (See below).
--- Matching Saturation
Matching the saturation of the subject to the background is important, as otherwise it will look wrong. Use a Hue/Saturation Adjustment Layer and adjust the Saturation slider. Then mask it to the appropriate area.
You can visualise how closely the saturation within an image matches by using a Saturation Check Layer. There are two ways of creating these.
Saturation Check Layer (Type 1)
To create this add a Colour Adjustment Layer, and go into each colour, make sure Absolute is checked and not Relative, and take the black slider all the way to the left to -100. For the Whites, Neutrals and Blacks take the black slider all the way to the right to +100. This can be saved as a Preset in the Colour Adjustment Layer. (Ref, Ref).
This now gives a black and white image where the lighter areas are more saturated and the darker areas less saturated.
You can add a Curves Adjustment Layer or a Levels Adjustment Layer and Clip it to this to increase the contrast and make it more visible.
This Check Layer can also be used to highlight how far you can go in increasing the vibrance or saturation of an image before losing detail. It also highlights any areas that may cause problems in printing by being over saturated.
Saturation Check Layer (Type 2)
To create this add a Solid Colour Adjustment Layer and choose Red as the colour, as RGB 255,0,0. Duplicate this twice to create three identical layers. Change the Blend Modes to Hue, Colour and Luminosity, and make a Group of them.
The more red the section the more saturated it is.
This one didn't work for me as I just ended up with a totally red screen.
However having two red layers with just Hue and Luminosity worked, and adding a Hue/Saturation layer above this with Saturation set to +50 increased the saturation of the image making it easier to see. (Thanks to Kirsteen Titchner for this).
Colours should usually harmonise in a composite, and you can adjust colours in several ways. Using a Curves Adjustment Layer and going into the individual Channels, and then using the Finger Picker to select and adjust individual colours is a popular way of doing this.
Using a Hue Check layer allows you to see just the colours in an image. Hue is the same as colour.
Hue (Colour) Check Layer (Type 1)
To create this add a Solid Adjustment Layer of 50% Grey, as RGB 128, 128, 128. Change the Blend Mode to Luminosity. (Ref).
The result looks a bit strange but can be very useful to see which colours stand out and do not match. Grey areas have no colour. You can increase the contrast of the image by Clipping a Curves Adjustment layer to it.
Hue (Colour) Check Layer (Type 2 - Better)
To create this add a Solid Adjustment Layer of Red as RGB 255,0,0. Change the Blend Mode to Luminosity. (Ref). Adding a Hue/Saturation layer above this with Saturation set to +50 increased the saturation of the image making it easier to see. (Thanks to Kirsteen Titchner for this).