Terminology
- Exposure Blending – Is normally the blending of two or more images shot with different exposure values (EV). To create an image that captures more of the dynamic range of a scene that a single shot alone cannot capture.
- Luminosity Mask – Is a mask used to isolate parts of an image based on shared luminosity values. You can use Luminosity Masks to do Exposure Blending.
- HDR (High Dynamic Range) is a software driven process of blending two or more images with different exposure values.
What can you do with a Luminosity Mask?
Luminosity masks are used for image blending. Luminosity masks are an alternative to using HDR (High Dynamic Range).
Luminosity masks allow for more control in the editing process than HDR.
Whereas HDR requires two or more images. It is possible to use a luminosity masks on a single image that has been copied and edited with differing exposure values to the original. This is especially true with raw files.
Ideally you should shoot at different exposure values, normally three shots at 2EV apart, by using, for example, Exposure Bracketing.
What is Apply Image?
Apply image is a calculator for digital images.
Whereas the Calculations command only works on channels. Apply image can work on channels, layers, and masks.
Apply Image applies a source to a target using a blend mode.
The source and the target are usually from the same document. But it can be another document with the same pixel dimensions.
The Source
The source layer options are:
- A single layer
- Merged, (all visible layers).
After you have chosen your layer options you have to pick a channel and that can be one of the following
- The RGB composite channel (all three colour channels).
- A single colour channel, red, green, or blue
- An Alpha Channel (mask).
You can also just choose the overall transparency of the chosen layer, or layers, instead of a channel.
Finally, you can choose to Invert the source values you have chosen.
The Target
The target is a single Layer, Layer mask or channel to which you apply the source values via a blend mode.
You can also output the target via a Mask using a Layer/s Channel (Colour or Alpha), or via a selection.
The permutations with Apply Image are not infinite, but it can be confusing for anyone not familiar with digital imaging.
Apply Image is not easy to use for beginners to Photoshop. That said it is immensely powerful in the right hands.
The Apply Image Process for Luminosity Masking
Put three images shot at different exposures into one document. Or create three images from one image (especially true for raw files). Then edit two of the images to make a darker version and a lighter version.
If you have shot the images with exposure bracketing you will usually need to align the layers using the method below.
Select the three layers and go to Edit > Auto-Align Layers…. and select ‘Auto’.
Use the crop tool to get rid of any transparent pixels at the edge of the image making sure that all layers are still selected.
Name all the layers according to the exposure values. For example: 0 EV, -2 EV, 2 EV. Or, Normal, Bright & Dark.
Put the balanced exposure at the bottom of layer stack.
Make sure you only have the balanced exposure visible. This will be important if you use “Merged” as a source.
Add a layer mask to the darkest exposure, and make sure it is selected.
Then go to Image > Apply Image and use the following settings.
Add a layer mask to the bright 2 EV layer and leave it selected and run Image > Apply Image again but this time tick Invert.
The result should look like this.
You can if necessary, run apply image again on either mask to strengthen the mask.
How does this work
What’s happening here is that the overall luminosity of the balanced layer is being used to make a mask for the dark layer.
You can see the mask is brighter in the sky area this will allow more of the sky from the darker layer to show.
When you make the mask for bright 2 EV layer it is the inverted version of -2 EV darker image layer masks. In this example this will allow the brighter foreground to show through.
Is luminosity masking better than HDR, yes sometimes?
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