Luminosity Masks – James Allan

Sisters of Kintail

A month ago the club had a guest speaker, Dylan Toh from Everlookphotography.  His stunning landscape images were inspired and beautiful.  He described his technique as follows.

“I set up my camera on a tripod. In live view, I select 2 or 3 different exposures for the light, dark and midtone objects in the image.  I then combine the resultant images in the software (photoshop) using luminosity masks.”

So here is a question for you.  What is a luminosity mask?  I will warn you that the term luminosity is used quite widely in image management.  Don’t get confused, it has nothing to do with luminosity noise reduction.

Well to start with lets try and understand the term luminosity.  Luminosity comes from astronomy where it relates to the brightness of a star (or other stellar object).  In photography it is one of the three variables, “Hue” “saturation” and “luminosity” (HSL) that can describe any colour or tone.  Refer to our article on colour theory.  When you alter the luminosity setting you adjust the brightness without altering either the colour (hue) or saturation of an image.  This is a very useful feature of image manipulation software as you are no doubt aware.  When an image is reduced to just the luminosity values, it becomes a monochrome or black and white image.

So how did the term luminosity come to be used in noise reduction filters?  It is simple.    Digital image noise comes in two varieties, colour noise and black and white noise (called luminosity noise).

So what is a mask?  I tend to think of masking tape.  When I apply white paint over an image, the paint obscures everything, except for where the masking tape protects the underlying image.  Masking tape is what I call a hard mask, giving good protection and having a sharp or crisp edge.   Alternatively if I used a piece of tule, I might get paint going through the tule to only partially obscure the underlying image.  This I would call a soft mask.

In photoshop you can layer two images, one on top of the other and use a mask to protect certain areas of the underlying image from being obscured.  These masks can be hard or soft.  They can have sharp or soft edges.  You can create masks by using the selection tools.  Refer to our article on selection tools.  It can be quite laborious and time consuming, and at the end of the day the results may be disappointing if you are left with watermarks and tell tale signs of your attempts at manipulation.

Luminosity masks are derived from the luminosity values of the image itself.  They have a combination of soft and hard elements to resemble a black and white version of the image.  They have the advantage that they are quick to generate and quite seamless without any tell tale watermarks.  There is a bit of a learning curve, but once mastered they offer a superior way to mask your image.

Let me demonstrate: Two different exposures of a beach scene.  One is exposed for the earthen cliff and the other for the sea in the distance.


If I layer the second image on top of the first and apply a simple gradient mask, this is what I can achieve:


However with a luminosity mask I can achieve this result:


The luminosity mask gives a softer, but also more realistic merging of the two layers.  What do you think?

Luminosity comparison

Detail from Graduated filter left, luminosity mask right

How do you create a luminosity mask?  There are several methods.    I have found it simplest to download an “action” that will do it all for you.  The masks are loaded on as channels and can be converted to selections by Crtrl clicking on the appropriate mask in the channel dialog.  I will refer you to a tutorial on Evantotuts.  This will show you how to create the mask and also contains a link to download an action set.

Another tool that creates a luminosity mask is the “select colour” dialog in the selection menu.  This is very different in it’s properties to both the “magic wand” or “intelligent selection” tools which both create a hard mask.  Experiment with it for yourself and see the difference in the mask that is created.

What else can you do with luminosity masks?  The tutorial at Evantotus also describes sharpening and adding colour cast to specific tonal ranges.  It is worth experimenting.  The results are quite evocative.

I can’t guarantee that your photos will turn out like Dylan or Marrianne Toh.  However it is worth learning about luminosity masks.  At least you can use the same tools that he uses when processing your landscape images.