Today's DUM question on PS CS6 - What is the difference between a layer and a mask?


TPF Noob!
Apr 21, 2011
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I've been working with PS CS6 for about 6 months but have had very little actual "hands on" time to work on photographs. I have watched a lot of videos on Youtube but I am not sure that I understand the difference between a layer and a mask, and when to use each one.

My understanding is that a layer works like a piece of clear plexiglass laid on top of the image. Whatever edits you make to the layer does not affect the underlying image because the layer protects the photograph in the same way that a piece of clear plexiglass laid over an image would protect the image and not let any changes made on the plexiglass "soak through" on to the underlying image.

On masks, I have seen videos where a mask is added to an image, and then certain effects are painted onto the mask (e.g., exposure is increased just on a person's face and not on the background) and only the area where the edit is made is affected. Wouldn't you accomplish the same thing by adding a layer (instead of a mask) over the image and then painting with the exposure brush to increase exposure on the desired part of the photo?

What am I missing?

Can somebody please simplify this for me?

By the way, I know how to spell "dumb." It was a joke. :mrgreen:

I am pretty sure that a layer pretty much what you said. It's like a piece of plexiglass that applies an effect. Pink plexi makes the photo pinkish, to push the analogy a bit far.

The mask is a black&white image that determined where the effect is applied in the 2D space of the picture. I always get black and white mixed up, but one of them means "apply the effect full on, here" and the other means "don't apply it at all here" and greys means "apply it but only in proportion to the grey level here"

So you might wind up with, I dunno, a blur effect in the layer. This will blur the whole picture. Then add a mask that's all whatever the "don't apply the effect" color is. Now your layer+mask does exactly nothing. Paint the other color ONTO THE MASK to allow the blur effect to apply here, here, here, but not there. Airbrush to apply the blur lightly here, more over there, etc.

ETA: I use GIMP which has some but not all of the same properties, so I am extrapolating a bit. Wiser heads may chip in!

Look at it like this. Humor me as I try to boil it down to it's very basics.

You have the exact same physical film photo. You're holding both of them in your hands. You develop the same photo two different ways. You put one on top of the other. Those are your layers. You have two photos stacked on top of eachother at this point.

So now you have two layers. One photo stacked and aligned exactly on top of the other photo.

Now let's say you want to use the sky from the top photo, but keep the foreground of the bottom photo. This is where you will use a mask. In our physical, real-world example, you would take out your scissors, and physically cut out the foreground from the top photo, so the bottom photos foreground shows through. When you look at the picture now, it will be a combination of the sky from the top photo and foreground from the bottom photo. There are definitely more sophisticated ways to blend exposures in the film darkroom, but you get the general idea.

So with layering and masking, you're doing the same thing in the digital darkroom, except with much more pixel precision. The same concept applies to any adjustment layer you make. You can apply an adjustment layer, such as saturation, and using the layer mask on that layer, you can selectively apply it across your photo.
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This was a dum question and you should feel bad for asking it.
Painting on a layer mask creates a grayscale image that controls how the layer is seen. In the case of an adjustment layer, which is the most common place to use a layer mask, white areas get the full adjustment (increased brightness by a certain amount, color balance adjustment, etc.), black areas are unaffected and gray areas get partial adjustment proportional to their lightness.

One could probably achieve some effects, like brightness adjustments, by painting on the layer itself, but there is really no advantage and one gets used to doing this with layer masks, which will control adjustments you really can't make by painting on a layer itself. The other factor is that the grayscale mask doesn't take much memory and you can add several adjustment layers with layer masks and probably increase file size less than by adding one layer with actual pixel content on it.

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