Strength is peculiar. When set to zero your image becomes all white. Any fraction is blending the image with white. Any value over 1 is exaggerating the difference from white. So - basically everything that is below max value goes lower.

If you're starting with all values below 1, increasing strength makes them all go lower, with some of them going negative. This is not your desire.

To keep 0 at 0 and increase all values, you multiply the map with a number greater than 1. Hard to do when you are only allowed to enter colors up to 255, right?

Your image is not color so use math nodes. Connect a math node to your mask image. In that math node, enter any number you want, such as 2.

Then 0 stays 0 because 0 times 2 is still 0. Where the mask is .5, you'll actually aget .5 * 2 which is 1.

In fact, if you know the min and max value of your input mask, a single math node can normalize it to the range 0 to 1. Here is how.

Suppose your mask is faulty and its luminance values go from A to B, and these are not 0 to 1, but some other numbers.

Connect this to a Math:Subtract node in Value_1.

Value_1 should be 1 / (B - A) and Value_2 should be A.

This node will then change the mask range from [A, B]  to [0, 1].

For example, suppose the mask luminance goes from .2 to .6.

You use Value_1 = 1 / (.6 - .2) = 1 / .4 = 2.5. And Value_2 would be .2.

 

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