Conny Wallström posted a video on YouTube about “Brightness vs. Luminosity inside of Photoshop”. As always, a somewhat more technical topic, but fascinating:
It is about the possibilities of transforming an image into its black-and-white representation.
Primarily when you use this as a help layer while retouching, you have to be extremely careful — otherwise, you get massive color problems very quickly.
Conny shows the different interpretation possibilities in Photoshop:
The initial picture:
Here we have six color areas of different colors. If we look at red and green in Photoshop, we have the following values:
In the RGB color model:
RED: R=255, G=0, B=0
GREEN: R=0, G=0, B=0
RGB = Red, Green, Blue.
In the HSB model, this is what it looks like
RED: H=0°, S=100%, B=100%
GREEN: H=120°, S=100%, B=100%
HSB = Hue, Saturation, Brightness.
If the saturation is set to 0% in HSB mode, pure white is the result.
If you make the example shown above black and white via Photoshop’s “Reduce Saturation”, this picture results:
This indicates that this function is based on the HSL (Hue, Saturation, Lightness) color model. In the HSB model, all surfaces would be white; here they are neutral grey.
The “hue/saturation” adjustment layer also uses the HSB model.
Image Mode “Grayscale”
When our image is desaturated via the menu item “Image -> Mode -> Grayscale”, a different image is created: It is not based on the HSL or the HSB model. What we see here is perceived or subjective brightness (luminosity).
Blending mode color
One way to look at an image in the luminosity equivalent of the colors is to have a black or white color layer above the image in the “color” blending mode.
Why should the difference between luminosity and brightness matter?
Especially in Dodge&Burn work, it is helpful to hide the colors via a help layer temporarily. However, it is important to choose a help layer that interprets the subjective brightness. This is not the case in HSB mode.
If you work on an image in HSB grayscales, you can change hue (doesn’t matter how much), you won’t see any difference in the image — at least not as long as you have the help layer active. This can destroy a few hours of work.
Conny has provided a lovely example:
Yellow is always perceived brighter than blue.
The same image desaturated in the HSB model, both colors appear equally bright.
In the HSL model desaturated, the subjective brightness is correct again.
Another example of his own:
A gradient across the color gamut.
This image is also only neutral grey in the HSB model.
With the black layer in blending mode “color”, the perceived brightness is maintained.
Here the HUE value was increased by 180° with the hue slider.
HSB conversion: everything turns grey again.
Here again with the layer in “color” blending mode.
Especially in portraits, where we are careful not to get color shifts and/or use a lot of time to correct them, the knowledge of these help layers is very important. Photoshop works in different places with these three modes – you should always be aware of which one is used where.
This article is a partial translation from the video “Brightness vs. Luminosity inside of Photoshop” by Conny Wallström.
Some things have been translated very freely and expanded with their own examples.
I strongly recommend you to watch the video above one or the other time – there you can learn a lot.
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