original RGB file ~ (embiggenable) • iPhone
grayscale file after RGB > LAB conversion ~ (embiggenable) • iPhone
TO USE A LANGE-ISM, RELATIVE TO THE, dare I say, "nonsense" being spewed out on T.O.P.* regarding the conversion of RGB color files to BW, aka Grayscale...
"He doesn't know whether to cry or wind his watch."
Mike Lange, a NHL Hall of Fame broadcaster, used that phrase when a goaltender had been so badly beaten by a goal scorer than he did not know whether to be discouraged or indifferent to the situation. Which is exactly how I feel about the bald-faced ignorance*, regarding converting color to BW in the digital domain.
I am willing to admit that I may being way to judgemental using the word "ignorance" but ....
.....Photoshop has a reasonably niffty tool-IMAGE > ADJUSTMENTS > BLACK & WHITE-for color > BW conversion. It does not require a degree in Rocket Science to use - it's a set of sliders, the use of which is immediately visible on screen. Pretty much the textbook definition of WYSWIG. I sometimes use it when I want a BW picture. And, FYI, after using it, I often do a bit of localized fine tuning with CURVES. HOWEVER .....
.....if what one wishes is 100% tonal value accurate conversion from color to BW, there is only one method I know of and it is as simple as it gets. And, it is hiding in plain sight in Photoshop under the name LAB Color - IMAGE > MODE > Lab Color.
I will not even begin to explain exactly what LAB color space is other than to write that it is very different than RGB color space. Consequently, very few digital domain picture makers have taken the steps to understand how to use it, aka: the "ignorance" I mentioned. Nevertheless, for use as a BW conversion tool, there ain't a lot to understand.
The key thing here is to know that, when a file is converted to LAB color, you end up 3 channels - L (Lightness channel), A (magenta / green channel) and B (blue / yellow channel). It is the L channel where the "magic" happens inasmuch as the Lightness channel contains only the tonal values of the image absolutely and completely independent of color values. Drag the A and B channels to the trash and you are left with an Alpha 1 channel which can then be converted to Grayscale and voila, there you have a "perfect" BW conversion from a color image file.
CAVEAT When I write "perfect", it may not be perfect for how you might want the picture to look. However, while still in LAB color space and before converting to Grayscale, you can use the CURVE tool to make adjustments to the file. There are other methods to use to get the look you want but this ain't no online workshop.
Addendum I use LAB Color space for most of my global color processing / corrections. It's the best way to do so inasmuch as, like the L channel, the A and B color channel contain only color information absolutely and completely independent of tonal information/ values. Also, as I have mentioned previously, I do all of my sharpening in LAB because when a high amount of sharpening is called for, you can apply much higher values of sharpening without getting the halo-artifact effect you would get when the same amount is applied in RGB color space.
* re: nonsense / bald-faced ignorance - hyperbole in action. I realize that, as is usually the case on T.O.P., it's just people trying to help out other people along the picture making road. Nevertheless, I do stand somewhat, but obviously not completely, dumbstruck with the fact that so few picture makers understand LAB color space as the very powerful processing tool that it is.