Greyscale conversion versus desaturate for color to B&W

JN
Posted By
Joel_Nisson
Oct 7, 2005
Views
327
Replies
5
Status
Closed
I need to convert a large collection of RGB images to greyscale and don’t have time to do anything fancy (like channel mixing). Can anyone tell me the relative advantages/disadvantages of using Image>convert to greyscale versus desaturate?
Thanks.
–Joel Nisson
CN
Cybernetic Nomad
Oct 7, 2005
Converting to greyscale results in a file with a single channel

Desaturating results in an RGB file with equal values of R, G, and B

Both can be automated with actions and batch processing
RK
Rob_Keijzer
Oct 7, 2005
And there is a significant difference:

Desaturations <http://home.planet.nl/~keijz198/TFAC/Grey_diff.jpg>

Desaturation strips the colour info, while Convert to Geyscale does a calculation aiming at maintaining lightness.

Rob
AC
Art Campbell
Oct 7, 2005
Also, channel mixing isn’t going to take you any more time, assuming you’re going to build an action to do the actual grunt work. And depending on what your subject is, it may be better…

Most women’s skin, for instance, would look better if you grab the red channel; men’s usually looks better on the green channel. And the same type of thing would happen with landscapes or whatever, if there’s a common thread running through your originals.

Art
C
chrisjbirchall
Oct 7, 2005
Rob. Thanks for the Greyscale/Desat chart. Its going to make me rethink the way I convert to B&W.

I generally use the two Hue/Sat layers method for complete control over tonality when doing general work, but for my portraiture I’ve been tending to simply Desaturate. Now I realise that I must be missing many of the subtlties I would otherwize get using greyscale.

Chris.
RK
Rob_Keijzer
Oct 7, 2005
Chris,

The fact that in the middle graph all colours except black and white produced the same tone does not mean that this produces contone greyscales that also miss a lot of tones.

The bars in the excample are pure primary and secondary colours (WYCGMRBK) that are far from the real world.

Still, as you say it’s better to not use Desat/Convert to greyscale to make fine B/W images. I too use the double Hue/Sat layer technique for this. It has potential I could only dream of in the old days when we had a bag full of filters and had to push-process film even twice sometimes.

Dont get me started. Oh, I did that myself! 🙂

Rob

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