Photoshop convert to grayscale. How does it work?

Hi Folks,

 

I'm giving a talk to my Camera Club in a few days time, about techniques for converting from color to mono. I

always use the channel mixer myself, but I want to cover as many techniques as I can.

 

I'd always assumed that converting to grayscale (image>mode>grayscale) would be exactly the same as desaturate,

but I was surprised to find that it isn't. I spent quite a while using the channel mixer to try and replicate the

results of converting to grayscale, without success.

 

In case you thought, like me, that grayscale and desaturate were the same, the attached example shows how they're

different. In the color image, the background is 128,128,128 (RGB). The colored words have their color settings

alongside them. However, if the color channels for each word are averaged, you'll end up with 128. When the image

is desaturated (lower right), the words just disappear as they end up the same color as the background. The lower

left image is the result of converting to grayscale.

 

Does anyone know what recipe Photoshop uses for converting to grayscale?

 

Thanks for your help

Steve

 

PS. Don't take this the wrong way - I'm grateful for your help - but I don't want to start a discussion on the

relative merits of the different techniques, there's enough coverage of that elsewhere.<div></div>

Hi Steve, this is the best discussion on this argument I've ever found: <a href="http://www.zuberphotographics.com/content/blackwhite/color2bw-intro.htm">link</a>

Regards, Alberto.

hmmm, funny i never see this kind of test, its cool : )

 

On the other hand, on a real image the difference are not the same, the result look pretty close. As for the number the grayscale use, i think i remember somewhere way back a recipe in channel mixer that was a base for it, 24-68-8, a recipe that give good bw to start, but need some tweak to be really good..level, curve etc...

heres a channel mixer vs a grayscale<div></div>

now that another way i use channel mixer instead of grayscale.

 

+ the ability to keep your color info, the use of all the filter.....

 

dont really answer your questions, but give you more material to think about it : )

Don't forgetthat in Photoshop CS3 and possibly other programs, there are built in contrast filters (red, blue, green, orange, yellow) and you can vary the intensity of the effect within each filter. As a safety mechanism it is best to make a duplicate of your master iamge first and then try the conversions as layers on the duplicate.

> Alberto: Many thanks for the link, it's going to take a while to digest all that info. I'll probably pass that

link to my audience if they want to research the subject any further.

 

> Patrick: Thanks. As I said, I use channel mixer myself, and I've got that technique plus several others covered

already.

 

> Ellis:I won't have access to PS for several hours, and right now I can't quite visualise the filters you're

referring to. If they're a CS3-onwards feature, I won't be covering them as the Club's laptop only has CS2.

However, I've got CS3, so I'd be interested on a personal level.

if you go layer adjustment there is an adjustment called black and white, awesome little filter, calculations can also be used for some fantastic black and white images.

>>> Photoshop convert to grayscale. How does it work?

 

I believe photoshops's simple convert to grayscale (desaturate) method is simply value-based. Probably calculated as the root-mean-

square (RMS) of the

individual R-G-B component values to get the resulting gray value result.

 

ie, Gray value= SQRT( R^2 + G^2 + B^2)

 

Which is why it's a lousy B&W conversion method. Equal weight is given to the individual components.

 

Lightroom's B&W conversion with input color filtration is tons better...

Hi Steve,

 

You might take a look <A HREF="http://www.northlight-images.co.uk/bwfromcol.html">here</A>

for different techniques of bw conversion.

Another approach relies entirely on Lightroom, which, in addition to contrast and "black" control (seems like QTR's ink loading control) offers total continuous color filtration control (as if you were using an infinite variety of color filters over B&W film). Lightroom is easy and rational, much better documented than Photoshop.