I want to convert a (colorful) picture to a black/white image. I know i can do that with the following options: * convert to grayscale * Desaturate * Channel mixer But what is the best technique ? Are there others ? Should i use RGB or CMYK ?

I use greyscale conversion or desaturate, depending on what I want to do with the image later (e.g. desaturate if you're planning to colourise parts of it). Then it usually needs some adjustment of the contrast and brightness to give it a more convincing monochrome feel. RGB or CMYK depends on how you're planning to use it: RGB for web, CMYK for print.

I found a tutorial which uses calculation by duplicating de red and green with 50%. Is this a good conversion ?

I suspect that Channel Mixer is the best for absolute control. Image->Mode->Grayscale uses an algorithm to perform the conversion based on channel brightness (I think). This normally results in the green and red channels having more influence than blue. An alternative is to simply choose one of the RGB channels, but usualy one alone isn't good enough, hence Channel Mixer.

the best way to convert an image to grayscale, is to first conert it to Lab colour mode. then discard all but the Lightness channel. then convert mode to grayscale. compare this with conversions using any other method and differences will be obvious. to further convert to pure B/W use the Bitmap mode (use the dither option).

humm... hi there! =) If i can say my word... well... first.. never work in cmyk... CMYK is not a good profile to work with,... better RGB or LAB. There is not a unique technique to obtain the perfect result. Depends. Sometimes working with the lightness profile under LAB leads to good results, sometimes this is completely unuseful since it can lead to imperfect images. The Lightness under LAB gives only the luminance of the single colors of the image and sometimes does not contains the differences and the characteristics of the colours themselves. This kind of information could be kept if you try to convert a colour image to B/W. For me the best way to proceed could be to observe the various results from Lightness (in LAB) and from Greyscale conversion+some channel mixing and meld 'em together.

I use CMYK for resizing images. So i thought i would also be a good idea to use it for black/white conversions. I will try LAB in combination with different techniques.

Desaturate just erases the color data from the pixels(Period.) You end up with as fine a quality image as you started with. The adobe manual says the first thing you should do is Image>Adjust>Levels. When you open this up you'll notice a curvy black area. If you set the top sliders to the beginning and the end of the darkened area, you'll have a finer quality image(Provided it's not there already.) If you want you can open the Image>Adjust>Variations dialog. Then you're going to want to apply the Unsharp Mask. Another way to achieve a nice grayscale is with actions. There ae actions out there for light greyscale-heavy grayscale. Some of which use the Duotone, Tritone, etc. methods.

Wow! Everyone has interesting techniques here, and they all seem valid. My question is what is the final output going to be to? That (along with the nature of the image itself) should be the determining factor in what is going to be best for a specific image. BTW... Cainbrogan has what I consider the most reliable method with making sure the levels are adjusted. Generally when I have a color image that is going to press in B&W, I do a level adjustment on the color image, then convert to grayscale, and tweak the levels again as the image values change.

Thanks Alpha! Another note...It's not that CMYK is a bad mode. it's that RGB is more intricate/extensive. CMYK is what Kodak, your local film processing, and professional photo labs use for color photographic negatives and prints! When I took Color Photo at school, we had to make sure...let's see if I get this term right...the 4 Toners:Cyan, Magenta, Yellow, and Black were full in the developing machine before we printed...oh, and after we'd exposed the color(CMYK) film to color(CMYK)photo paper(In the darkroom) using the CMYK gels and dials under the exposer light! So, it makes sense for Photoshop to have a CMYK color mode. It's the most accurate way proof your images, when you know you'll be going from digital to film. What Artak is trying to say is that all of the CMYK colors are included in the RGB spectrum, though the case is not so in the reverse! Thre are thousands if not millions of more colors in the RGB specrum. But it's use is limited to digital digital work, with regards to proofing(When you show a professional printer what you expect, before a job goes through a printing press. = )

What I do: 1) In RGB space, adjust curves/levels/brightness/contrast to get a good looking color image. 2) Use the channel mixer. [Actually, what I usually do is just look at the separarate Red, Green, and Blue channels, and pick one. This simulates using the B&W filters on the camera to alter image tones.] For precise work, or something in between, use the Channel Mixer. 3) [Image should now be grayscale.] Maybe adjust brightness/contrast again. Convert to duotone, tritone, etc., and try loading different presets until tones look pleasing. 4) Convert to RPG before saving as jpeg.