change cmy to k

[Urraco37]

Hello

I have a document (cmyk mode) with a bunch of layers

I want to convert a single layer (not  text) from all  cmyk chanels to black channel only?

How to do that?

thanks

[Myra Ferguson]

You can make a single layer grayscale by adding an adjustment layer and clipping it with the layer that you to be affected while leaving the other layers as they were. Here's how:

1. Change the color mode to RGB temporarily by going to Image > Mode > RGB (otherwise, the Blank and While adjustment layer will be inaccessible)
2. If you have smart objects, a dialog will ask if you want to rasterize them. Most likely not, so click Don't Rasterize
3. Select the layer you want to make grayscale (selecting it will make Photoshop place the adjustment layer above this layer)
4. Go to Layer > New Adjustment Layer > Black and White (a dialog will open to name it. You can click OK)
5. With the adjustment layer still selected, go to Layer > Create Clipping Mask
6. Select the adjustment layer and the affected layer and go to Layer > Smart Objects > Convert to Smart Object (if you leave the adjustment layer, going back to CMYK will remove it)
7. Go to Image > Mode > CMYK to go back to CMYK

[Urraco37]

thanks for help

the documents are native cmyk, so I wouldn't want to do any conversions..

[Stephen Marsh · Accepted solution]

Use channel mixer directly to the layer, or even better as a clipped adjustment layer above the layer, without changing modes from CMYK in the original. 

Otherwise I'd work on a grayscale mode copy and paste the results back into the K channel of the original layer, removing the originaldata from the CMY channels of the target layer (again, without changing modes from CMYK in the original).

[Urraco37]

thanks for help

channel mixer work well