Hi,
Can someone explain me when to use the different channels. In which situation are they most usefull?

Thanks for any advice,
Photophil
http://www.wildeyes.be

Cheers,
Photophil

GALLERY / WILDEYES / ARTERRA
Before You Attempt To Beat The Odds. Be Sure You Could Survive The Odds Beating You.

There are several different kinds of channels:

The colour channels are what you’re really working with when you’re editing an image in Photoshop. They break your image into its component colours according to the mode you’re in, so RGB has three (Red, Green, Blue), CMYK has four, greyscale has one and so forth.

You might want to edit just one of the colour channels to deal with a particular problem. For instance digital cameras often suffer from blue channel noise, so by doing any cleaning up on the blue channel only you’re losing as little detail as possible. Another example is sharpening: if you convert an image to Lab mode you can sharpen the luminance channel only which avoids the problem of the halos you normally get around the edges of objects.

The composite channel (RGB or whatever mode you’re in) isn’t really a channel – it’s just a quick way to select all the channels at the same time and return you to normal editing mode.

Alpha channels are saved selections: they define which sections of your image are transparent when you’re working with layers or saving GIF files - so you can edit the selection by editing the channel.

Spot channels are generally used for printing with Pantone inks: they allow you to specify colours other than the standard four (CMYK).

Hope that's some help,

Alan.

Hi Alan,
Thanks for the info. I guess I don't have to bother much about them for normal, every day amateur use. It looks more like tools for a pro.

Cheers,
Photophil

GALLERY / WILDEYES / ARTERRA
Before You Attempt To Beat The Odds. Be Sure You Could Survive The Odds Beating You.

I wouldn't say that. The other day I was restoring a grainy old photo for a friend that I scanned from a negative. I could not get the grainy appearance out even using the GEM feature on my CoolScan IV ED. Finally, I looked at the blue channel alone and that is where most of the problem was. I Gaussian Blurred that channel to a greater degree than I could have the whole image and this helped a great deal. (Is this a proper way to use channels? It worked anyway.) So now, I at least look at the individual channels after scanning.

George (N80)
http://Web.InfoAve.Net/~gebarron/photography/

George Barron

Hi George,

Great tip. Might be handy one day.

Thanks,
Photophil

http://www.wildeyes.be

Cheers,
Photophil

GALLERY / WILDEYES / ARTERRA
Before You Attempt To Beat The Odds. Be Sure You Could Survive The Odds Beating You.

LAST EDITED ON Nov-08-01 AT 09:47 PM (GMT)

That's a legit way to deal with blue channel noise. The blue channel has the lowest image fidelity and highest noise of the the RGB channels (a problem common to all imaging systems to varying degrees).

Another common way to use channels is to delete the blue channel when converting an image to monochrome. You can use only one of the other channels or a combination of red and green without the noisy blue channel.

BJ

BJ

Zenfolio gallery

Hello,

Here is my sharpening tip with usage of the channels (well not actually mine, adapted from www.designsbymark.com)

Here we go....
Load image to photoshop and set color space from rgb to Lab color. Now go to the channels tab and select luminance channel only (A and B are color channels). Apply unsharp mask to the luminance channel (e.g. 200%-0.2-0). In my opinion this gives much better results than applying unsharp mask to e.g. all RGB channels by reducing possible oversharpening & color burning. After sharpening select all the channels and change color space back to rgb. Same way this can be applied to CMYK color space by selecting only the channel with highest contrast. (don't have photoshop here so hopefully tip is right)

Yours,

Janne

P.S. here is link for DBM's phototip:

http://www.designsbymark.com/pstips/photo_enhancing.shtml

Thank you all for the very interesting tips. I feel I still have a lot to learn about Photoshop.

Thanks,
Photophil

http://www.wildeyes.be

Cheers,
Photophil

GALLERY / WILDEYES / ARTERRA
Before You Attempt To Beat The Odds. Be Sure You Could Survive The Odds Beating You.

LAST EDITED ON Nov-10-01 AT 06:49 PM (GMT)

Just do your regular sharpening on the RGB channels, then choose, Edit...Fade Unsharp Mask and choose the Luminosity channel in the pull-down menu. Does the same thing without having to convert your image to LAB and back to RGB again! Sharpening the luminosity channel only avoids potential color fringes in the sharpened image.

Cheers.

LAST EDITED ON Nov-13-01 AT 06:34 AM (GMT)

There are a couple of intricacies to this method. You have to do it immediately after doing the unsharp mask or the fade menu goes away. You don't have to worry about that limitation if you duplicate the image layer to be sharpened and do the sharpening on the duplicate layer set as a luminance layer. Here's a good quick description of both methods:

http://www.desertdolphin.com/sharp/Photoshop2.html

And one that argues the merits of using layers rather than making unrecoverable changes to your base image:

http://www.creativepro.com/story/feature/11242-2.html

BJ

BJ

Zenfolio gallery

LAST EDITED ON Nov-13-01 AT 01:22 PM (GMT)

Hello BJ and others,

Though the sharpening wasn't the original topic thanks for great links! It's great to find and learn new tricks. I recently discovered similar layer usage for quite an opposite thing, to create soft focus effect. Might be old and known trick but here we go... Create copy layer of original image and gaussian blur the copy layer buy 5 or 10. Adjust copy layer opacity to e.g. 50% or 30% and thats it. I guess fade can be used instead as well.

Yours,

Janne