Two Pass 'Lab' Sharpening Technique
I don't usually sharpen my images before submitting them for consideration as stock imagery. However there are times when a photo can benefit from a little sharpening. Here's a method, in Photoshop, to sharpen your images using a two step process that deals only in the luminance channel of your photo, and employs a two step process that first addresses contrast, and secondly edge detail. Also using this method results in a much lower color shift if preserving color balance is important to you.
The first step is to bring your image into photoshop and immediately duplicate the background layer by pressing Ctl + J on a PC or Cmd + J on a Mac. With the duplicate layer active, go to 'Image/Mode' on the top menu and select Lab color as your color mode. (It will ask you if you want to flatten the image, select 'do not flatten') Then go to the 'Channels' tab and highlight just the 'lightness' channel. Go back to the Layers tab and make sure that your 'Layers 1' layer is still active. Your image now will appear as a monochromatic image, don't worry its supposed to at this point.
Now go to 'Filters/Unsharp Mask' and apply a sharpening pass using settings that employ a high Radius setting and a relatively low Amount setting. For example a Radius of 145 and an Amount of 11. (Depending upon your image you may want to increase or reduce either of these settings till the contrast sharpening looks acceptable to you. Don't overdo it.)
If you wish, you may duplicate the Layers 1 layer (or not) and then, with the new layer copy active, run another Unsharp Mask filter pass. This time however, use just the opposite process for sharpening. In this case use a low Radius setting of about 1.2 to 1.5, and a higher Amount setting of maybe 80 to 100. After you've applied this second sharpening, go back to the Image,Mode menu and change the mode back to RGB. You will be once again prompted to flatten your image or not. If you wish to retain your image with the ability to adjust the two separate sharpening layers then select 'do not flatten.'
The resultant image will once again be displayed in color and, in most cases, render a sharper picture with less noise and color shift than if you employed a standard sharpening using RGB mode with all color channels active.
Be judicious no matter what sharpening you use as it can be overdone easily and any sharpening process if not reigned in will garner you a over filtered rejection.
Photo credits: Lightart.