How to squeeze the utmost quality out of your film/scanner combination.
By Tom Ang
Variation of grain with color channel
Suppose we scan our black-and-white negative in RGB mode. We obtain what appears to be a grayscale image. Now, inspect each of the red, blue, and green channels. We observe that there are slight differences in the appearance of the grain. This is due in part to a phenomenon called Rayleigh scattering. Some of the light incoming to a film is re-emitted by the silver particles-this is scattering-but the amount of scattering varies with the wavelength of light. It increases as wavelength decreases, i.e., as the light becomes more blue. This is what gives us blue skies (scattering from small air molecules) and gives sunsets their glow. Long wavelengths are scattered less than short ones by dust particles so the reds and yellows penetrate the atmosphere, whereas greens and blues cannot.
Now, the differences visible in the channels in our black-and-white scans are consistent with this theory. In general, you will find that grain in the red channel is less than in the blue and green channels.
But it means you can take your pick. If you wish for the finest grain, select the red channel and simply delete the others. In theory, if you need even finer grain, you could try scanning through a red filter (but the quality fanatic should be aware that CCDs are usually filtered against infrared). To obtain coarser grain, of course, the blue or green channels will deliver.
In Photoshop, you can invoke the Split Channels command in the Channels palette to create three separate files, each one containing a separation. Just save the one you want to keep and discard the others. But of course ensure you do this to a duplicate file.
Tom Ang has been writing about photography for more than 20 years, and his own photography-both digital and film-has been internationally exhibited. This information has been adapted from his recent title, Advanced Digital Photography, and is courtesy of AmPhoto Books, an imprint of Watson-Guptill Publications (www.watsonguptill.com).
Rotating Crooked Scans
You can straighten up a poorly aligned scan in some scanner drivers and software by using the Crop tool and rotating it so it lines up, accepting the crop. Another more accurate method, available in Photoshop, is to use the Measure tool: Call this up and drag along one edge of the image. Then, rotate the image under Image > Rotate > Arbitrary. You will find a figure for rotation already entered in the box. Ensure the rotation is in the right direction (clockwise or counter-clockwise) and hit OK. Your rectified image is delivered. Note that the image canvas increases to encompass the whole image; you will then need to crop off the excess.