So instead of keeping it to e-mail, I thought we'd try to share some of these questions and responses to benefit others as well.
Every few days, I get a request to describe what I mean by "masking". I'll say this particularly when a picture needs selective sharpening.
The term "masking" comes from "masking tape", that paper-type adhesive tape which you can use to cover (or mask) things that you don't want to get paint on, like the door handle when you are painting the door. Masking in a photo-editing program works the same way, allowing you to select parts of a picture to which you will do some processing, like sharpening, and conversely which parts will be "masked", or protected from any changes.
This comes very useful is certain situations. For example:
Photo © Colin K. Work
On this plane, there are thin stripes close together along the fuselage. If the picture was blurry, and sharpening is applied to the entire plane, stripes such as these quickly start to look like hell, and ends up in a guaranteed rejection. Similar things happen when you see thin daylight between the flaps, or simply on the edges of an airplane, like the wing's leading edge.
Masking allows you to cover up these lines and apply sharpening to the entire picture except the lines. I consider masking to be a key tool needed to make your pictures look the best they can.
The same logic can be used in order to adjust the plane without touching the sky, or vice versa.
I am not familiar with many different softwares, and maybe other people can jump in to describe others. I use Corel Photopaint 8.0, which is roughly the equivalent to Photoshop.
Assuming that similar tools exist in other programs, you will have several masking tools.
Firstly, you will have a positive or negative mask. A positive mask means that the area you select WILL be affected by subsequent processing. A negative mask means that the selected area will be protected from subsequent processing.
Secondly, you have the different masking tools. The most common and easily understandable tool is the rectangular masking tool, where you click on one place in the picture, drag the mouse somewhere else, release the button and end up with a rectangular area surrounded by a wavy border. In Photopaint, by default, this will be a positive mask, meaning that whatever you do after this (sharpening, lightening, whatever) will only be applied to what is inside the rectangle. If you had pushed the selector to make it a negative mask before defining the shape, processing would only be performed on the area outside the rectangle.
There are other masking tools other than the rectangle tool. The 2 I find most useful are the paintbrush and the "Magic Wand". The Paintbrush is easy - you select the tool, select the size/shape of the brush, and run the brush over the area you want to apply the mask to. This is what I would do to the picture above to mask the thin lines so that I could apply sharpening to the rest of the picture.
The Wand is also interesting. Let's say you have a picture like this:
Photo © Frank Schaefer
If the plane was blurry, and the sky looks like it will get grainy if you apply sharpening to it, you can use the Wand to make a negative mask over the sky, leaving the plane available for sharpening and leaving the sky alone. In Photopaint, I would select the negative mask selector, select the Wand, and click somewhere in the sky. The program will select all the areas of the picture which are of the same color as the point you clicked on (you can define how tolerant the Wand is of slight variations in color). After maybe 2 or 3 clicks around the sky, the entire sky should be masked, and you can go one with sharpening the plane without causing the sky to go grainy.
Lastly, an interesting tool is called "Mask Feathering" (at least that's the name in Photopaint). This allows you to create a fuzzy border around the mask, so that the processing you do in one area does not cause an overly abrupt transition border between the masked and unmasked areas.
This is meant to be just a brief primer to masking. I highly recommend those who are not familiar with these tools to read their program manuals and learn how to use them (especially if you have been getting a lot of rejections).
If there are any other recurring issues like this, which the screeners see, we can do another post. As far as masking is concerned, and questions on this can be posted below.