1. SIZING THE IMAGE (DOCUMENT SIZE)
When we talk about sizing an image for printing, we have to think in units like inches or centimeters, actual physical dimensions. You want, for example, and image that is 4x6 inches on 5x7 inch paper. Image resolution is also important. In order for an image to be printed in photographic quality, 300 ppi (pixel per inch) resolution is the general rule.
Open the original JPG image from your camera. In the example below this image consists of 4032 x 3024 pixels.
In the greyed-out Document Size field you can try out at what dimensions the printed image will have at what resolution and vice versa. This picture has enough resolution to be printed 13 inches wide at 300 ppi. (4032 : 300ppi = 13).
2. SHARPENING THE IMAGE
Let's say you want to scale the image to 6 inches wide, so that you can have it printed full-frame on 5x7 inch paper with a border. Make sure that Constrain Proportions is checked. Next you fill in the desired image resolution. Use 300 ppi. Then you fill in either the desired width or height of the new dimension. The second dimension will be scaled automatically. In out example, your image is 6x4.5 inches with a resolution of 300 ppi.
In the greyed out field you can see the actual pixels dimensions of the new and smaller image. The new file requires much less memory.
Next, you want to sharpen the image. This process is critical, because unsharpened images may be too soft, while over-sharpened images cause ugly sharpening artifacts. The proper tool for sharpening is the UNSHARP MASK (USM), or the more modern version that Adobe calls SMART SHARPENING. For the sake of simplicity, I will say the USM mask raises micro contrast along edges, thus increasing the perceived level of sharpness. The USM can be found in the menu under FILTERS (in Photoshop, possibly under EDIT etc) and has three options: Amount, radius, and threshold. As a beginner, use only amount and radius. For printing is a good starting point for the radius is 1 or 1.5 pixels. The amount required varies, and is best tested by trial and error. Avoid over-sharpening as it tends to be more unpleasant than a slightly soft image. Generally, images that are to be printed require considerably more sharpening than do images for monitor display. This is especially the case if the image is going to be printed with an inkjet printer.
3. COLOR SPACE
Make sure the file is in the sRGB color space, because most labs will only print properly in this color space. If your image is not in the sRGB color space, convert the file to it.
4. SAVING THE IMAGE FOR PRINTING
Save this image as a TIFF file or SAVE AS JPG at the highest quality setting. Do not use SAVE FOR WEB. Some photo finishers will allow you to upload a TIFF file, but all printing places accept JPG files. Upload your TIFF or JPG files to your photo finisher.
Be advised that aspect ratio of paper and image rarely match. You will either have to crop part of your image, or you have to print with borders on at least two sides of your image. Steps beyond what I have lined out in this basic tutorial are required if you have to deal with aspect ratios that do not match between image and paper.
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PREPARING IMAGES FOR FOR THE WEB