If you request 4×6 prints from Costco, they will print your images at 4×6, cropping to fit if the photos do not have a 4×6 size ratio. The number of pixels you supply basically doesn’t matter–Costco will reduce or enlarge the image as required to fit in the 4"x6" print size (or whatever print size you request).
I’ve only scanned a few photos, but I’ve found that the resolution to use when scanning depends on the photo and especially on the type of paper. I usually have to experiment to find the scan resolution that produces the best result with fewest artifacts.
After scanning the photo, clean up the photo in any way needed — reduce noise, increase sharpness, color saturation, etc. etc. Crop the image to a 4"x6" ratio (set 4 in and 6 in as dimensions in the Photoshop crop tool; leave resolution field blank). If the color space is Adobe RGB or anything other than RGB, convert the color profile to RGB.
At this point, if your image size is less than 4"x6" at 300 dpi, or 1200 x 1800 pixels, you’re done and ready to save the files as JPG, level 12, and take them to Costco.
If your image size is larger that 1200 x 1800 pixels (again, for a 4"x6" print), downsize using Image Size to 4"x6" at 300 dpi. I usually use the default bicubic resampling, but might try bicubic smoother if the image is noisy.
Be aware that when you print at Costco that they will enlarge the image very slightly (1% to 2%) to ensure that the image reaches the edges of the paper. In other words, expect a 1% to 2% crop to occur in printing. If you don’t want that to happen, enlarge the Canvas Size by about 1.5% so that what they crop off is excess you added by enlarging the canvas. But you might end up with a white border on one or several sides. (You can enlarge the canvas in gray or black and the border might be less noticeable.)
If you have a calibrated monitor, and if you have adjusted the image in Photoshop to have the exact color, saturation, etc., that you want, tell Costco to make "no adjustments" when printing. Otherwise, they will automatically try to improve your image. You may also want to proof the colors by using the color profile for your local Costco printer (color profiles available at <
http://www.drycreekphoto.com/icc)>.
Guy
