I just read Larry Berman's discussion on his website about image size and quality. Here's what I don't understand--When I'm in photoshop I set the size of the image, say 1920 x 1920 or whatever. But when I go to save it as a jpeg the program asks me what quality image I want, and that has a big impact on the size of the image file. What gives? Why doesn't the number of pixels determine the size/quality of the image?
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For your jury images, always save your jpeg at the highest quality possible that still keeps the file size under the maximum allowed (for ZAPP it's 2MG). That will assure you of the best quality image when projected for a jury.
This gets into the math of the jpeg format. jpeg is a compression formula. When you reduce the quality, it basically throws away a lot of information that it doesn't think you need anymore. The lower the quality, the more information it throws away. That information may not seem important at the time, and the on screen image may look same at first, but they aren't.
One way to explain this is to consider a stripe that varies from light red to dark red and is 1000 pixels long. The original image may have 1000 different pixels colors. When it's reduced in jpeg, every other pixel may be a different color instead of everyone. It still looks the same, until you blow it way up on the screen.
That's a crude description of the way jpeg works, but hope it helps.