In a nutshell, the purpose of image optimization is to find the best compromise between file size and image quality, depending on the image’s intended use.
This actually starts in your camera, the moment you pick the format, size and quality level. You will or should continue to worry about it up until the time comes where you put out the final product.
For the purpose of this course, I want everyone to keep three attributes in mind:
To be specific we will concern ourselves with pixel resolution (or pixel dimension). Photos are two-dimensional images, whose height (rows) and width (columns) are measured in pixels. While there are no carved-in-stone rules regarding actual numbers, we have gravitated towards certain de facto standards based on the frame sizes we talked about earlier. For 4:3 images, I usually resize to 640 x 480 (standard VGA dimensions), 1200 x 900 or 1600 x 1200. For 3:2, I’d go by 600 x 400, 900 x 600, 1200 x 800, 1800 x 1200 and so on. Sometimes, I would also follow TV/video-based standards like DVD (720 x 480), HD (1280 x 720) or full HD (1920 x 1080). It will really depend on your purpose.
This is essentially the amount of detail in a given area. We already know that a digital image’s absolute size is measured in pixels. But when talking within a specific context, pixel density comes into play.
Pixel density is commonly measured in pixels per inch (ppi). While not accurate, it is also referred to as dots per inch (dpi). There are admittedly a number of things to understand regarding pixel density, but for the sake of simplicity, let us only concern ourselves with prevailing standards. Images intended for print requires a high density at 300 ppi for best quality. However, for more mundance purposes, we can usually get away with 150 or 200 ppi. If an image will only be used for web publishing or on-screen viewing, 72 ppi has become the standard (96 dpi is also used, sometimes).
While some of us, particular the DSLR users, would likely start with pictures in RAW format, it ends up being converted to a more open format, eventually. JPEG is still, by far, the most ubiquitous format there is. Unfortunately, it is a lossy compression format. That is why we need to be conscious of the JPEG image quality level parameter found in your image editing software. Because image editors employ different optimization algorithms, there is not across the board standard levels to guide us.
Most editing applications, at the very least, gives us a bit of qualification for all the optimization numbers it throws at us to make things a bit easier. But in its absence, if space and file size is not an issue, we have the luxury of maximizing quality level. But if it is, or if it is going to be published on the Web, we have to eyeball it.
A note on pictures intended for print:
If you look at the screenshot above, the image size window includes parameters for document size, these are the dimensions of the picture if it is to be printed at 100%. While it can be fairly large at more than 11×16 inches, the picture will not look at its best on print as it is optimized for the Web at 72 ppi/dpi. It is important to keep you pixel density in check as you tweak the document size according to your purpose.