Digital/Online

Note: for digital/online images, the measurement unit for resolution is PPI (pixels per inch) and the colour mode is RGB (red, green, blue), which is an additive colour mode (i.e. when no colours are present, the result is black, but as more colours are added, the result becomes brighter and more saturated until it eventually becomes white).

When optimising files for digital/online use, the primary consideration should be to reduce the file size to make it easier to upload, download and distribute over the internet. This can decrease website loading times, increase transmission speed, and allow files to be sent using communication methods that have a file size limit (e.g. emails). To achieve smaller file sizes, you can employ techniques such as cropping and resampling, or selecting compressed image formats such as JPEG and PNG (or even WebP, which offers an even higher rate of compression, albeit with limited compatibility). It is also crucial to choose a widely compatible file format that can be shared and opened across different platforms (e.g. PDF).

Print

Note: for print images, the measurement unit for resolution is DPI (dots per inch) and the colour mode is CMYK (cyan, magenta, yellow, black), which is a subtractive colour mode (i.e. when no colours are present, the result is white, but as more colours are combined, more light is subtracted/absorbed, making the colour darker and less saturated until it eventually becomes black).

Optimising files for print involves ensuring the final printed product looks high-quality and true to the original design. First of all, this means changing the colour mode to CMYK so that the colours on the printed product appear as intended. 

Additionally, lossless file formats are preferred for printing because they preserve all the necessary information needed for high-quality and accurate prints. Some commonly used file formats include PDF, PSD, and TIFF. PDF specialises in retaining the original format across all platforms, so nothing is affected regardless of which device the file is printed from, while PSD, the native file format for Adobe Photoshop, and TIFF are both excellent options for high-quality print graphics. It is also important to set the DPI between 300-600 for optimal resolution and clear and crisp output. 

Another set of factors to consider when printing are margins—to ensure that no content is cut off—and bleeds, if there are elements that extend to the edge of the page. As printers cannot print right up to the edge of the paper, it is necessary to add extra space around the edges of the document that extends beyond the final intended size. This additional space is known as bleed, which requires printing on a larger piece of paper, and then trimming off the excess space around the edges.

Image: Margins and Bleed, Image by Adobe (https://helpx.adobe.com/be_en/indesign/how-to/set-print-bleed.html)