We will learn more about compression and file formats later. For now, just accept what is said below.
You will need to save your picture to the hard disc at various times:
- when you first input the picture
- while you are working on it, so that you can come back and do more later
- when you've finished working on it and you want to save it for posterity.
Images can be saved in different 'formats' - the ones you will be concerned with are .jpg, .tif and .psd (.psd is for Photoshop, other software will have its own 'native' format). The different formats enable you to have the option of storing your image with the maximum quality (detail, colour, etc.) but a large file size, or with lesser quality but a much smaller file size.
When saving in .jpg you may be given the option of setting a quality level. High quality means large file size, low quality means small file size.
If you input from a camera then you must store your initial image in whatever format the camera determines - probably jpg.
While you are working on the image always store it away in the software's standard format - .psd in the case of Photoshop. This will ensure that nothing is lost next time you come to work on it.
When you have finished work then, and only then, store it in the most suitable format for long term storage:
if you may want to make high quality prints again, choose a lossless format - probably the software's standard format.
if you want it for a computer slide show, or just to print off some copies for Auntie Flo, then save it with jpg, probably high(ish) quality.
if you want it for a website or to email, the you will probably want to reduce the file size to minimise downlaoding times. Store in .jpg with low(ish) quality.
In Photoshop you can watch the effect of varying the amount of compression (i.e. the quality) when you save the image. If you select 'save for web' you can select the type and degree of compression while watching previews of both the original and the compressed image. It also gives information on the resulting file size.