When an image is captured by a camera or a scanning device it has to be saved or memorised in a 'file format'. The information can only be read and understood if the software recognises the format.
The dominant files are:
- JPEG (Joint Phoographic Experts Group) .jpg
- TIFF ( Tagged Image File Format) .tif
- PSD (Photoshop document) .psd
- RAW (Raw, unprocessed information)
File > Save As - This opens up numerous options for the photoshop/pc user. for simplicity it is advisable that you save your work as either a TIFF or PSD file because they support all of photoshops features. NOTE: Not all formats support layers.
TIFF - This format is generally regarded as industry standard because it is the most universally recognised format. Photoshop's variant of this format supports image layers, adjustments, layers, paths, clipping paths and ICC profiles. It;s the most popular format for photographers delivery RGB and CMYK files to clients. It's good practice to flatten files because only photoshop supports layered TIFFs.
PSD - Photoshops native format supports everything photoshops can do including ICC profiles. Until version 6 it was the only format that supported layers. This format is only recognised by photoshop so it is not advisable to hand PSD files over to clients.
JPEG - The most commonly used file format. It has 'lossy' compresion that transmits what it deems to be 'important contend and discards colour information that it considers redundant, there is no option to save without compression but embedding ICC profile is optional. JPEG allows you to make a trade off between image file size and image quality. JPEG compression divides the images in squares in 8x8 pixels which are compressed independently. As you increase compression the squares become more visible.
100% quality JPEG is very hard to distinguish from the uncompressed original typically taking up to 6 times as much storage space.
80% quality JPEG still look good although it is 10 times smaller than the uncompressed original. Some detonation will occur however.
60% quality JPEG is 20 times smaller than the uncompressed original. sufficient for website use.
10% quality JPEG will show serious image degradation with visible 8x8 JPEG squares. It is unlikely that you ever see this level of compression but it illustrates what JPEG is doing at higher quality levels.
Photoshop - PDF (Portable document format) This format can support layers. Files can be reopened and edited in photoshop. A group of images can be saved as a multi page document. PDF supports various compression amongst them are ZIP, JPEG and JPEG 2500.
Upon close inspection of a compressed JPEG block of 8x8 pixels can become evident. Because of this degradation is it advisable to avoid saving in this format for high quality images.
JPEG can use several layers of compression dramatically reducing the size of closed files compared to their open state. If n image is shot on JPEG avoid resaving as a JPEG as it will further degrade the image. Save as a TIFF.
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