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"Save as a jpeg image, being sure you do not overwrite the full-size image in the process."

Sometimes you will be asked to save an image for projection in a format other than jpeg - tiff, for example - but jpeg is by far the most widely used so that is what we will concentrate on here.

The most important thing is to make sure that in saving your smaller image, you do not overwrite the full-size one it came from! This might sound obvious but it's a surprisingly easy mistake to make. If you DO save and realise what you've just done BEFORE closing your image, don't panic! You may be able to step backwards (or undo) until you reach a stage where you have recovered what you lost.

When saving jpeg images you normally have an option to choose the level of compression applied. Put simply, the more aggressive the compression, the lower the quality and the smaller the resulting file size. In Photoshop CS we are able to choose from a scale with "file size" at one end and "quality" at the other. This scale represents a compromise between these two extremes. A small file size means minimal quality, and high quality means a large file size. For digital projection it is advisable to go for as high a quality as you can. In Photoshop CS this would be a setting of 12 (on a scale from 0 to 12), though 11 or even 10 will usually give decent results if file size is a concern.

low quality jpeg

This is part of a jpeg image which was saved with a quality setting of 1 (on a scale of 0 to 12). It has been magnified to let you see more clearly what horrible things have happened. Note how much "blocking" occurs because of aggressive file compression. There has also been some unwanted colour introduced (see the top left part of the image). This image will project poorly.

 

high quality jpeg

This is a close-up of exactly the same image, magnified in the same way, but this version was saved with a quality setting of 11 (on a scale of 0 to 12). The whole image is shown below so you have an idea of the magnification used. The screw head above is on the boat's bow near the bottom left corner of the full image. This image will project well.

original jpeg image

© David McHutchison (Deemacphotos)
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