How to create an "Actual Size" image in Photoshop

Occasionally its required to create a 1:1 sized image i.e. its the same size printed as it is in real life.

This is fairly easy to do in Photoshop by utilising the power of the crop tool. For example if we take this picture of a diskhead the length of the dashed area in real life is 2cm, we want it to still be 2cm when printed at 300ppi.

The Area marked with a dotted line should be 2cm wide
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The first thing to do is to unlock the background layer so that it becomes a normal layer. Then take the crop tool and select the hide option on the toolbar. Now crop the image to just show the piece we have an exact measurement for.

The completed crop in Photoshop
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Now we resize our file based on the size of this image. Go to Image>Image Size and enter the size this part of the picture should be.

Image Size Dialogue
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Now all we need to do is select Image>Reveal All and our image is life size.