PARALLELISM DEVICES

As you know, accurate focus is important for photography. But focus cannot be accurate over an entire image unless CCD (image), lens, and copy are already parallel to each other.

Think of those three planes as independent from each other:

• The most accurate reproduction and edge-to-edge uniformity of an image always lies in an imaginary plane that is parallel to the copy.
• A lens's optical axis is always normal (90 degrees) to the imaginary plane of uniform focus (the focal plane). This is the plane of the lens' best performance.
• When those two imaginary planes are precisely superimposed, focus on one spot will “catch” both the focal plane and the plane of most accurate reproduction simultaneously, giving you uniform focus.

Or you can think of the lens' optical axis as normal to CCD and copy. When it is, the lens performs as designed, resulting in the best possible image quality.

The requirement for parallelism by scan backs is especially important because of all the detail uninterpolated color makes available.

So if you want your image to have equal sharpness and shape as your copy, you need all three planes -- image, lens, and copy -- parallel to each other.

The best way to obtain parallelism is by using an optical system that displays an alignment pattern made from repeated images. This makes extremely small errors in parallelism “painfully” obvious.