what's an alignment pattern?
Picture a round mirror with a hole exactly in the middle and a ground glass ring around the edge. Then imagine looking through that hole to a plain mirror that shows the round mirror's reflection on it. You should see at least one circle, hopefully several circles, and ideally many concentric circles. Examples are to the right.
the need for alignment patterns
Lack of parallelism will change the shape of an alignment pattern like compound interest changes principal. Principal does not increase noticeably for the first few interest periods, but as the number of interest periods increase, the changes in principal become more and more obvious.
In a similar way, each additional ring repetition in zig-align alignment patterns increasingly magnifies any error in parallelism of the planes being checked.
To take full advantage of digital photography's capabilities when copying flat art, pay careful attention to the parallelism of all three planes of your imaging system. Zig-align alignment patterns are an excellent complement to the performance of digital systems, increasing the value of your investment.