Importance of Parallelism
Precisely parallel planes maximize both uniformity of sharpness and lens performance, and poor parallelism degrades both. It's just that simple.
This chart illustrates the old and new roles of parallelism:
A definition
What is parallelism? Parallelism is the angular relation of two or more planes. In photography, this usually refers to the optical axis of a lens being exactly normal (90° ± 0°) to both image and object.
In-Depth
If non-parallel planes are extended, they eventually converge. The angle at the point of convergence is called the included angle. When parallelism is perfect, the included angle is 0°.
Error is three-dimensional, but as long as parallelism is almost perfect, the two-dimensional included angle concept defines parallelism very well.
Different opinions about the need to check parallelism
Many photographic equipment manufacturers seem to think that setting parallelism at their factories is sufficient forever, because they do not publish parallelism specs. So the manufacturers imply that their parallelism will not change. But by not providing a method for you to set your final alignment, they do not acknowledge your need to fine-tune the original parallelism. They minimize, perhaps unknowingly, the impact that precise parallelism makes.
Every photographer needs to be able to see when the included angle is
zero – not just assume that it remains near zero. Only alignment
patterns made from repeated images are sensitive enough to show
you when the angle is extremely small. And when it is, there is absolutely
no need to examine all four corners of an image, and you have a level
of parallelism that your equipment alone simply cannot provide.
How parallelism is lost – but can be restored
In a perfect world, all photographic equipment would remain well aligned forever. In reality, a process of unavoidable change begins soon after equipment is designed. It is manufactured, shipped, relocated, set up, and used. You raise and lower your enlarger head, move your view camera standards, and adjust your copy setup. Even the temperature and humidity change. All these things contribute to degradation.
So degradation is inevitable. That is the bad news.
The good news is that you can check your parallelism accurately, easily, and quickly. Read on to learn how.