"Good Enough", or "Perfection"
Lenses have a tolerance for misalignment. But the better lens alignment is, the better the lens will minimize aberrations and flare, variables that are highly dependent on the designed geometry of both lens and other equipment.
The existence of lens tolerance leads photographers to accept a "good enough" print. It does not lead to achieving the best print; after all, the one they just made looks good enough. But by accepting the print, they minimize their return on investment in a lens (as well as other equipment) and make prints that are not as good as they could be.
Figure 10 in Advanced Topics on this site shows that parallelism set for 8x10 prints according to a symmetrical zig-align pattern is within 0.001 inch of perfection for the LED module and 0.002 inch for the ring module. The 0.001 inch (a linear measure) shows parallelism that is different from perfection by 21 seconds of arc (an angular measure) for the included angle. But since one can notice a change of 20 seconds of arc in the LED module's pattern (Figure 12, same page), it is possible for that module to show when parallelism is perfect..
These are very small numbers and easily could lead someone to believe that any procedure for using zig-align is "good enough". However, the quality of an actual print only measures performance of your entire printing system. That includes your personal printing skill, as well as the actual geometry of your lens and other equipment. But you need to separate these variables. Just as the bars in The Basics show, looking at prints is not the way to go.
You may think your print is fine, but you won't know if further improvement is possible unless you go through the cycle of: 1) realigning more accurately than before; and 2) making another print. All of this can be time consuming and requires a careful diagnosis, during which mistakes historically are made.
The multiple-repeats in a zig-align alignment pattern give you visual information about the actual state of geometry of your equipment. The pattern shows you nothing about your printing skill, but a perfect pattern does show you when planes are parallel. This maximizes lens performance and guarantees your lens will operate as designed.
Suppose you want to make a print, despite the fact that your enlarger is covered with slithering rattlesnakes. Then you would accept your enlarger's present alignment as "good enough". But if you had started to use zig-align and seen a not-quite-symmetrical alignment pattern, and if the snakes weren't really there, then a "good enough" print isn't what you are capable of making. Don't ask the "good enough" question until you have tried to make a symmetrical zig-align pattern and then print.
When you ask if "good enough" is "good enough", you imply, perhaps without realizing it, that you are already familiar with a standard by which to make judgements. But zig-align can help you reach a higher standard, one that may be unfamiliar, but newer and better.
Asking the question also assumes making an accurate diagnosis so you avoid a photographic mishap. Hopefully, you do not want to risk repeating the mistake of Western medicine where over 60% of diagnoses are incorrect and where medical mishaps (next to heart attacks and cancer) are the third leading cause of death. Zig-align certainly can help your prints live on.
You can avoid the risk and time consumption of diagnosis and evaluation of prints in the first place simply by making all your equipment planes parallel. Then you know you are getting the best results from your lens. Zig-align deals with that technical issue, and when you make its alignment pattern symmetrical, you can create the best images possible for you and your equipment.
In twenty years of professional photography and fourteen years of zig-align, I have never seen nor heard of any photo equipment in "reasonable" condition that would not permit a user to achieve a symmetrical zig-align alignment pattern. But I have seen lots of equipment with poor alignment. If zig-align is used as intended, it makes your enlarger, view camera, copy or taking lens, and copy setup perform as accurately as designed, perhaps even better than manufactured.
Sooner or later all equipment will change alignment to some degree. You move your enlarger's light head on the upright, you change the bellows draw for the enlarger's lens, the temperature or humidity varies. Just normal wear and tear can create degradation. It is impossible for any manufacturer to prevent those changes
Only the end-user can overcome them. The easiest and best method is to "fine-tune" them away by depending on what zig-align shows you.
Bill Ziegler