Alignment Checking Methods
Techniques to check alignment fall into two broad categories.
| Method | Description |
| Mechanical | These include rulers, carpenter's squares, devices of fixed length (even pencils!), and jigs with gauges (such as tramming rods). Mechanical methods are not mathematically precise. |
| Optical | These include bubble levels, test negatives, grain inspection, one mirror (lasers), and two mirrors (zig-align). Optical methods are usually more precise than mechanical ones, although use of either bubble levels or test negatives is time-consuming. |
Mechanical methods are not precise enough for photographic needs, so
let's examine some of the optical methods in detail.
Grain Inspection
Grain inspection is commonly used to check parallelism. But unless parallelism is much more accurate than grain inspection permits, a lens will not perform as designed. Here is why the technique is misleading:
Look at these bars from a distance
of ten feet, then up close.
All the bars appear the same at a distance. Up close, they do not.
The bars at ten feet are like appearance of grain. Even in huge prints, it is impossible to see the edge of a grain. You actually see only the grain body and must assume that all edges and bodies are related in a consistent way throughout the entire image. But you just saw that edges, as judged by their bodies, are not always what they appear to be. Edges, not bodies, show sharpness.
Judging parallelism by grain inspection – "eyeball uniformity"
– by either an unaided eye or a magnifier leads to prints that are
too "soft." The method is not mathematically precise, leaves much room
for improvement, and makes it impossible to follow the basic rule of test
and measurement – to check with ten times more accuracy than you
need to achieve.
Lasers
Lasers may sound high-tech, but as used in photography, their results are not. Laser devices for photography cannot detect small changes in parallelism
because the laser beam is reflected just once. These laser devices use
only one mirror, so amplification of error is minimal. And unless the
laser beam is normal (90°) to the bottom of the box containing the
laser, results are inconsistent.
In-Depth
zig-align
Zig-align's systems use two mirrors which maximize amplification and saves you time. In addition, their alignment modules form symmetrical alignment patterns only when planes are absolutely parallel.Therefore, you can judge parallelism easily and confidently when using zig-align systems. Their alignment patterns make even small error obvious. You can decide when planes need adjustment, then adjust to any desired degree of accuracy. You can even see how much a thin piece of paper will change parallelism.
The distinction betweeen accuracy and precision is discussed in Advanced
Topics.
In-Depth