Two Viewpoints about Parallelism

Photography needs parallelism. This is especially true for the three planes (neg, lens, and paper) that must be parallel for the best printing and copying.

Many photographers become aware of their need for better parallelism when they see that the corners of a print are not equally sharp. This sort of checking, "eyeball uniformity", has led to a strong association between parallelism and grain inspection – the idea that parallelism is good enough because prints appear to be uniform enough. However, images are actually much better if parallelism is checked with mathematical precision.

Two ways to think about parallelism are considered below, primarily in the context of an enlarger.

the first way: Think of the three printing planes as independent. When you do, you emphasize how focus depends on parallelism to obtain uniformity throughout an image.

• Regardless of a negative's orientation, the negative is always parallel to the imaginary plane of its most accurate reproduction and edge-to-edge uniformity.
• Regardless of a lens's orientation, the lens's optical axis is always normal (90degrees) to the imaginary plane of its uniform focus (the focal plane).
• Focus should superimpose those two imaginary planes, but it can do so only if the imaginary planes are already parallel to each other. When printing, focus is on one spot in the focal plane, but one spot can define the focal plane only if all planes are already parallel to it.

Parallelism between the NEGATIVE and PAPER pair results in uniformity and accurate reproduction. Without adequate parallelism, a shape in the negative will not be the same shape in the print.

But it takes more for the LENS and PAPER pair; both parallelism and focus must be "dead-on". Only when the focal plane is already parallel to the paper plane can focus on a spot superimpose both imaginary planes and give you a uniform image.

A print should accurately reproduce information available in a negative. Transferring information from one flat plane to another requires more exacting parallelism than does recording three-dimensional subjects. Minor variations in a 3D-subject plane are not as obvious because of depth-of-field protection.

• Precise parallelism of lens, negative, and paper, especially for an enlarger, will improve uniformity that results from focus and maximize accuracy of reproduction.

the second way: This time, think of only the lens plane as independent, so position of negative and paper are dependent on the lens. This way emphasizes lens performance, and maximizing lens performance is the primary reason precise parallelism is so essential.

An enlarging lens is highly specialized and performs best when used under the specific set of conditions for which it is designed. One condition is optimum aperture, usually two stops from wide open. Another condition is having the lens's optical axis normal to neg and paper, not almost normal. When the optical axis is normal to both, the lens performs as designed, resulting in the best possible image quality, not just satisfactory uniformity. As one of the many benefits of zig-align parallelism, you will always get four sharp corners, not just one, two, or three.

Copying flat artwork with a camera lens also transfers information from one flat plane to another. Camera lenses have less-flat fields than enlarger lenses, so copying by traditional photography has a similar but less rigorous need for parallelism than does enlarging. However, the requirement for parallelism is even more rigorous if you want to capture all the detail available from the uninterpolated color produced by scan backs.

• Either way you consider parallelism, its full benefits are available only when focus is exact. That is possible only when all three optical planes are extremely parallel. But because one's perception of sharpness is restricted to being able to see an edge, the second way best describes why highly accurate parallelism gives unexpectedly good results.