In designing optical systems, the optical engineer must make tradeoffs in controlling aberrations to achieve the desired result of the design. Aberrations are any errors that result in the imperfection of an image. Such errors can result from design, fabrication or both. It is impossible to design a perfect optical instrument that is aberration free. Besides any aberrations from the optical design, air turbulence and other environmental factors limit the resolution of optical instruments especially at long distances.
When you buy a binocular, spotting scope, or riflescope, your expectations are that it should be sharp at the center and at the extreme edges. However, in reality this does not happen due to design compromises (except in some high-end products). Some aberrations of a minimal amount are present in all optical instruments and generally are less apparent as the price of the instruments increase. So, do not be too concerned unless the aberrations are severe or bothersome. A few aberrations are briefly discussed below:
Chromatic Aberration – CA (Color Aberration or Color Distortion) — is the failure to bring light of different wavelengths (colors) to a common focus. The result, mainly in a faint colored halo (usually blue or yellow but on rare occasions you can see it in red or green) around bright objects, bright stars, the Moon and planets, or dark objects against a bright sky. Color aberration (CA) is present when looking at telephone wires, poles and straight lines of roofs or buildings especially at the edge of the field but it is actually throughout the field. It reduces contrast, resolution, and color saturation.
Note that purple or blue fringing (seen on the Moon image above) is not CA but color fringing caused by light scatter from a digital camera lens.
In optical products you will see some color aberration when they use achromatic lenses which is inherent in the optical design. Chromatic aberration is reduced in many optical designs using low dispersion objectives (ED glass or fluorite elements). Carson Optical recently developed a unique CA test. It uses a chart with blue and red strips that will merge and appear purple when there is Chromatic aberration present.
Distortion — is generally caused when magnification varies from the center of the field to the edges. Looking at an object with straight lines (telephone wires, brick walls, doorframes, etc.) that covers the entire field of view, you will notice a curving of the lines as you look near the edges of the field.
Best binoculars, best spotting scopes, and best riflescopes can have specific distortion called rectilinear distortion and it has two sub types – barrel or pincushion distortion. If the lines curve inward, this is barrel distortion, (the most common form of distortion) – where the object magnification decreases with distance from the optical axis and the appearance is of looking at a tradition large wooden barrel. If the lines curve outward, this is pincushion distortion – where the object magnification increases with distance from the optical axis and the appearance is like a pincushion.
For binoculars and spotting scopes, you want the complete field of view to be free of distortion for higher-end products with the expectation that distortion will be present for lower priced units. For riflescopes, the center of the field is most important but distortion can dampen your overall view and it is annoying to have obvious distortion in your field of view.
Field Curvature/ Flatness of Field— caused by the light rays not all coming to a sharp focus in the same plane. The center of the field may be sharp and in focus but the edges are out of focus and vice versa. All low and mid-priced products have some degree of field curvature. Higher-end products normally have flat edge-to-edge fields due to their having additional field flattener correcting lenses in the objectives or eyepieces or both.
Again, do not be concerned with these potential problems. Usually, if any of them are severe, you will know it just by using them.
Collimation (Alignment) The biggest problem with binoculars that consumers have is poor collimation. Collimation is the alignment of the optical elements of the binocular to the mechanical axis. Good collimation prevents eyestrain, headaches, inferior (or double) images and improves resolution.
The manufacturer normally tests binoculars for proper collimation, but some (usually lower priced units) slip by the quality processes. Rough handling or shipping can cause de-collimation. The majority of collimation problems are with Porro prism types but roof prisms can also have this problem. A high percentage of binoculars sold are out of collimation (mainly the inexpensive units). Most people do not realize this severe problem because their eyes try to compensate for the misalignment. A couple of quick tests to determine if collimation is okay; First focus on an object in the distance (like a street lamp or sign) about 100 yards or 100 meters or further away. Next, have someone hold a book or other solid object over one objective lens. Now look through the binoculars (with both eyes open). Have the solid object removed from the one objective very quickly. If two images appear and then blend into one, the binoculars are out of collimation. Another test is to look at the top of a brick (or similar) wall or roof top at a distance. Looking through the binoculars with both eyes, slowly pull them away from your eyes out to about 12 inches (30cm) in front of you (keep looking through them). At first, you see one view but as the binoculars are pulled away, you will then see two views. These two views should be similar and in line. If one side or the other is higher or lower, you have a vertical collimation problem. If one side or the other is on top of the other, then you have a severe misalignment. If one side or the other is further apart from each barrel, then you have a horizontal collimation problem, which is rarer than a vertical collimation problem. If you first look through a binocular and have double images, you need not do any other tests, as obviously something is severely wrong.
There are many articles you can find on the internet about how to collimate your binoculars. I would not encourage you to try them as many do not have the correct information and done improperly can make your binoculars worse. It is true that some people can do a collimation (called conditional alignment) at his or her IPD (interpupillary distance) and the binocular may give acceptable performance but it may not be usable by others, which have different IPDs. The conditional alignment may seem ok for the user but if the optical paths are slightly out of parallelism, your eyes may be accommodating themselves but after observing continually for a long period of time your eyes may become irritated or you get headaches. If your binoculars are in need of collimation, contact a reputable, independently owned repair facility or factory repair facility. Collimation is very important to how your binoculars perform and they should have the proper collimation machines to do the work. A true collimation is on 3-axes so that the hinge movement (one axis) for different IPDs (extremes of the hinge and mid-point) are all meeting alignment standards for both optical axes. This can only be done with a collimator, regardless of what others may tell you. Further information is available through William J. Cook’s SPIE paper (8491-14), Binocular Collimation vs Conditional Alignment.