Telescope Reviews: ABERRATIONS AND DEFICIENCIES

Nikon provides a brief explanation of the six major aberrations here
Nikon Sports Optics - Aberrations and Deficiencies

Vladimir Sacek provides a much more in depth discussion of the major aberrations in his web article here
AMATEUR TELESCOPE OPTICS See INTRINSIC TELESCOPE ABERRATIONS

Tom Trusock wrote a paper with some easy to undersatnd explanations
Telescope Optic Aberrations

MAJOR ABERRATIONS

ASTIGMATISM
Astigmatism makes it difficult to ever achieve a precise focus. The telltale sign of astigmatism is found when you view the out of focus Airy disk image. On one side of focus it will be elongated in one direction. On the other side of focus the elongated orientation of the image will change by 90°. There can be several causes of astigmatism. One of those causes can be a miscollimated or misaligned optical tube. It can be in the eyepieces. It can be in the objective lens. If the optical axis is perfectly aligned and the if the images are merged between the two barrels and astigmatism is still present, the problem is not one the user can fix. At any rate, the affect of astigmatism is that you will notice elongated images off axis. If astigmatism is severe, then on-axis focused images will show a very small cross, never a fine pinpoint. The affects of astigmatism are generally eliminated with high power and small exit pupil, but that is usually not achieved in binoculars.

SPHERICAL ABERRATION
Spherical aberration is a displacement of the rays along the length of the optical axis. Rays originating in the lens further off axis do not reach the same focus point as on-axis rays. If present, even on-axis images may look considerably soft and no amount of refocusing or flattener will help to improve. The best you can achieve is focusing on what is known as a least circle of confusion.

Stopping down or masking the outer edges of an objective will help suppress spherical aberration. This would have the effect of eliminating improper focal length rays generated at the extreme outer portion of the lens. This would improve or "reduce in size" the least circle of confusion at or near the focal plane. This would give a more pinpoint image to focused stars, but, at the expense of lost aperture.

DISTORTION and FIELD CURVATURE
Q - I was hoping someone could explain to me the difference between distortion and field curvature? At the very edge of fov in my binocs I am noticing the stars stretched out a hair compared to very tight dots in the center to about 90% out.

A - By Bill Tschumy
Distortion is of two types: barrel distortion and pincushion distortion. It isn't related to sharpness of the image, but rather image scale across the FOV. If you look at a square centered in the FOV, with pincushion distortion, the sides of the square will be bowed inwards. With barrel distortion, the sides will bow out. Generally, when looking at a star field, the distortion is not much of an issue. Sometimes, when panning, barrel distortion can give a "rolling" effect that makes some people dizzy.

Field curvature is when the outside of the field of view has a different focal point than the center. You can't get both in focus at the same time but you can get one or the other in focus.

What you describe as the stars being stretched out near the field sound more like coma to me, yet another aberration.

Bill T dedcribed distortion and curvature pretty well. As Bill said, "It (distortion) isn't related to sharpness of the image". Distortion does not affect the image of the star, it simply moves it in the field of view. Generally it cannot even be seen in astronomy, but can usually be seen when the binocular is used for looking at terrestrial objects where we are accustomed to seeing nice straight line edges of things like buildings and poles.

Pin Cushion Distortion discussion explaining reasons for pincushion

PINCUSHION DISTORTION IN BINOCULARS drawings showing pincushion

FIELD FLATTENERS TO REDUCE CURVATURE
Curvature of field (change in focus off-axis)can be reduced by using a field flattener lens. Without the flattener, the outer portions of the field will be slightly out of focus when the center is in focus. Instead of pinpoints in the outer field, this will have the tendency to enlarge and blur the circles of light that form the stars in the outer field. They would be slightly out of focus. (Slightly enlarged circles, that is assuming no other aberrations are present).

A field flattener can be used to insure that the entire field is in focus at the same time. Stars in the outer field will be focused and appear just as stars in the center (again, assuming no other aberrations are present). The field flattener does not eliminate spherical aberration, astigmatism, coma or chromatic aberration.
Field Flattener vs Field Aberrations

Field Curvature Affect on Depth of Field
Of the aberrations mentioned above only one can be changed by refocusing, field curvature. Field curvature appears because the outer portion of the field of view may have a different focal point than the center. The focal length increases as you move further out in the fov. You can't get both the center and the edge of the fov in focus at the same time but you can get one or the other in focus.

An astronomer would prefer all binoculars to have as little field curvature as possible so the entire plane of the fov has as little distortion as possible. It is not unusual to find that a binocular with much less sharpness out at 60 or 70% in the fov has a much greater amount of field curvature than a binocular which appears sharp out to 80-90% of the fov.

A terrestrial viewer might want to consider how field curvature affects the view, and may in fact find field curvature to be a useful aberration. While it does nothing to add to depth of field further distant than the plane of focus, field curvature provides for closer objects to appear in focus as they range out across the fov.

It seems clear from this correlation that the binoculars with greater field curvature have the perception of greater depth of field in front of the focus point. Therefore, it may stand to reason, a binocular with more field curvature may provide a greater “near” depth of field for the terra user. (of course that won't do you any good if all the birds you want to see are in the same tree sitting on branches between 45m-55m away). Also, it may be that a field flattener lens, which would be desirable for an astro viewer to get as flat a field as possible, might work against a terrestrial viewer looking for the greatest depth of field.

Curvature Affect on Apparent Depth on Field

COMA
Coma is generated by off-axis light rays. The intersection of light rays from different points along the objective is not symetrical. Coma is present when you see images of stars begin to spread out in a fan shape. Rather than see a tight spot of all the rays forming the image, a diagram would show most of the rays forming the bright point and the remainder of the rays spreading out from that point to form the recognizable fan or comet shape. Coma increases toward the outer edges of the lens. A star will have its brightest point towards the center with the fan shape extending outward.

CHROMATIC ABERRATION
this topic has been moved to its own thread

All achromats show some CA. It is a function of lens design. It is also a function of focal ratio. Very few binoculars are designed specifically to suppress CA. BTW, CA is not considered a major aberration. Longitudinal CA will produce softness of image focus of perfectly on-axis objects because all colors in the spectrum will not come to focus at the same point. Lateral CA increases as you view objects further off axis and is more pronounced towards the outer edge of the lens. Lateral CA is seen as the outer edge of the object bleeding, usually a different color, generally observed in large bright extended objects. Significant CA (on the moon or planets) may cause some edge softness on both and loss of contrast, but it should not be confused with flaring.

Spherical Aberration usually is referred to as an aberration in the optical elements. However, when referred to in the exit pupil, usually what is meant is "Kidney Bean". Read the explanations here.
Spherical Aberration of the Exit Pupil

Binocular deficiencies to watch out for:

GHOST IMAGES
Coated vs Multi-coated vs Fully MC will have an affect on total light transmission and contrast. Lack of coatings may produce ghost images internally. Ghost Images and internal reflections is a deficiency that will destroy contrast.

PRISM LIGHT CUTOFF
The following post has a photo of a binocular through the objective lens. look closely at the image and you can see light blockage at the edges. This is known as prism light cutoff. The net effect = this is reducing the effective apperture of your binocular.

Here's the photo of prism light cutoff
Prism Light Cutoff shown in photo

In this image above here you see the perfect example of prism light cutoff.

This is not related to collimating binoculars. This is not what you would see if one side or both sides of the binocular were out of collimation within the tubes themselves. In that case you might see ovals or cats eyes or the images would not be merged.

In this picture, either the prism housing infringes in the path or prisms themselves are so large that they infringe in the light path, or the prisms are to small to accept the entire light path from the objective and you see the edges of the prism cutting into the light path and cutting off the edges of the exit pupil.

Here's a link to the discussion and the description of what is going on in this binocular system
Prism Light Cutoff ...

Here's a link to another discussion as relates to another model binocular
Prism Light Cutoff - What to look for - How to calculate

COLLIMATION
A binocular that is out of collimation will show double images. It is corrected by mechanical adjustment.
This has been moved to it's own post.

FLARING
Q - I see flaring of the stars in my binocular? What is it?

A - I'm not sure what you mean by flaring. In fact I think people have different meanings for this term. I think some people are referring to radial spiking and some to ghosting. Flaring in photographic images refers to something similar to ghosting, as in lens flare. There are other things people might mean when they say flaring.

What some people are calling flaring might be lateral chromatic aberrtion. Lateral CA increases the further off-axis you view the object and is more pronounced towards the outer edge of the lens. Lateral CA is seen as the outer edge of the object bleeding, usually a different color than the inner edge, generally observed on very bright stars or large bright extended objects and would have a smooth outer edge shaped to the object.

Coma is another good example of an aberration that people might refer to as flaring. Coma would be seen as a off-axis wedge with the point of the wedge towards the center. Generally not noticed on-axis, the wedge would probably get larger as you move the object further off axis. Coma would be seen on all stars.

Severe spherical aberration could be producing an on-axis image that looks like a star point encircled by a diffuse circle of light. The inability to focus all the light into a fine point creates a diffuse glow around the point that cannot be focused out, either on or off axis. This might be interpreted as flare by some people.

However I use the term flaring to mean radial spikes, spikes of light flaring off of the central image disk, more than just a few, and in all directions. The binocular that I had with this problem displayed maybe 30-40 flares coming off the star.
In this thread About two years ago I described flaring as radial spikes.
More recently, the discussion came up again and much discussion was devoted to problems in the eyes in this thread about
Radial Spikes Around Bright Stars

RADIAL SPIKING
On bright stars, I'm seeing a rather nice radial pattern of spikes around the point star. The pattern doesn't flare or move like atmospheric abberations, it remains static as long as I hold my eye still. As I move my eye around the spikes move around as well, but never disappear.
When using both eyes, I cannot get stars to focus to points no matter how I set the diopter and focuser. I have 20-20 vision BTW.

New 20x80 binos - Radial Spikes

The problem Gary describes has nothing to do with mis-collimation nor mis-alignment of any portion of the optical train. Gary describes radial spikes, something akin to radial flaring. This is an anomoly of the lens.

This abberation is similar to some degree to what you might see in a normal lens on a very bright star. Focus on Sirius or Vega and what do you see. In many binocular lenses, there will be what looks like a few, maybe 6 to 8, small diffraction spikes eminating from the bright star. This is usually seen ONLY on the most extreme bright objects.

What Gary has described is radial spikes all around a star. I have seen this aberation exactly as described. This is an unusual abberation that is not normally seen in a lens.

A star test, if viewed on a group of faint 4th /5th magnitude stars would not pick up this abberation. By the time the view moves over to 5th or 6th magnitude stars, there is not enough light for this anomoly to be seen, but it is still there. It will interfere with resolution of bright close objects, especially if the companion is faint, and it will interfere with resolution of extended objects made up of point sources such as very dense open clusters and all globular clusters.

If this is indeed a lens condition, the only kind of test that would detect it is a severe star test, similar to that used to test the diffraction pattern in a telescope. This is not a condition that would normally be discovered by any QC.

I have tested a number of binoculars and looked closely for any appearance in the diffraction image in all of them. At powers of 15x or below, separation in the diffraction rings cannot even be seen. At 16x it can barely be noticed. At 20x the first real definition of the diffraction rings begins to appear.

I would suspect flaring is a result of the figuring of the lenses use in the manufacture. It could be in the objective lenses or the eyepiece lenses. Figuring of the lenses could also be producing soft focus and not appear as flaring.

After much discussion and searching by many people, it seems several tend to agree that a strong contender as a source may be that this problem is caused by pinched optics. It may be a result of uneven pressure during the cementing of a doublet or it may be the retaining ring around the objectice being too tight. Read page 150 of Suiter.

It is also a simple fact that very bright objects produce some flare or minor spikes in the image. Here again, I would expect a more highly figured lens would reduce this flare.

VIGNETTE
Vignette, or the blocking of light, by whatever means, reduces the amount of light delivered by the optical system before it eventually reaches the receptor which in this case is your eyes. There are several different deficiencies that could be causing vignette. Identifying what is causing it is more difficult than identifying how much of a light loss is present. This is a very lengthy thread. I have singled out very specific passages for the reader. Fell free to read the entire 100+ posts if you wish.

Vignette in Binoculars!! YES it's there

Test procedure using a laser light to check Vignette in binoculars

Vignetting in All Binoculars!! the prism is not fully lit by the entire objective

Why can't we see the light drop off?

Graphic representation by field of view

Graphic Representations by % light in exit pupil (see attachment)

Graphic Representation showing affect of Off-Center Exit Pupil on the Overall Light Distribution in the Exit Pupil

Things you should know about "too small" prisms

Comments on use of stops

More on stops

Post Extras