Usual optical problems in binoculars

A binoculars is designed to gather more light than the unaided eye does and to enlarge the image of what you are looking at. In designing the optical system for a binoculars, 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 the design, fabrication or both. It is impossible to design and absolutely perfect binoculars.

There is a brief description of aberrations in binoculars.

1. Astigmatism: This result from light rays focused as two lines perpendicular to each other rather than as a single point. It is detected by moving the binoculars in an out of focus and watching stars at the edge of the field. If present, the focused lines go from vertical to horizontal and vice versa.

2. Chromatic aberration (color aberration): This is due to the different wavelengths of the different colours of light. An image with chromatic aberration will appear blurred and suffer colour 'flaring' around the fringes of objects. This is because the different colours are not all focused at the same point as they should be.
When the image viewed is geometrically different to the object, it is said to be distorted. There are two types of distortion; pincushion and barrel. Both types are caused by different parts of the image being magnified by varying amounts, instead of it all being magnified uniformly.

3. Coma: Generally caused by poor optical manufacturing. Coma affects the edges of the field of view and produces a V-shaped blurred image.

4. Distortion: It is generally caused when magnification varies from the center of the filed to the edges. Looking at an object with straight lines that covers the entire field of view, you will notice a curving of the lines as you look near the edges of the field. If the lines are curved outward, this is called barrel distortion; and if the lines are curved inward, this is called pincushion distortion.

5. Field curvature: A flat object does not appear flat when viewed using the binoculars. -This means that the centre of the image can be in focus whilst the periphery is out of focus and vice versa.  

6. Spherical aberration: here light rays at different distances from the optical center come to focus at different point on the axis. This caused a blurred image and you will not be able to focus sharply on any object.