Curved Focal Plane and Spherical Aberration

Distorted stars from a curved focal plane and astigmatism are seen here in the corner of the field.	Stars show spherical aberration and axial (longitudinal) chromatic aberration in the center of the field.

Curved Focal Plane - Most telescopes do not have a flat focal plane because the distance from the lens to the corner of the sensor is larger than the distance to the center. In most scopes, the focal plane is curved because of this. Stars at the edge of the field will be out of focus.

• Solution: Use a field flattener.

Spherical Aberration - Spherical aberration is caused when light passing through different zones in the lens or mirror's periphery is not brought to focus at the same point as light passing through the center of the lens. Stars will look like a bright spot inside of a large diffuse halo. Many fast camera lenses will exhibit spherical aberration, especially when used wide open

• Solution:
  • Stop the lens down.
  • Use a better made optical system.

Stars exhibit astigmatism in the corner of the field in this image.	Stars exhibit coma in the corner of the field in this image.

Astigmatism - Light from different planes focus at different points. Stars will elongate into an ellipse in one direction on one side of focus, and at right angles to that direction on the other side of focus. Sometimes they can look like crosses or lines in the corners of the field in an image.

• Solution:
  • Stop the lens down.
  • Use a better made optical system.

Coma - Most fast Newtonian telescopes and camera lenses will exhibit coma in the corners of the field. Stars will look like comets or seagulls.

• Solution: Stop the camera lens down, or use a coma corrector on a telescope.

Axial (longitudinal) chromatic aberration is seen here in the center of the field.	Lateral chromatic aberration and astigmatism is seen here in the corner of the field.

Chromatic Aberration - There are two common types of chromatic aberration: axial and lateral.

• Axial, or longitudinal, chromatic aberration causes stars to have blue or purple, and in some cases even red, halos around them. Because different wavelengths of light each have a different refractive index, they each focus at a slightly different place when they pass through a substance, such as glass, that refracts light. A single lens is not capable of focusing all of the colors of light at the same spot.

• Lateral chromatic aberration causes stars in the corners of the field to have a red or blue star image that is shifted in relation to the star image formed by the other colors. It is caused by prismatic dispersion from light passing through the edge of the lens where colors spread out and shift sideways depending on their wavelength.

Lens designers use multiple lens elements and configurations, and glasses with different refractive indexes to try to solve these problems. Apochromatic refractors are specifically designed to bring the three primary colors to focus in the same spot. But some apochromatic refractors are designed to do this for visual use of the telescope and are not apochromatic for photography because digital cameras have different sensitivity than human eyes.

Most refractive (lens) optical systems will suffer from some chromatic aberrations. Reflectors, such as Newtonians, do not suffer from this problem because light is not refracted by a mirror, it is reflected.

Lateral chromatic aberration is more common in extreme wide angle lenses and shows up as stars with shifted red or blue images in the corners of the field.

• Solution:
  • Use a minus violet filter in a telescope or camera lens, or stop the lens down.
  • Use a reflector instead of a refractor.

For most camera lenses, optical aberrations can be greatly improved by stopping the lens down one or two stops from wide open. Really fast lenses improve more from stopping down. Slower lenses are generally better corrected and usually work better wide open, but even they will improve when stopped down.

The drawback to stopping down a camera lens is that you lose light gathering ability. In cases where optical system speed is most important, such as with fast moving comets, aberrations in a fast lens can be tolerated more easily. There is no substitute for aperture and system speed, and the price you may have to pay is some aberrations in the image.

Well designed and constructed telescopes usually do not suffer from much spherical aberration, and it is usually not necessary to stop them down when a field flattener, coma corrector or minus violet filter will fix their associated problems and allow you to use the maximum aperture of the scope.

Mouse-Over f/1.4           f/2           f/2.8           f/4           f/5.6

Hold your mouse cursor over the f/stops below the image to see a Canon 50mm f/1.4 lens' performance at different f/stops as the lens is stopped down from wide open at f/1.4 to f/5.6 in one stop increments. These images are enlarged to 100 percent magnification.

In the mouse-over examples above, we can see how the lens' performance improves greatly as it is stopped down.

The first image was shot at f/1.4 with a Canon 50mm f/1.4 EF lens. It shows large halos from spherical aberration and color fringing from axial chromatic aberration as the dominant problems in the center of the field. Spherical aberration, chromatic aberration, coma, astigmatism and vignetting are all present in the corners of the field.

The second image was shot at f/2, one stop down from wide open at f/1.4. It shows that the spherical aberration is greatly improved, but that chromatic aberration, coma and astigmatism are still bad in the corners.

Also note how the sky background gets brighter in the corner of the frame from f/1.4 to f/2. This is because vignetting has been improved by stopping down.

The third image was shot at f/2.8. All aberrations are becoming much better controlled and when viewed at a normal magnification would probably not be too objectionable. This lens is definitely usable for astrophotography at f/2.8, stopped down two stops from wide open.

The fourth image was shot at f/4, three stops down from wide open. Notice that the performance continues to improve. Stars are still getting sharper across the frame. Chromatic aberration is almost gone, but not quite. Coma is much improved in the corners of the frame.

The fifth image was shot at f/5.6, four stops down from wide open. The lens' performance is very good at this point. All of the optical aberrations are almost completely gone. Diffraction spikes from the aperture blades in the lens' diaphragm are becoming prominent around bright stars.

Notice how even though the lens has more aperture and light gathering power wide open at f/1.4, more faint stars are recorded as the lens is stopped down. This is because images of faint stars are not even recorded when the lens is used wide open because the light is spread out too much because the aberrations are so bad. More light is focused into a tighter spot that can be recorded as the lens gets sharper as it is stopped down.

If Canon sold this lens with a fixed stop at f/5.6 or f/8, you would think it was an excellent performer. It is not really fair to judge the lens harshly for its performance at f/1.4 on a star field. A star field is the single most difficult test for a lens, and it is an unusual subject that really reveals the lens' aberrations when used wide open.

For normal daytime photography, the lens would rarely be used wide open like this because there normally is plenty of light. For extreme low-light terrestrial applications where a higher shutter speed is required and a fast aperture is a great benefit, optical performance is traded for focal-ratio speed.

For long-exposure photography of star fields, performance can be greatly improved by stopping the lens down. The cost of this improved performance is longer exposures because of the slower focal ratio.

Problem: Optical Aberrations - The Bottom Line There are many types of optical aberrations present in most camera lenses and telescopes. Most can be improved and corrected in camera lenses by stopping the lens down to a smaller aperture. In telescopes, optical accessories such as field flatteners and coma correctors can help with these problems.