Porter's Camera Store March 2005 Newsletter

Lens advertisements often use (but seldom define) buzzwords such as apochromatic, low dispersion, aspherical, etc. Here are a few definitions to help you make more sense of the terms.

Chromatic Aberration:
As light rays pass through a lens, they are bent, or refracted. Since each color of light doesn't refract exactly the same way, the colors don't all focus on exactly the same plane. This aberration is more of a problem with telephotos than other lens types. A lens with little or no correction for chromatic aberration will produce photos with color fringing around the areas of the subject when viewed under high magnification. Apochromatic lens elements made of special low-dispersion glass have different refraction characteristics and help focus all the colors on the same plane to control chromatic aberration.

Spherical Aberration:
Light rays passing through the edges of a lens are bent differently than those refracting through the center and focus at different points along the lens axis. This causes the picture to be less sharp in the center when the lens is focused for sharp edges and vice versa. Aspheric lens elements are precisely curved around the edges to focus the image sharply on the image plane and eliminate spherical aberration. As an added bonus, lenses incorporating aspherical elements are more compact than similar lenses that use conventional spherical elements to correct this aberration.

Distortion:
Have you taken a picture of an object that had straight lines and noticed that the picture shows the lines bowing in or out? This is distortion and a rectangular object may exhibit pincushion distortion if the lines bow inward, or barrel distortion if they bow outward. Aspheric lens elements and the proper placement of the aperture in the lens design help control this fault.

Digital-only Lens:
Since most manufacturers of digital SLRs use sensors that are smaller than the 35mm film format, camera makers and independent lens manufacturers are producing lenses that cover the surface area of the CCD or CMOS sensor, but not 35mm film. While most of these digital-only lenses can be mounted on a 35mm SLR of the same brand (Canon EF-S lenses being an exception), there will be quite a bit of vignetting since the optics don’t cover the full size of 35mm film. They are not recommended for film cameras for this reason.