Spherical aberrations are responsible for the “softer” images we often get when shooting at or close to wide-open on a fast lens. Moreover, the challenge of focusing is made all the more difficult because the refractive index also depends on the wavelength! Spherical Aberrations Unfortunately, lenses are not perfect and they cannot always make light rays bend as we want them too. Consequently, light bends more entering glass from a vacuum than it does entering water from a vacuum. Glass, depending on the composition and quality, has a refractive index typically between 1.5-1.7. Snell’s Law shows how the change in the refractive index from one medium to the other relates to the angle at which the light is refracted but, essentially, the greater the relative refractive index the greater the angle of refraction. This means that light travels 1.33 times slower through water than it does through a vacuum. For instance, the refractive index of water is 1.33. … where c is the speed of light in a vacuum and v is the speed of light through the substance in question. The relationship between the speed of light in a vacuum and the speed through which it travels in another medium is given by its refractive index, n: For example, light will travel faster through a vacuum than through air or glass. This bending happens because waves travel at different speeds through different mediums. Through refraction-the bending of light when it passes through a different medium-the light that enters the lens is focused onto the sensor or film. A concave piece of glass is the most basic part of the lens used to achieve this result. ![]() The job of the lens in any photographic system is to focus the light entering the lens onto the sensor. There are two types of aberrations that concern us in photography-spherical and chromatic. Different lenses will be affected by aberrations to varying degrees and in some cases these unwanted effects can be mitigated by stopping down or through lens correction algorithms built into higher-end cameras. Aberrations are imperfections in the optics of the camera lens and result in image deterioration.
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