APO & Aspherical?!?!?

[sdgmusic]

What is the difference between the lens designations APO and Asperical? I thought they were the same.

From what I understand APO is a special asperical piece of glass that allows all colours to focus the same so that all colours are vibrant in the picture, not just the ones in focus. Is Asperical the same thing?

 

[Helen B]

'Apo' refers to apochromatic correction - an apo lens does not have to use aspherical elements. There are various definitions of apochromatic that range from a vague notion that the lens is better corrected for chromatic aberrations than most to very specific technical definitions.

In general a lens that does not have an 'apo' designation will be corrected at two points in the visible spectrum - known as an achromatic lens. The focal length of a lens varies with wavelength (ie the colour of light). By designing a lens with different types of glass the focal length can be made to be equal at two different wavelengths. At other wavelengths the focal length will be different. There is no rule of thumb that says how much or in what direction the focal length will vary.

Designers of apochromatic lenses may make the focal length equal at three wavelengths of light (this is not an accurate definition). The idea is to try to keep the focal length pretty much the same over the range of wavelengths that the lens is intended for use with. There are also lenses that carry the correction beyond the wavelengths of light into the infrared (longer wavelengths) and ultraviolet (shorter wavelengths). Apochromatic correction tends to be more important for long-focus lenses.

Aspherical elements allow correction for other aberrations, in particular spherical aberration. Spherical aberration can be corrected by combining different glasses, as with chromatic aberration. However, full correction may not be possible by combining elements with spherical surfaces, hence the use of aspherical surfaces.

It is often the case that short focus (wide angle) lenses need aspherical elements and long focus lenses benefit more from apochromatic correction.

All the above is highly simplified, and please do not quote me.

Best,
Helen

 

[Garbz]

The Aspherical element also is used to correct for some barrel distortions.

 

[Alpha]

I'd also like to add that the APO designation often, but does not necessarily mean that a lens is great, though it will make it better than the same non-APO lens from the same manufacturer. For example, Vivitar makes a 70-210 APO that is actually really crappy. To say that it's better than the regular 70-210 doesn't say much.