In this article we look at the low-dispersion lens element, providing a guide to what it is for and how it works

low-dispersion-lens-elementYou will know the low-dispersion lens element by various labels, such as ED, ELD, SLD, UD and other acronyms. It is a low-dispersion glass lens element. ED and ELD stand for Extra Low Dispersion, and S stands for Super or Special. Here are some examples that incorporate low-dispersion glass elements, identified by the ED tag in their names: AF-S Nikkor 18–35mm f/3.5–4.5G ED and Olympus M.Zuiko Digital ED 12-40mm F2.8 Pro.

Not all lenses reveal their use of low-dispersion glass, like the Sigma APO 70-200mm F2.8 EX DG OS HSM or the Canon EF 300mm f/2.8L USM, for example. The former incorporates two FLD glass elements, that Sigma claims have performance equal to fluorite glass, and three SLD (Special Low Dispersion) glass elements, while the latter uses both UD (Ultra low Dispersion) and fluorite elements.

So what does the low-dispersion lens element do?

All types of optical glass disperse light in varying degrees. This is the separation of colours, ably demonstrated by a prism, caused by the different phase velocities of the waves representing different colours. This is bad for lenses because it causes chromatic aberration, where the different colours end up being focused at different positions compared to their origin. This causes unsightly colour artefacts that can reduce sharpness, resolution and colour accuracy.

Low-dispersion glass can be used to minimise the mis-registration of colours via an optical system. Some low-dispersion glasses ensure the two primary colours can be focused accurately and more exotic low-dispersion glasses can correct the projection of all three primary colours.

Telephotos have mostly benefited from low-dispersion glass, but with their increased complexity, low-dispersion glass is now widely used in standard and wideangle lenses today. Low-dispersion glass is employed even in relatively inexpensive lenses, and when used in conjunction with aspheric lens elements it can give impressive image quality compared to older lenses.