What is Special Low Dispersion Glass? (SLD)

Special Low Dispersion (SLD) glass

Special Low Dispersion (SLD) glass becomes labeled as an APO lens at Sigma. While the glasses refractive index is dependent upon the light's wavelength, color aberration happens when different colors develop images at unique points. This issue often happens with telephoto lenses. When using normal optical glass, correction can only be for two spectral primary colors by combining convex low dispersion and concave high dispersion lenses. All Sigma Apo lenses employ (SLD or the new Extraordinary Low Dispersion (ELD) glass to provide superior sharpness, elevated Contrast and color adjustment by minimizing any chromatic aberration.

SLD Diagrahm

Low dispersion glass (LD glass) is another type of glass containing low dispersion glass elements, the main use being in lenses, everywhere SLD elements minimize any chromatic aberration of an entire lens compilation.

LD glass

Special low dispersion lens glass (SLD glass) plus extraordinary low dispersion lens glass (ELD glass) are lens glasses with even lower dispersion (plus a higher price tag). Other llens glasses in this category are called extra-low dispersion lens glass (ED glass) Extra-low Dispersion" lens glass is employed in reducing chromatic aberration along with ultra-low dispersion lens glass (UL glass). Some lens glasses have a unique property labeled anomalous partial dispersion. They were pioneered by Leitz. for employment in telephoto camera lens assemblies.

Prior the availability of these lens types, calcium fluoride composed of fluorite crystals was the main material used for these lenses; although calcium fluorides low refraction index created a high curvature lens requirement, therefore intensifying spherical aberration. Fluorite also features poor shape preservation and is awfully fragile.

AD Glass


Glass containing added thorium dioxide also has high refraction plus low dispersion was used prior to WW2, although its radioactivity was a factor in it being replaced with other compositions. Even during World Way II, Kodak was able to make high-performing thorium-free glass of optical quality for aerial photography use, although it had a yellow-tint; as it was employed with black & white films, this trait was actually useful for that particular purpose.

Leitz laboratories determined lanthanum (III) oxide is also a suitable replacement for thorium dioxide; although other elements must be added to maintain the amorphous nature of this glass and thwart crystallization that causes striae defects.

Additionally fluorite is used in place of glass in a few high performance camera lens elements and telescopes. Other high-performance optical glass contains high contents of zirconium dioxide; although its high liquefying point necessitates use of platinum layered crucibles to preclude contamination with crucible matter.

Another good high-refraction substitute for calcium fluoride used as a material for lenses is fluorophosphate glass, when a relative amount of fluorides are stabilized using a metaphosphate, with titanium dioxide being added.

The high price of these high-performance glasses are chiefly dependent on the supply of enough pure chemicals in large enough quantities, and on related technological challenges.

XR Technology. Extra Refractive Index Lens Glass employed with Tamron lenses (decreases sizes of the lens). The first major rationale is that the lens becomes very compact by employing high refractive index lens glass. High refractive lens index glass bows light more than regular or lesser refractive index glass lenses. Usually, glass material containing a refractive index over 1.69 is labeled high refractive index lens glass.

ELD Glass

The A03 optical lens system contains glass material featuring a refractive index above 1.69 is installed in the front lens group. Additionally, the refractive index for the entire optical system is augmented. By employing high refractive optical index lens glass of the front lens group, the elements contained in the first cluster are made thinner, and reduces an element of the second group. This allowed the total length of the entire optical system to be shorter.  updated article Jun 16, 2011

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