Lenses are often said to incorporate u201clow dispersionu201d glass but what exactly is dispersion and why must it be minimised?
In an earlier ‘blog I wrote about apochromatic lenses, which are designed to bring all three colours of light to a focus at the same distance from the lens. That explanation overlooked the cause of the different-foci problem, which is dispersion.
When light enters a lens it is bent but the amount of bending depends on the colour of the light. White light contains a mixture of colours and each of these bends a different amount: this is called dispersion and it is the effect that allows a glass prism to project a spectrum of colours onto a wall when sunlight is shone into it. It is also the same effect that creates a rainbow when sunlight is dispersed within raindrops in the atmosphere.
The difference between the amounts of bending is a measure of the dispersive power of the medium. The medium is for creating a rainbow is water and the medium in a lens is glass. There are different types of glass and a type that separates the colours by a greater amount would be said to have high dispersion whereas a glass that hardly separates the colours at all has low dispersion. In all cases, blue light is bent the most and red light is bent the least but the amounts of bending vary with the type of glass that is used.
Although it can be helpful, in terms of reducing chromatic aberration, to manufacture lenses using low dispersion glass this is not the end of the story. In fact you don’t necessarily need low dispersion glass at all if you can arrange for the colour-separation that is created in one part of the lens to be exactly matched by a complementary colour-combining effect elsewhere. This is fairly straightforward for a prime lens but it can become much more difficult when designing a compact zoom, where the large amount of bending needed to make the lens compact also risks introducing greater colour separation due to dispersion.
As you will probably be starting to deduce, the use of low dispersion glass is something that may indicate a manufacturer’s desire to produce a top quality lens but it may also suggest that the lens has been ambitiously designed and could need extra help to achieve the same image quality as a previous lens achieved with less effort. In short, it isn’t necessarily true to say a lens that features low dispersion glass will be better than another that doesn’t, nor to say a lens that lacks low dispersion glass must also be a lacking in terms of image quality.
When it comes to lens performance, labels can help but only live testing will reveal the true level of image quality in practice.