segunda-feira, 11 de maio de 2015

The benefits of lens coating on complex design lenses

The other day I was changing some ideas through a internet forum  dedicated almost exclusively to manual focus lenses with a friend, Ian Greenhalgh, and he made me aware about the benefits of a good coating on complex designed lenses. He also made me aware to the fact that chromatic aberration on lens is mostly a design issue - a collateral effect of design decisions - and nothing on lens coating can correct this issue, as coating is intended to deal with lens flare only, as follow:

Hi Renato

When light passes through the air-glass interface some is lost to scattering and reflection. Some of this non-image forming light still hits the film/sensor and causes reduced contrast and flare. Coating reduces the loss of light at this air-glass interface and therefore increases contrast and [minimizing] flare.

Coating is of most benefit in complex designs with many elements. For example, take a double-gauss type lens like a Planar, it has a pair of cemented doublets and a pair of meniscii making a total of 6 elements in 4 groups. It has a total of 8 air-glass interfaces. If we assume a typical loss of 5% at each of those interfaces, you are losing overall, 40% of the light that enters the lens, which has a massive impact on contrast. That is why double-gauss lenses like the Planar only became widespread after the advent of coating.

Chromatic aberration is when the different wavelengths of light converge at slightly different points. Most lenses are achromatic - they focus the green and blue light at the same point, this is why most lenses exhibit red/magenta colour fringes in difficult lighting situations such as bright highlights in direct sunlight. A lens that focuses green, blue and red light at the same point is apochromatic, few lenses that claim apochromatic correction are actually fully apochromatic. In most lens designs, an achromatic doublet with a crown glass element cemented to a flint glass element is used to correct for chromatic aberration. Apochromatic designs use exotic low dispersion glass such as fluorite in order to achieve full correction of CA (or more often, close to full correction).

Source: Wikimedia

Source: Wikimedia

From Camerapedia:

Coating against reflections between lens groups
Modern lenses are coated with a very thin layer of antireflective material (like magnesium fluoride or calcium fluoride). This lens coating is applied to the each element of a lens which has a surface exposed to air. The only purpose of coating lenses is to reduce lens flare by eliminating reflection off the surface of the glass; this has the effect of increasing contrast and giving images more "punch". Lens coating thicknesses are typically of the order of a few wavelengths of light, - a few nanometers (nm).
Lens coatings have nothing to do with color correction of lenses, as is widely thought.

Nenhum comentário:

Postar um comentário