DS Coating
One of Canon's main goals in lens development is to get better optical performance for both parts of the image that are in focus and parts that are out of focus. This is especially important when trying to get a bokeh effect (an out-of-focus background); the quality of the picture will depend on how well it shows the areas that aren't in focus. Bokeh is a very powerful effect in photography, and one of the best things about lenses with large apertures is that they make bokeh. In general, as a lens's optical performance gets better, the edges of all parts of the image get clearer, even the ones that aren't in focus. In some situations, this outcome might not be a good one. There are times when the sharper edges can mess up the bokeh effect and make the subject stand out too much. Canon learned that photographers would rather have more choices about how sharply each part of the image is rendered when they are trying to create the bokeh effect.
Canon's proprietary Defocus Smoothing (DS) coating, a vapour-deposition film technology, is the result of work to solve this problem. The RF85mm f/1.2L USM, a great portrait lens and a symbol of Canon's optical technology, was the first lens to use this coating. This lens can create a unique bokeh effect that softens the edges of a blurred subject while keeping its high resolution, high contrast, and ability to block out chromatic aberration. From the centre of the lens to the edges, the coating gradually lowers light transmittance, which means that more and more light is blocked. This makes a lens that can make bokeh pictures with soft, smooth edges.
For the RF85mm f/1.2L USM DS, a DS coating was put on the outside to make the effect stronger. The areas of a subject that are in focus have a very high resolution, but the edges are softly blurred. The lens can be used with an aperture setting of up to f/1.2, which lets you capture even more expressions in portraits.
