The Sony 100mm f/2.8 lens is just as sharp as all the other 100mm lenses in its class but it has a trick up its sleeve in the form of Smoooth Trans Focus (STF), which gives a more subtle and soft bokeh. This lens is purposely designed for portrait work with a shallow depth of field.
100mm lenses are all pretty optically flawless these days and although the Sony 100mm f/2.8 STF GM OSS isn't a macro lens, it is purposely designed for portrait work by utilising Smooth Trans Focus to get pleasing, soft bokeh in the defocused areas of your image.
Smooth Trans Focus means that there is an internal apodization filter in the lens. This is a filter with a coating on the outer edge which is essentially an ND filter and this causes the edges of the bokeh to blur much more than normal, creating a softer bokeh.
This makes your out of focus backgrounds look amazing but it does come at a cost - the coating on the lens element reduces the overall light transmission through the lens. This means that wide open you are getting an equivalent aperture of f/5.6 and as such the aperture ring is marked as T5.6 to represent the light transmission of the lens.
As this coating is only on the outer edge of the element, the light loss is only in the aperture range of f/2.8(T5.6) - f/8 because as your aperture gets smaller, light passes only through the clear centre of the filter.
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