F and T aperture settings

[Father Ted]

I have very recently switched from Canon to Sony A7R111. I bought 3 lenses, a couple of zooms which cover most of the range I want plus, the f2.8/100 stf gm prime, this mainly as my portraiture lens although I will also use it for sports and dance

I was confused by this lens as the biggest aperture you can set is 5.6. I then read the blurb (which I hadn't done before!) and find that it says that the biggest aperture is indeed 2.8 but the lens is built such that this only lets in the same amount of light as 5.6. 

I will generally use it at the biggest aperture, and I suppose as the finder effectively always gives a DoF view I can just shoot and ignore what the camera says the aperture is

but i'd be interested to hear from anyone who uses this lens, why it is built this way and how they use it e.g I understand the difference between the f stop and t stop reduces as you close it down but I don't know by how much......

[2xbass]

The lens is built that way in order to get particularly smooth bokeh. The apodization filter is basically a radially graduated ND filter that is darker on the outside of the filter so but clear in the centre. This means that once you stop down the lens where the aperture is smaller, the filter is effectively not being used since it’s only seeing through the clear part of the filter and it behaves like any other lens. However when shooting wide open the filter is used and since it’s darker around the edges it reduces the total amount of light coming in. Usually you would only buy this lens knowing exactly what you are getting and why you are buying it as it has a very specific look which some people love and others hate but the loss of light is the price you pay.

Edited April 27, 2019 by 2xbass