In article , android
wrote:

Why does my macro lens (Tamron SP90) open up to 2.5?

Does it ? as it seems to state f2.8 on the web.

It could be that the firmware of the camera has access to the lense T
value in its database. PC uses a fairly modern EOS 6D mark II, IIRC.
There is according to DxO Mark two different Tammies 90SP one T/3.0 and
the other, newer T/3.3. The latter would be more like 2.5 rather than 2.8
if the former is used for reference... HTH!

or it could actually be f/2.5
http://www.adaptall-2.com/lenses/52B.html