F-Stop vs. T-Stop: What's the Difference?

With the Canon 85mm f/1.2, for example, you really can’t see a whole lot in the way of lenses inside and it’s quite an expensive lens, but you’re still losing a little light. This lens has a t-value of 1.4, which means that even though it will open up in terms of aperture, with an f-number of 1.2, you’re losing .2 on the way through the lens.