Please help my little ficus!

Almost as soon as I bought it, the leaves started getting tiny brown dots that grow into brown circles and eventually envelop the whole leaf. When the spots are big enough, a sort of greenish tinge can be seen at the center. I have no idea what it could be; my first thought was that it was some kind of fungus, but I wanted to see if something else was causing it before jumping to any conclusions. Over the course of several months, I experimented with the amount of watering and light it received. Nothing has completely solved the problem, and the leaves drop, one by one, after the brown "blight" takes over each in turn.

Most of the large leaves have now fallen off, but there is new growth at the top; the leaf drop has slowed down a good deal since it started occurring. However, the brown areas are still there and take over the leaves (although much more slowly than at first). Some of the new leaves are developing the tiny spots, so clearly the problem persists.

Please tell me what this is and how I can save my plant before it's too late!

Series of photos of the blight when it started causing leaf drop a few months ago:

Photo 1

Photo 2

Photo 3 (Probably the clearest picture of it)

Any help would be greatly appreciated. It's a beautiful plant and I'd love to make it healthy!