You're making a mistake. In fact, it would greatly appreciate a summer vacation in full sun, as long as you acclimate them gradually.

The leaf loss in your tree is pretty much assured because it sounds like you cannot provide adequate light & the transition from bright light outdoors to dim light indoors causes leaf loss. Since this seems inevitable, wouldn't it be better for the tree to be robust & full of energy after a summer in the sun as it goes into winter, instead of just limping along because you had it growing in the shade?

Plant's don't really adapt to much of anything. They're genetically encoded with limits to what they can tolerate in the area of cultural conditions. The plant will tolerate various adverse cultural conditions as long as it is able to produce more energy than it uses. If this is reversed, and the plant begins operating under stress, using more energy than it produces, it leads to strain. Strain, uncorrected, will cause death of the organism.

From something I posted on another forum:

"Well, there is an abscission zone at the base of each petiole (leaf stem) that allows water and nutrients to flow freely from roots to leaves and for photosynthate (the plant's true food) to flow freely from leaves to sinks (areas in the plant that are "calling" for more food so that they can grow or store it for further use).
There is a growth regulator (or hormone), auxin, produced as a product of plant metabolism, primarily in the apices (growing tips of branches) and leaves that must flow across the abscission zone to keep it open. If photosynthesis/metabolism is slowed due to a decrease in photoperiod or light intensity, the flow of auxin is slowed and a corky abscission layer forms at the base of leaf petioles. This corky layer is impermeable (a barrier) to the flow of water/nutrients and food, so the leaf dies and separates from the tree at the abscission layer. This is part of the mechanism by which deciduous trees' leaves change colors and then fall (abscise/abscission) during autumn in temperate regions.

We frequently talk of leaf abscission in Ficus trees as if it's a great mystery, but technically, anything that slows photosynthesis and the flow of auxin can cause leaf abscission. Chill (particularly sudden chill), lowered light levels or exposures, wet or dry soil, and other stress agents that affect photosynthesis all cause leaf abscission.

Often, people will tell us that any cultural change will cause a Ficus to shed foliage, but not so. Changes like increased light, warming temperatures (when it's been cool), better soil, etc., things that make the plant want to grow with better vitality, do not cause leaf loss. Improving conditions will encourage better growth and more biomass."

Al

I have a bright southern sunroom in which to put it in. However, the problem remains. The tree gets southern and eastern light, as well as a grow light, as well as four skylights. It still sulks and drops leaves. I have since moved it to an eastern window which also gets southern light from another window (not the sunroom.) It seems fine now. I just want it to find its equilibrium.