Tv, on most cameras is the designation for Shutter Priority mode, a mode that will allow you to select a shutter speed that stays constant and forces the Camera to select an appropriate aperture to compensate. Well, I was wondering today why it was labeled Tv?
So I googled. The answer is it stands for Time Value. The little “v” in all those designations stands for value. For instance, the Av mode stands for Aperture Value and allows you to select a specific aperture (that would be the diameter of the hole that opens up to let light onto the film/sensor) and adjusts the Time Value/Shutter Speed to compensate for a optimum Ev (Exposure Value)
So what about the Ev setting? Well it does get complicated. Before I get into the complicated part, the easy answer is that on most cameras, the Exposure Adjustment option labeled Ev+/- allows you to change what the camera thinks is the optimal exposure, and does this in either 1/3 or 1/2 stop increments. Now onto some of the complications. ;)
It seems that all these (X)v acronyms came from a system of determining exposure called APEX which stands for the Additive System for Photographic Exposure. Now, APEX is interesting, and you should read up on it. But the gist of the whole thing is that all the elements that determine an optimal exposure (Av, Tv, Sv) typically move in 2:1 scale, i.e. they double when going up and halve when moving down.
Aperture value goes from f/1.0 to f/1.4 to f/2.0 (this will make sense if you google how f/stops work, the gist of which that an f/stop measure the diameter but doubles/halves the area, thus the odd numbers in the scale). Time values go from 1/2 to 1/4 to 1/8 and so on, and Sv (the Speed Value of film, usually measured in ISO) goes from 50 to 100 to 200 and so on.
What does that mean? Well, it means that all three elements are moving on the same scale, which means that reducing the ISO from 200 to 100 will give you a full stop of exposure and to compensate you could either increase the Av (aperture value) one notch, say from f/2.0 to f/2.8 or increase the Tv (Time value) from 1/100 to 1/50.
There is a bunch of other things that APEX can tell you, but not much of it matters of a practical level to the amateur. You can read about it from Douglas A. Kerr who wrote a pretty easy to understand guide to APEX if you like. I recommend it.