Problem
thats because the scene u are shooting is very dark.
So suppose now u are using iso 200 and Apeture priority mode in ur camera. When u shoot at the model, u will aim ur camera at the model rite? when u aim the camera at the model, the camera will automatically determine the optimum shutter speed to shoot the model properly.
As the scene is very dark, very likely u will be forced to use shutter speeds of 1/15s or 1/3s or 1s. At these slow speeds, human handshake will cause ur photos to be blur. and the WORST thing is, the model is moving!
Model movements + human handshake + slow shutter speed = TERRIBLE DISASTER !!!
Solution
there are 2 solutions to ur problem.
a. the zero-cost option.
increase the iso sensitivity of ur camera. For example as mentioned earlier, i assumed u are shooting at iso 200. If u increase ur camera to iso 800, the camera will have an increased of light sensitivity of 4 times. iso 200 to iso 400 = twice the light sensitivity. Then from iso 400 to 800 = another twice the light sensitivity.
what does increase in light sensitivity means? Now suppose u are shooting at iso 200 and apeture priority and the camera determined that the shutter speed to use is 1/15s to shoot the model properly. if u increase the iso to 400, u will be able to shoot at 1/30s. and if u further increase ur iso to 800, u will be able to shoot at 1/60s. For ur 400D camera, u can go to iso 1600 or iso 3200. Meaning, u can shoot at 1/120s or 1/240s !!!
As u can see u can shoot at faster and faster speeds, thus ur handshake problem will be much much reduced.
b. the more expensive option.
For this expensive option and the better option, buy a fast lens. normally a fast lens are those at apeture of f/2.8 or bigger.
Suppose u are using a lens that is f/5.6. A f/4 lens allows 2 times more light to enter the camera, while a f/2.8 lens allows 4 times more light to enter the camera!
So now suppose u are shooting at iso 200, apeture priority, f5.6, 200mm and the camera determined that the shutter speed to use is 1/15s to shoot the model properly. IF u have a f/4 lens, u will be able to shoot at 1/30s while maintaining at iso 200 and 200mm. BUT IF u have a f2.8 lens, u can shoot at 1/60s !!!!!
SO how do people shoot in dark scenarios?
They normally use high iso (800 or more) and use a fast lens (f/2.8 or faster)
for example, if u use iso 800 and f2.8 for the above scenario, u will be able to shoot at 1/240s.
IF u use iso 3200 and f2.8, you will be able to shoot at approximately 1/1000s, which is very very very very fast.
Note:
High iso maybe attractive, but it does have drawback. Generally at high iso (800 and above) u will start to see a lot of grains and the picture become noisy and ugly. Thus the best option is to shoot at low iso and use a fast lens.