If I take an indoor shot with f/3.5 and 1/400 shutter (some ambient coming in), I need at least ISO 3200 for a proper exposure.
But then for an indoor shot of an interior room with just the overhead light on (zero light from outdoors), I need ISO 12800 for a proper exposure.
Your camera has a maximum of ISO 3200. And you most likely wouldn't want to use that as it would have quite a bit of noise. But, I now suspect the reason for your dark images is that your camera just cannot compensate for that 1/400 second shutter when shooting indoors. I also suspect flash isn't allowed. So your only option is to drop shutter speed. However, if you're zoomed in, your lens' aperature will close down to as much as around f/6.5 which almost 4 times less light coming in.
Do study up on the Exposure Triangle to get a better understanding how shutter, aperature and ISO are all interconnected.
I do feel your pain... for my first point-and-shoot cameras, I ended up with either blurry and/or noisy images when wanting to shoot anything indoors where flash wasn't allowed. Sometimes the only way to maximize your options for capturing images with existing (ambient) light only is moving up to better equipment. For example having a lens that could let in much more light. And cameras with more ISO options and lower noise even for high ISO values.
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Ricky
Camera: EOS 5D IV, EF 50mm f/1.2L, EF 135mm f/2L
Lighting: Profoto Lights & Modifiers
