FujiFilm Finepix S8500. It is capable of shooting high FPS videos. It works fine in broad daylight but when indoors, the video is always dark. What settings I can adjust to make the video brighter? I tried to adjust ISO and aperture but that does nothing. I also notice that the videos are darker if FPS is higher. I understand why this happens but is there anything to get a brighter video?
1) Larger aperture (the lower the number the larger the aperture) letting in more light, but also narrowing your depth of field).
2) I would suggest a higher ISO, don't understand why that does not work for you. The current crop of camera's now are absolutely amazing at capturing low light environments that you cannot see with the naked eye.
One of the best cameras for this is the Sony a7s, just released (s for sensitivity).
It needs to be seen to be believed.
The best way is to add more light, I see you mentioned not having the equipment to do so. There are a couple of home hacks to get around that.
1) Remove lamp shades and use the highest wattage bulb you can. I also prefer the clear glass bulbs sometimes with a diffuser over them. I find that tends to be brighter than the hazed bulbs but I have official evidence to prove this just experience.
2) This is my favorite home light hack... Go to home depot and buy your self the the double halogen work light with stand. These work great for video as they are bright, easy to move around, come with a rotating head and are on a stand already, not to mention the best part, they are cheap!
3) Use halogen track lights in the room. This is also a way to get a nice light array for not a ton of money. Again they are super bright, easy to move around, just mount the track to a piece of 2x4 and you have a mini light array right there.
Reasons not to use a higher ISO: In all cases I avoid increasing the ISO unless absolutely necessary. A higher ISO will increase noise and there are no 2 ways about it. I always exhaust my other options first, aperture, exposure time and simply turning on a light if I can.
Only with a larger aperture to let in more light if a higher ISO isn't helping. Though the camera probably already uses a very large aperture for this high frame rate mode. The more FPS the less light per frame obviously.
Your only other option would be to adjust the brightness in post which is a very limited option as it increases noise and can introduce artifacts but it might improve the footage a little bit. This usually works better if you can shoot in MJPEG instead of h.264/AVC. Try to shoot with the highest bitrate possible to have the biggest flexibility in post.
Usually you want to shoot with plenty of light when filming with a high frame rate, especially in-doors.