I also think your camera will expose the 500T at 250 asa. The film should already be notched correctly, but since you camera only reads till 250 asa your film is getting 1 stop overexposure. The 200T reads at 160 asa so it gets 1/3 extra light. When you set the filter to daylight, the filter takes about 2/3 stops of light. Because of the light loss of the filter the effective film speed is only 160 asa, but the film is still exposed at 250 asa; the camera will open the aperture 2/3 stops to compensate. I would't worry thought. The film will easily tolerate 1 stop more light. The grain is finer when you overexpose film. When you underexpose film and/or push process it, the grain will be more obvious. I think the reason your footage came out grainier in low light is because there was just not enough light, so the camera keeps the aperture wide open but beyond that it cannot let in more light, so the film may actually been exposed at 1000 asa. You can only get more light on film by shooting at lower fps. Or setting up some lighting. Hope this helps.