The longest possible exposure is at ISO 160. As the ISO increases, the maximum exposure time decreases. I don’t think there is a way to get more light into sensor than 125 seconds at ISO 160 or the equivalent combination if time and ISO rating. Also, the resulting images look pretty much the same as far as noise goes – regardless whether it is long exposure/low ISO OR short exposure/high ISO. I discovered this while trying to photograph fireflies in the dark.:)
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