What I mean is that your effective exposure is not 13 minutes unless the slit uncovers the entire frame for 13 minutes.
For example, if the slit is moving from right to left, your effective exposure is the time it takes for the right slide of the slit to cover the width of the slit. On a large scale, if the slit is one inch wide, and the slit moves at one inch/second, you have a one second exposure. Since this is in front of the lens, the light is spread, so the actual exposure will be a little more.
How much more will require some experimentation. |