I'm a bit more than a bit bored with calibrating tractors. I am not an expert with digital imaging, but I know a little bit more about engineering, so thought I would explain in diagramatic terms how the shutter works on your camera. 1st Diagram - shows the shutter at rest with no light getting through to the sensor. 2nd Diagram - as the shutter fires the 1st curtain begins to open and allows light through. 3rd Diagram - 2nd curtain begins to close shutting off light to the top part of the sensor, 1st curtain is open enough to allow light to the middle of the sensor . 4th Diagram - 1st curtain is fully open allowing light to bottom of the sensor, 2nd curtain is blocking light to middle of the sensor. 5th Diagram - 1st curtain begins to close blocking light to the sensor and they both return to rest position. All this happens very quickly sometimes fractions of a second. As you can see the light from the lens actually travels across the sensor (or film) in a horizontal band that moves down, not all the sensor (film) is exposed to the light at anytime. Two curtains are necessary because it would be physicaly impossible to get one to open and shut quick enough for modern cameras. In real life the curtains are made of very fine leaves of material, often metal, in some high end cameras of carbon fibre composite. They are controlled by tiny magnetic pulses which will open and close them very quickly.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html