Don, I agree that the shutter opens from the center and that part is the last covered on the return trip of the blades. However, consider this. Imagine you are standing in the center of the image plane with a light meter and a friend of yours is standing on the corner of the frame with a light meter. The diaphragm aperture is set to f/5.6. Now imagine that the shutter is closed. Neither of you will register any light. Now imagine that the shutter has opened slightly. Maybe to a point where the opening created by the blades is the equivalent of f/32. Both of you at that time take an incident light meter reading. Note that both of you "see" or are illuminated by the same effective lens area. That associated with f/32. Both meters should read the same. Now the shutter opens some more. Maybe to f/11. Both meters register an increase in light level. It is 3 stops more than the level was at f/32. Now the shutter opens some more. Maybe to f/5.6. Both meters register an increase in light level. It is 2 stops more than the level was at f/11. Now the shutter continues to open further until it gets to its maximum opening. The meters are still only registering the level associated with f/5.6 because the diaphragm limits the amount of light that can pass through the lens. Once fully open the blades stay there for a while depending on the exposure time set. Then they start on their return trip. Once they pass f/5.6 the blades start to diminish the effective aperture of the lens and gradually the light level as read by the meters drops, simultaneously, until all light is extinguished. Will there be a discrepancy between the readings between the two meters metering at the same time? Absolutely. But this is due to the fact that from the corner of the frame one is looking at the circular hole of the aperture from an angle and the nice round circle is not a circle anymore but an oval shape of smaller area than the full circle and so transmits less light towards the edges. Also, note that the distance from the center of the lens to the center of the frame is less than that to the corners, so more or less according to the inverse square law, the light level has to be less at the corners. Focal plane shutters were not developed to correct for uneven illumination at high speeds but because they did offer high speeds and only one shutter was needed if ready and relatively inexpensive lens interchangeability was desired. FP shutters have their own problems, however, with providing an even exposure across the frame! more or less IMO, ;) Andy
