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15. There are three different conditions in which a flash picture can be taken:

1 - In high ambient light.

2 - In low ambient light with the subject near the camera.

3 - In low ambient light with the subject far from the camera.

16. Basically, the Model 636 uses either a light measurement alone to fire and shut down the flash, or a combination of light measurement and time measurement to fire and shut down the flash. The method used depends upon scene brightness.

17. After the flash is charged and the exposure button is pressed all the way in, three things happen simultaneously which decide how the flash is going to be controlled:

1 - The solenoid releases the shutter blades and they start to open;

2 - The photocell starts to measure scene light;

3 - An internal clock in the camera logic is started.

18. From this point, there is a race between the internal clock and the photocell to decide which is going to control the flash. Either the internal clock will time-out before the photocell measures a pre-determined amount of light or vice versa. Whichever occurs first will fire the flash.

19. Now, lets go back to the three conditions in which a flash picture can be taken. The first is in high ambient light. In this condition, there is so much scene light available that the photocell always wins the race to fire the flash. This is the Fill Flash mode of operation.

20. In the high ambient or Fill Flash mode, the photocell measures scene light. When it sees 75% of the total light required for a proper exposure, the photocell fires the flash.