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Setting the aperture selection switch to "M" places the 177A in the manual flash mode. The automatic flash circuit is cut off and flash and camera are switched to regular synchronized flash photography. In this mode the shutter speed of each A-series camera is still switched automatically to the X-sync speed of 1/60 sec. (unless the shutter speed is set on"B") when the pilot lamp glows. The aperture must be calculated for each change in shooting distance using the guide number formula and then set manually on the lens' aperture ring. The procedure for manual flash photography is a as follows:
- Set the 177A's aperture selection switch to "M"
- Make sure that the proper ASA film speed is set on the flash
- Focus then subject
- Read the shooting distance from the lens' distance scale
- Calculate the aperture using the following formula: Aperture = guide number / shooting distance
- Set the aperture on the lens
- Turn the main switch ON
- Do not press the shutter button until the pilot lamp glows
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The 177A's guide number is 41 at ASA 100
