Canon Speedlite 155 A
The Canon speedlite 155A is an automatic electronic flash developed especially for AE flash photography with the Canon AE-1. It also makes AE flash photography possible with the A-1 and serves as an ordinary computer flash with the Canon AT-1, AV-1 and other cameras. When the A-1 and AE-1 is set for automatic exposure photography and the pilot lamps lights up, the epoch-making system of the 155A functions to automatically adjust the camera's shutter speed to the X synchronization speed as well as the aperture to the prescribed aperture. The exposure is automatically determined by the amount of light reflected back from the subject. This flash employs a unique averaged light sensing system which reduces excessive reflections from the central area to give a better overall exposure. Thus, the 155A offers more appropriate exposure than do other current flash units. An outstanding feature particular to the 155A is the automatic switching from flash photography mode to the normal AE photography mode when the flash is not charged providing  the AE exposure is within the AE coupling range. In addition, the speedlite 155A can be used with the Power Winder A or the A-1 for sequential flash photography.
Type: Electronic flash unit with a series control system
Contact: Clip-on type with a directly coupled contact. With lock mechanism.
Guidenumber: 17m or 55ft. (ASA 100). The guide number is obtained 30 seconds after the pilot lamp lights up when new batteries are loaded
Flash coverage Angle: Adequately covers angle of view of any lens with a focal length of at least 35mm on a 35 mm camera.
Flashing intervals: (Interval between firing of a flash and relighting of pilot lamp using new batteries)
Alkaline-manganese batteries: less than 7 seconds
Ni-Cd batteries: less than 5 seconds.
Number of flashes: (When the flash is fired at intervals of 30 seconds with new batteries loaded)
Alkaline-manganese batteries: More than 300
Ni-Cd batteries: More than 90
Color temperature: The equivalent of average daylight. Correction made by a Canon-exlusive green tinted diffuser
Flash duration: 1/1000 sec. to 1/50000 sec.
Flash Control system: By measuring the amount of light reflected back from the subject. A series control system saves unneeded energy. The flash offers an averaged light sensing distribution.
Auto/Manu: Switch: Achoice of three differnet positions: twp Auto positions (red and green) and a MANU position. The red position is f/2.8 at ASA 100 and green position is f/5.6 at ASA 100.
Auto working range: 0.5 m to 6 m with the AUTO/MANU. switch set to the red position and 0.5 m to 3 m with the AUTO/MANU. switch set to the green position.
ASA Film speed:  ASA 25 to ASA 800
Aperture scale: f/1 to f/32
Distance scale: 0.5 m to 15 m or 2 feet to 40 feet.
Power source: Use four size AA alkaline-manganese batteries (AM-3). Ni-Cd batteries can also be used.
Pilot Lamp: When charged, the pilot lamp lights up automatically switching the camera over to flash photography mode. There is no flash if the pilot lamp does not light up. This lamp goes out when the main switch is turned off. The pilot lamp also works as a flash testing button.
Size: 70 mm x 51 mm x 105 mm (2-3/4" x 2" x 4-1/8")
Weight: 300 g (10-9/16 ozs), including batteries
Accessories: Synchronisation cord A (Sold separatly) and Case