A camera’s flash synchronizes the shutter so that light passes through to the film or image sensor. Cameras with mechanical shutters use an electrical contact within the shutter, which closes the circuit at the appropriate moment in the opening process. As someone who shot part of a job, back in the film days, with a blown sync switch this produces no photos. Most just call it Flash sync is different for every camera and that’s the critical part.
In order for the firing of an electronic flash to coincide with the shutter opening and allow light to strike the sensor, the shutter speed must be set at or below the maximum sync speed for your camera. If it’s set faster, only the ambient light—not the flash—will be captured by the image and in low light settings you may only see part of the photograph that was exposed by the flash. The dark background in the above somewhat hides this faux pas but it’s not vignetting. Exposure was 1/60 sec at f/11 but I needed to shoot it at the Pentax 6×7’s sync speed of 1/30 sec.
This was more of a problem with film cameras when we had to wait for the film to be processed until seeing you only had half a photo but it can be a problem when using flash for fill in high ambient light conditions. Most cameras have the ability to magnify the image on the LCD screen. Use it when making portraits to enlarge part of the photograph where the subject’s face is to see if their eyes are open and if the flash is doing its job. Some cameras may blink a warning when a shutter speed exceeds the sync speed. If your viewfinder display is blinking don’t make a photo until you know why.
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