Flash and the beauty of no synch speed.

Top:  no flash was used.  Bottom:  built-in flash used.

I have spoken on the merits of fill flash, and will likely do it again because of how much it improves an image, even when its use is not mandatory.  Today I want to talk about the difference in using a DSLR camera which is limited by something called a flash synch speed and using a non-SLR-type camera which is not.  Normally I would sing the praises of the DSLR because of the larger sensor and the ability of it to take accessories such as other lenses and external flashes.  This is one place where non-SLR cameras have an advantage.

To understand what a flash synch speed is, you first have to comprehend why it even exists.  DSLR cameras use a mechanical shutter to allow the camera's sensor to be exposed.  It is a physical mechanism which opens and closes using "curtains", which block light when present in front of the sensor.  When a photo is taken, the first curtain travels across the sensor plane leaving the senor open for an exposure to take place.  The first curtain remains open until the exposure is complete, then the second curtain moves and finishes off the job.

They travel independently of each other as long as the shutter speed is at or below the flash synch speed.   This tends to be between 1/180th to 1/250th of a second, with most cameras using 1/200th of a second.  At shutter speeds faster than the flash synch speed, the shutters move in tandem, the space between them guided by the shutter speed.  Faster shutter speeds require that the two shutters are closer together.  To see a video on this, click here.

The problem starts when flash is used.  At shutter speeds at or below the flash synch speed there is no problem because the entire shutter is open.  At speeds above this, only a portion of the shutter is open at any moment and the flash will illuminate only a portion of the image.  To prevent this your camera will limit the shutter speed when the flash is popped up or an external one is attached.  If you use an off-camera flash which is not dedicated, the camera will not be aware of a flash.  This means you can go above the flash synch speed, but with a consequence.

Enter non-DSLR cameras.  Compacts usually use non-mechanical shutters.  They are electronic in nature and use no moving parts.  They do not have a flash synch speed and so higher shutter speeds can be used in situations involving flash.  This is where the photos above come in.

Both shots are done with an ISO of 80, an aperture of f/3, and a shutter speed of 1/1000 of a second.  The top photo is not an issue as no flash was used.  The bottom image does involve flash, however.  The camera was a bridge camera; it is an all-in-one camera that uses an electronic shutter instead of a mechanical one.  A shutter speed of 1/1000 of a second is not a problem.  The question you may have at this point is, "So what?   Why is it a problem?"

The short answer is aperture.  It was very bright, and the shutter speed of 1/1000 of a second allowed me to use the low aperture of f/3.  This means the flash can further and impact shadows a greater distance from the camera.  With a shutter speed of 1/200th of a second, my minimum aperture would have been around f/7.  Light travels less than half the distance at that aperture, and it is likely there would not be enough of it to properly illuminate the scene.  The built-in flash of a DSLR camera would not be able to work to its full potential because of the limitations of flash synch speed.  There are two ways around this though.  One is to use an external flash on a DSLR or to use a feature called high flash synch speed, which is not available on all cameras.

video link:  https://www.youtube.com/watch?v=v3SjH_Ja_E8