Why is part of my image underexposed when using flash with shutter speed faster than sync speed? [duplicate]

Question

This question already has an answer here:

• What is sync speed? [duplicate] 3 answers
• Why doesn't a shutter speed of ¹⁄₂₅₀th freeze motion when a flash of that duration does? 5 answers

Hardware

• Neewer TT560 SpeedLite
• Cannon T6i

Shutter Speed

• 1/200 shows full photo
• 1/400 shows 90% photo
• 1/640 shows 50% photo
• 1/1000 shows 10% photo

From what I can tell, the shutter is faster than the flash. I tried modifying the external flash settings but I get this message:

Flash Control > External flash func. setting

This menu cannot be displayed. Incompatible flash or flash's power is turned off.

Question

How can I get the shutter to time better with the flash if I'm not able to modify the settings? I've tried modifying the settings when the flash is powered on and attached to the hotshoe as well as when it's not attached.

Answer 1

The subject of your question is flash synchronization. A little background information will help you understand.

Your camera sports a “focal plane” shutter design. This mechanism uses a curtain, not unlike a window shade to cover the entire image sensor chip. Thus its normal state is closed and since it is opaque, it prevents light from the lens from playing on the surface of the image sensor.

When you press the “go” button, the shutter curtain begins its journey. It travels the span of the image area. Now the curtain contains a slit. This slit uncovers a portion of the imaging sensor. That portion is the actual width of the slit. When the shutter is set high, say 1/1000 of a second, the silt width is tiny. The image sensor only sees light from the lens through this slit. The slit travels the entire distance exposing all entire image area but the travel time is longer than 1/1000 of a second. The shutter speed is the time it takes for the slit to travel just its width.

As you change shutter speeds, you are actually changing the slit width. Each time you slow down the shutter you are actually enlarging the slit width. On your camera at 1/200 of a second the slit width is just wide enough to expose the entire image area to the light from the lens.

When using an electronic flash, the duration of the flash is super quick. If your shutter is set faster than 1/200 of a second, the blitz of the flash catches an image of slit. Sorry to report that if you choose a shutter speed greater than 1/200, the result will be partially uncovered image area.

By the way, we call the setting you need “X” synchronization. In the early days of flash bulb photography, a voltage was applied to the flash bulb in advance of the opening of the shutter. This setting was called “M” synchronization. The “M” is for medium delay which was 20 milliseconds in advance of the shutter reaching full opening. Since electronic flash requires no delay, this zero delay was labeled “X”. Check your camera manual, you will find that your camera works with electronic flash at the “X” synchronization setting of 1/200 of a second shutter speed or slower. These slower shutter speeds provide a slit width that uncovers the entire image area.

Answer 2

At high shutter speeds, the closing shutter curtain chases the opening curtain across the frame, so the shutter is never 100% open. The flash has a very short duration and this means that while the flash is lit only part of the frame is visible through the shutter.

The exact shutter speed that you'll start to see this issue, depends on the model of camera - but it's often around 1/200s (which matches your observations)

If your flash has a "High speed Sync" mode, then you can try using that - this will lengthen the duration of the flash to be lit for the entire shutter duration. This will allow you to use a higher shutter speed, but the exact upper limit will depend on how good the timing of the flash burst is - this depends a lot on the model of flash you're using.

Another relevant question How does high speed sync mode work?

Answer 3

Despite your photographic "evidence" to the contrary, the flash is probably faster than the shutter; however, the synchronization between the two is not timed optimally for faster shutter speeds.

You don't have much choice modifying the settings to better synchronize the two. You'll have to leave the shutter open for the flash to occur before you close the shutter.

Metaphor: You use the same technique when you want to photograph lightning strikes. This time the "lightning strike" is your unsynchronized flash.

The technique normally used when this happens, is to use any shutter speed that will give you an illuminated frame and adjust the aperture (and ISO?) to control the exposure according to the demands of the flash as your main source.

In effect, the flash duration will determine the exposure time rather than the shutter.