Why do I see a shadow or ghost image when using my Canon 70D’s built-in flash?
Asked 3/29/2016
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I shot a RAW photo with a Canon 70D and the 18-55mm IS STM using the built-in flash at 1/80s. The subject seemed still, but the photo shows a shadow or ghost-like duplicate. What causes this, and how can I avoid it?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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It is either subject motion or camera motion. 1/80 second is pretty slow for human subjects not making a specific effort to be still. It is also pretty slow to use hand held if you are going to pixel peep at 100% magnification.
The flash is much shorter in duration than the shutter, so the motion that is only illuminated by the ambient light is much dimmer than the light that came from the flash and froze the motion.
For more about how Av exposure mode with automatic flash exposure (TTL) works in many cameras including pretty much all Canon SLRs, please see this answer to How do I get my Canon 60D to use short shutter speeds with flash in Av mode? as well as this answer to Why is flash TTL metering independent from ambient light metering?
The way Canon (as well as many other) cameras behave when using TTL flash with Av exposure mode is called slow speed sync or dragging the shutter. For more on slow speed sync please see this answer to What is "Dragging the Shutter"?
For more on how Canon's TTL automatic flash exposure systems works, please see this answer to How do TTL flash metering systems calculate how much power is needed?
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
0
Generated from our catalog & community — verify before relying on it.
This is most likely motion blur/ghosting from a mix of flash and ambient light, not a sensor problem.
At 1/80s, the shutter is open long enough to record some movement from either the subject or the camera. The built-in flash fires for a very short duration and effectively “freezes” one moment, but the ambient light continues exposing during the rest of the 1/80s. That can create a faint second image or shadow around the sharper flash-lit subject.
So the sharp part is from the flash; the blurry/shadow part is from movement recorded by ambient light.
To reduce it:
- use a faster shutter speed (up to your camera’s normal flash sync speed)
- lower ambient exposure if possible
- ask the subject to stay very still
- steady the camera better or use support
- check flash settings/mode if you’re in Av, since some Canon cameras choose relatively slow shutter speeds with flash in that mode
Image stabilization helps with camera shake, but it will not stop subject movement.
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