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I'm not sure how else to put it, but it is what it exactly sounds like. When I take consecutive pictures in Aperture priority mode, it so happens that the first image is probably brighter (high flash) and the second one is the right level and the third one is a lot low. It's not exactly in the same order, but I hope I explained it correctly. The flash intensity level doesn't stay the across consecutive pictures (same settings, light level, etc, etc). Can someone help me understand what am I doing wrong?

Adding some more information: Flash was in TTL mode. The pics were taken one after another with a gap of may be 4-5 seconds.

  • Body - Canon 6D2
  • Flash - Altura AP-C1001

Bright one Dark one

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    How quickly in succession are you taking these pictures? – mattdm Dec 26 '18 at 22:02
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    So is that camera shake from the whiskey you’ve been mixing with that ginger ale? – Hueco Dec 26 '18 at 22:17
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    TTL mode? Fresh batteries? Are you waiting to confirm the flash is ready to pop again? – Hueco Dec 26 '18 at 22:20
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    @deppfx We need more information before we can helpfully answer your question. There are several variables regarding how your camera and flash might be set that could affect the issue you are describing. How is the flash power being determined (manual or TTL)? Do you have 'Flash Exposure Compensation Bracketing' enabled? What (ambient) metering mode are you using? What flash metering mode are you using? Etc. – Michael C Dec 26 '18 at 22:40
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    In my opinion, you need to repeat this test. You were hand holding the camera, and with an object as close as this one, with metallic reflection, any slight change in position will give you a hotspot that the camera will interpret as a different reading. Take a photo of a further back object, not glossy, like a teddy bear or a pillow and use a tripod. If you really want to do controlled tests, do controlled tests. – Rafael Dec 27 '18 at 5:00
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The entire point of using E-TTL flash is to allow the camera to change the amount of flash power used based on changing shooting conditions. When you have a target as reflective as a soda can and you move the camera/flash in relation to that can, you're very likely to get different results.

If you want to test the consistency of your flash from shot to shot, set it at a manual power level to lock in the same targeted amount of flash power from shot-to-shot and use manual exposure control to lock in ISO, aperture, and exposure time. Also consider setting the camera on a tripod to eliminate distance variations between the camera/flash and subject from one shot to the next.

If you insist on using E-TTL flash control:

Some third party flashes are known to have issues with not calculating enough power when used with Canon cameras with the E-TTL metering set to 'Evaluative'. This is the case particularly when the flash head is set in a "bounce" position.

Changing the menu item [Shooting (Red Tab with camera symbol) Tab 1 → External Speedlite Control → ETTL II Flash Metering] from Evaluative to Average usually resolves the issue. You might also try switching main (ambient) metering to Center Weighted Average, Partial, or Spot metering modes rather than Evaluative. (Main metering and E-TTL metering are two different settings in two different areas of the camera's menu).

  • Why would you want me to switch the main metering from Evaluative to Center/Partial/Spot? Can you please explain? – deppfx Dec 27 '18 at 8:40
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    @deppfx So that the camera is metering on your main subject, and not weighting the background elements in the calculation. – Michael C Dec 27 '18 at 9:05

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