Why does lens focus distance affect TTL flash exposure at close range on my Canon R6 Mark II?
Asked 5/3/2023
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2 answers
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I’m using a Canon R6 Mark II with Godox TT350C or TT685C for macro photography, with the flash mounted directly on the camera. When the lens focus distance is below about 1 meter, images become heavily underexposed, almost as if only the pre-flash fires. Just above that distance, TTL exposure works normally and stays consistent when I change aperture while keeping shutter speed and ISO fixed.
I tested this by manually setting focus distance on several lenses. Around the threshold, slightly longer reported focus distances gave correct exposure, while slightly shorter reported distances gave severe underexposure, even if the image itself was out of focus. A fully manual lens does not show the problem, and using a Godox X2T-C wireless transmitter also works correctly at close distances.
Why would the lens-reported focus distance change whether TTL flash exposure works correctly at close range?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
3y ago
2 Answers
1
Another possibility may be that the pre-flash reveals to the TTL algorithm that even the minimum power the flash can be set to will be too bright. So it doesn't fire the flash at all.
You can test this hypothesis easily. Stop down a couple of stops and see if the narrower aperture decreases the distance at which the flash does or does not fire. Then open up to maximum aperture and see if the distance increases.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
3y ago
0
Generated from our catalog & community — verify before relying on it.
Most likely, the issue is in TTL behavior at very short subject distances, not the lens mechanically changing flash output.
From the reported behavior, two plausible explanations fit:
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TTL decides no main flash is needed: At close range, the pre-flash may tell the camera/flash system that even minimum flash power would overexpose, so the main flash is suppressed or reduced so much that the frame looks like only the pre-flash occurred.
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A timing/communication quirk with on-camera TTL: The camera may handle metering or flash timing differently when the lens reports a short distance. Since the problem disappears with a manual lens and with the wireless transmitter, the lens distance information and on-camera TTL communication appear to be part of the trigger.
A useful test is to vary aperture: stop down several stops and see whether the “failure distance” gets shorter; open up and see whether it gets longer. If so, that supports the minimum-power/TTL explanation.
For practical macro work, manual flash power or off-camera/wireless triggering may be the most reliable workaround.
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