Longshadow wrote:
I've never used square filters. Are the edges covered/coated so light can't get in sideways and bounce around inside the filter?
Irrelevant. The filters, as well as the lens are all in front of the shutter. The shutter would need to be lose or leak before any leakage would affect the sensor.
This looks much more like a light flare on the front element. That is most easily cured by using a lens hood or not shooting into the sun.
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