Which one is better? increase ISO from 6400 to 10k or push aperture to F1.8.
It depends on what look you want. If you don't mind a more shallow field of focus, then opening the aperture wider would be better because it would allow a faster shutter speed and lower ISO which equals less noise/ cleaner picture. If you need more of your shot in focus, then you'll have to push the ISO up along with the F-stop. If the 6D's noise performance is anything like the 5D3, then 10K ISO should still produce very useable pictures. Use what you need to get the shot you want.
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