Ideally, you'd measure from real keypress upto visible light; This, I think. When measuring latencies for e.g. reaction time experiments for neurophysiology I'

Ideally, you'd measure from real keypress upto visible light; This, I think. When measuring latencies for e.g. reaction time experiments for neurophysiology I'd always use a photodiode or similar to figure out when things actually get on the screen as that's the only thing which matters. IIRC this was always with V-Sync on, but still with custom DirectX applications it was impossible to get lower than 2 frames of latency on Windows 7. So it has always been a bit mistery to me how these articles talk about latency of keyboard to screen of < 1 actual frame. Maybe this explains it, i.e. they're not measuring actual latency from keyboard to screen? I just don't know enough about graphics but even with V-Sync off I'm not sure you can just tell the GPU 'hey here's a pixel now put it on screen right now'?