analog oscilloscope trace brightness

[Belrmar]

my problem is pretty simple and i just want to know if is normal on analog oscilloscopes. Wen reducing the time constant on my proma od-416 the trace brighness starts going down quite abruptly on the <1uS time constants

[Ian.M]

Your Promax OD-416 only has two timebase setting faster than 1us/div.
Once the sweep time becomes comparable to the holdoff time, the duty cycle with which the beam 'paints' the trace decreases sharply.   Make sure the holdoff control is set to as near minimum as possible for best brightness.

If you use the horizontal pull x10 magnification feature its even worse as that makes the X amp overdrive the deflection plates by a factor of 10, so the duty cycle of the visible portion of the trace is reduced by a factor of 10.