This is pretty good
[video]http://petapixel.com/2013/05/19/how-to-shoot-day-to-night-time-lapses-using-bulb-ramping/[/video]
Crop factor doesn't affest f-stop, only focal length. If a lens was made specifically for 35mm format (full frame) such as your old 50mm 1.8, on a crop sensor this focal length with be longer because the sensor is 1.6* smaller than the full frame sensor. Due to this, you pretty much just 50 *1.6 = 80mm focal length on a crop sensor.
You'll also find that crop sensors are less likely to vignette because they're only using the middle part of the lens (does this make sense?). You won't see the edges of the 50mm because it was made for a full frame camera. Obviously this is ONLY the case if you have a full frame lens on a crop sensor, specific kit lens etc designer for your crop sensor won't behave like this.