I did that last Friday. It was a night parade, no sky light, with inadequate street lighting, on a narrow two lane old-time urban street. 50 mm was definitely too long (on full frame) for many of the shots but I managed to place myself at a turn in the road so it wasn’t awful. I exposed for the highlights, which consisted of a multitude of Christmas lights on each float, and a lot of the shadows were indeed rather dark. The lens was a f/1.8, and most of my shots were f/2.2 at rather elevated ISO. My keeper rate was well below what I’m accustomed to. Definitely I’d use a wider lens were I to do this again, like maybe my 28 mm f/2.8, and I’d shoot it wide open.
Here is the basic equation explaining the relationship:
Focal length / Sensor width = Distance to subject / Width of field at subject
A normal lens for a camera is defined as one whose focal length equals (more or less) the width of the sensor. Often, the width of a sensor is measured from opposite corners. A focal length equal to the sensor width will give you a width of field—at the subject—equal to the distance to the subject. Doubling the focal length halves the width of view, doubling the sensor width doubles the width of view, etc.