Use the 50mm f/1.7. It’s the fastest lens you have, so it lets in much more light than the 135mm f/3.5 or 70-210mm f/3.5, which is critical for night shooting without flash. It’s also a more practical focal length for nearby trick-or-treaters and will give a more natural-looking perspective.

Under typical street lighting with ISO 400 film, you may be around 1/10 second at about f/1.7 to f/2.8, with light varying by a stop or two depending on the area. That means you could end up anywhere from roughly 1/5 to 1/20 second, or even slower in dimmer spots.

The problem is motion blur: 1/10 second is slow for both handheld shooting and moving children. A common handheld guideline is about 1 over the focal length, so around 1/50 second for a 50mm lens. In practice, expect blur unless subjects pause and you hold very steady.

If there’s any doubt about the battery or meter, replace the battery before shooting. Without flash, your best results will come from the 50mm used wide open, brighter locations, and moments when people are relatively still.