Controlled movement
Since the burning flame moves at a very quick rate. Increasing the rate at which the shutter closes captures a millisecond of the movement making it look like a still subject. But the problem is that increasing the shutter speed means letting less light in so we got to bump up the Aperture too to allow as much light as possible.
I am using the Nikon AF FX NIKKOR 50mm with a maximum aperture of 1.4 making it a perfect lens for low light imaging.
Camera : Nikon D5300
Lens : Nikon AF FX NIKKOR 50mm
Settings : f2 ISO 600 1/500sec
Type : Controlled movement
Location : Santa Maria del Fiore, Firenze, Italy.