Despite having less power output than flash heads in photographic terms, constant lights can still dazzle your subject and cause pupils to narrow when you’re shooting portraits.
Flash heads usually have a daylight colour temperature, whereas halogen constant lights have a tungsten colour temperature. LED panels can have either, and may be fitted with dual banks so you can switch between daylight and tungsten.
Depth of field
The narrower apertures enabled by flash heads can be good for group portraits of two or more people, where you need to keep everybody looking sharp.
You can often swap between halogen bulbs of different wattages for varying the power of constant lights, but you have to wait a few minutes for them to cool down after use.