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One big issue with light painting is accidently lighting parts of the scene that you do not want lit, or do not want brightly lit. Besides a variety of objects that may show up different than you desire, the worst problem can be the ground in front of you. Another issue can be not getting your subject evenly lit, where you can end up with blotches or streaks.
A typical flashlight with a narrow or very spot type beam also has a lot of light that spills out to the sides, so you end up with a light source that has one tight beam, and a secondary beam around it that is very wide. For use as a flashlight, that is usually desirable, so that you can see at your feet while searching in the distance for the trail or some object. When you use such a light to light paint a somewhat distant object, the ground in front of you that is in the secondary beam can easily become over exposed.
One way to reduce the secondary beam is to add a cone or nozzle on the flashlight, which will still allow the main beam to work well, especially if you want a very tight beam. But few flashlights offer a nozzle or cone attachment, so you likely will need to make something. But adding a nozzle is a compromise, and if you end up with a tight beam, it can be very difficult to apply the light painting in a uniform manner without getting streaks.
Without me actually looking for a flashlight to use for light painting, someone showed me one that seems perfect. The light brand is "Lenser", and they offer models that have a feature called "Advanced Focus System" that provides an adjustable beam pattern over a wide range of size. It uses an optical system that provides very uniform light over the adjusted beam size, but it also has a side benefit of almost no side spill. So you get the great feature of being able to adjust the beam to well match your subject, and at the same time you almost totally eliminate the secondary wide beam that can really mess up a lot of light painting scenes.
Lenser offers a range of flashlights with different light outputs. I chose a fairly strong light, and in my case I also desired it to use AA cells because I have a lot rechargeable AA's for speedlight and other uses. The model I bought is shown in this Amazon link:
I am amazed at how sharp a cutoff the beam edge has, and how very uniform the light is within the beam. And the spill light outside the beam is indeed very low. I guess you could add a nozzle to cut it even further, but I have not yet found the need. This flashlight is fairly new to me, but for the times I have used it for light painting, it has worked very well. Lenser offers lower cost lights with less light output, and also higher output and rechargeable lights. The cost is on the expensive side compared to other flashlights with out the special focusable, but for light painting use, I highly recommend this series of lights.
The M14 model I got with its 220 lumens may be too bright to use for some light painting projects, in order to have enough time to paint, and still balance with the exposure of the rest of the scene. I may even consider getting another smaller model Lenser to fill that need. Another thought I had was to temporally add a neutral density filter on the front to cut the light level for the situations that need less light.