The short answer is – the more the merrier! You can never have too much light.
The longer answer is, it depends…
Generally, video lights are not as powerful as strobes. Even through strobes aren’t measured in lumens, just as a rough comparison, an entry level strobe (20 GN) has a power output equivalent to about 30,000 lumens.
This is why if you’re planning on doing mostly photos, strobes are still the best way to go.
One scenario in which you might benefit from using a video light for photos, is when shooting macro. A continuous light allows you to use burst mode, and shoot several photos rapidly. If you do this while gently rocking back and forth, you will most likely nail the focus on one of the shots. This technique cannot be done with strobes due to recycle time.
Since you’re close to your subject, you still get plenty of light from your video light.
Even though it’s counterintuitive, the deeper you go, the less light you need! This is because you don’t need to overpower the sunlight, so you can make do with a weaker video light. As you shoot closer to the surface, the ambient light becomes very powerful, especially on a bright sunny day, your video light becomes less effective and gets lost in the strong sunlight.
For night dives, you’ll find that even a basic video light produces enough light for good photos and videos.
As a rule of thumb, when shooting with a video light, it’s recommended to be no further than 2-3 feet away from you subject.
Strong video lights (10,000+ lumens) may allow you to shoot 5-6 feet away as well, but the light fades away quite fast underwater so want to make sure it’s still effective by getting as close as possible.
Video lights are constantly evolving and getting stronger every year. Recently, a strobe feature has been integrated into some video lights, which allows them to trigger off a built in camera flash via fiber optic cables and let out a strong burst of light, stronger than the usual rating of the light. Kraken have started including this as a standard feature on all of their Hydra lights.
So, how many lumens do you really need?
As much as you can get within your budget!
As of 2018, 2000 lumens is an excellent entry level light, 5000 lumens is considered mid-range and anything above 10,000 is high-end and considered very powerful.
We will likely see these numbers increase in the future and LED lights may take over the strobe market.
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