Some Things to Watch Out For When Filming in Slow-Mo

 What do you look out for when you want to shoot in slow mo?

    There are couple for things you need to be aware of when dealing with 

    High speed shoots. Firstly...

• Your Action has to be over in a split second. 

         This works best for high speed. 

        “snap” of finger. 

         Thats how fast your action should be. 

• As we have limited recording time, you need to plan your shots. You can’t have a long action sequence. It has to be fast and quick.

        Secondly... 

    Lighting for Super Slow Motion

 

• Lighting plays important role in high speed. 

• The more light you have the faster you can shoot.

• You need to be aware of the lights you are using. And check for flicker from the lights.
• Generally you can’t use fluorescent lighting and tungsten filament lights below 2,000W. I.e the old workhorses like Arri Fresnel T2 lights or the "Blondies".

So how do we know the lights flickering? 

B&S Flicker tester

• Usually we do pre production tech recce where we bring along a flicker meter. This one is actually made by Arri’s sister company b&S who manufacture Arri ballasts for them. (Thats why they are the same color as Arri lighting)

        How do we use it?

• We point it at the light and this shows you how much flicker there is in a lighting          fixture. Its a handy tool we use on set. The lower the number, the lesser flicker there is.

!Protip
Rather than staring straight into the light and aiming the flicker meter at it, I bounce it off a polyboard or styrofoam and take the reading from the reflected light instead. I try to keep the range of light flicker on set to be below 0.6% for HMI lighting.

How does Lighting Flicker Look Like in Super Slow Motion?

    Here’s a good example below. Many years ago, I was working on a Non Profit shoot, where the            budget was tight, and we had to make do with the lighting equipment provided by the film school            we were filming in. The biggest light they had was a 1.2k HMI. 

    Before the shoot, I was told they had a high-speed ballast but, as always, things never quite go the        way you plan in production. Turns out, they didn’t have a high-speed ballast for any of their HMIs.        In this case, shooting at 100fps or slower would’ve been safe. That would’ve kept us in the safe            shooting range with the power cycle and avoided most of the flicker. But we were aiming for                   something more magical, and the director didn’t mind the flicker once I pointed it out. In fact,                it added a bit of character to the footage. So, we leaned into it and got away with a little creative            licence. But you might not get a such collobrative and open Director like i did. 

    So its best you double check on this things before shoot.

Why Does Flicker Happen with HMI Lights?

    Flicker happens with HMI lights because of how they interact with power. Standard HMIs run off        mains electricity, which alternates at either 50Hz or 60Hz, depending on where you are. This means        the light pulses, turning on and off, 100(50Hz) or 120(60Hz) times per second.

    Now, to the human eye, this flicker is usually invisible. Our eyes can’t catch it because the cycling        happens so fast. But when you shoot in slow motion, especially at high frame rates like 200fps, or        more, your camera starts to capture those invisible fluctuations in brightness. That’s when the flicker     shows up on screen.

    It’s even more noticeable if you’re using a magnetic ballast, or a non-high-speed electronic ballast.        These don’t stabilize the current well enough for high frame rate shooting, so the intensity of the            flicker becomes more pronounced the faster you go.

    The result?  A visible pulsing of exposure that looks like the light is breathing.

How to Fix or Avoid Flicker in Slow Motion

    1. Use Flicker-Free LED Lighting

    One of the most reliable fixes is to use flicker-free LED lights from the start. Brands like Aputure,        Nanlux, and Godox now offer high-powered LED fixtures designed specifically for high frame rate        and slow-motion shooting.

    These are becoming the industry standard for a reason:

• They’re flicker-free
• Energy-efficient
• Easier to power and control on set

    For anyone shooting super slow motion regularly, making the switch to modern LED lighting is one        of the best investments.

    2. Match Your Frame Rate to the Power Cycle

    If you’re stuck using traditional HMI lights without high-speed ballasts, your best option is to shoot        at frame rates that sync with your power frequency:

• In 50Hz regions: Shoot at 25fps, 50fps, or 100fps
• In 60Hz regions: Shoot at 30fps, 60fps, or 120fps

    While this limits your flexibility for extreme slow motion, it reduces flicker significantly. 

    3. Fix the Flicker in Post-Production (But Don't Give Your Editors More Work!!)

    If you can’t avoid flicker on set, it can still be handled in post. Most major NLEs (Non-Linear Editing     Software) offer flicker-reduction tools.

    Here are some options:

• DaVinci Resolve (Studio version): Built-in Deflicker effect
• Adobe After Effects: Use plugins like Digital Anarchy Flicker Free
• Premiere Pro & Final Cut Pro: Workarounds using temporal smoothing or third-party plugins

    Just keep in mind: post-processing can reduce flicker but may not always deliver perfect        results. It’s always better to catch it on set. 

    Don't have a fix it in POST attitude, FIX IT ON SET!