The following blog post is the transcript from the "Monday Morning Cup of Mo". You can listen to this on your favorite podcast app, or on the podcast website. There is also an embedded podcast player down below.
Today I want to focus on one of the most common bits of misunderstanding and commonly shared pieces of technical information when it comes to flash photography: High-speed sync.
Right off the bat, let’s dispel myth number one: high-speed sync is not for freezing motion. Repeat it with me, please: high-speed sync is not for freezing motion. In fact, in most cases, it absolutely sucks for freezing motion. Folks believe that the words “high speed” in the term “high-speed sync” mean that it’s meant for photographing high-speed subjects. No, no, no, no, no. And no. Uh uh. That’s a negative, Ghost Rider. Okay, smarty pants, so what does high-speed sync mean then? It refers to switching to a special mode on your flash so that you can use shutter speeds faster than your normal x-sync speed on your camera. The x-sync speed on your camera is the fastest shutter speed you can use with normal flash, and on most cameras it is around 1/200th or 1/250th of a second. If you don’t know what the x-sync speed is for your camera, here’s a great chance for you to finally open that manual that came with it. Or ask the Google. You can ask Siri, but she’s not much help. And Alexa will just play an episode of the X-Files.
So if the x-sync speed is the fastest shutter speed I can use with flash, what happens if I try to use a shutter speed that is faster? Have you ever seen a black bar appear on the edge of your photos? You were probably exceeding your x-sync speed. What happens is that your shutter curtain is traveling too fast for the flash exposure and you end up catching part of the curtain blocking your sensor in your exposure. Hence, the black bar.
This is where high speed sync comes into play. In normal sync, your flash fires once and dissipates. Boosh. Which is great up to 1/200th of a second. Once you go faster than that, you need to engage high-speed sync. Instead of firing once, you flash instead pulses at 30,000 plus times per second, so fast that you can’t see the pulsing. The result, however, is that your flash acts like a continuous light source; it no longer flashes once. It pulses 30,000 plus times. Which evenly illuminates your subject for those super-fast shutter speeds. But there’s the problem when it comes to motion. A quick flash of light is great at freezing motion. Continuous light, however, is typically pretty sucky at freezing motion. And by engaging high-speed sync, you are turning your flash into sort of a continuous light source. Now, you may ask, “Won’t the fast shutter speed freeze the motion?” Wellllll….maybe. A super fast shutter speed like 1/3200th of a second or faster might. But 1/400th of a second won’t freeze a dancer in the air, or a batter swinging a bat. You will get motion blur.
Another downside of high-speed sync is that it robs a lot of effective output power from your flash. It depends on the flash and the camera system, but in general just by crossing over into high-speed sync you will lose at least one to two stops of effective output. So your flash that you have set to full power will now have the effective light output of ½ or even ¼ power. In some situations, that may make or break the shot. And that’s from going from 1/200th of a second to 1/400th of a second. Every time you cut the shutter speed in half again you lose another stop of effective light output. So if you lost a stop at 1/400th, you’d lose two stops at 1/800th, 3 stops at 1/1600th, 4 stops at 1/3200th, and so on.
So, wait a minute…It sounds like high-speed sync sucks. It doesn’t freeze motion like everyone says. And it robs a lot of power. So what’s the point of high-speed sync?
Great question! High-speed sync is great for one thing: allowing you to use flash for fill when you are working in strong ambient light situations and your base exposure settings for the ambient light are forcing you to use a shutter speed faster than your x-sync speed. Here’s an example: you’re photographing a client outside in strong daylight and you want or need to use shallow depth of field to blur a distracting background. And you don’t want to blow out the background or the sky. And you’re trying to use the sun as a kicker light behind the subject. So, let’s assume that your base settings are f2.8 at 1/3200th of a second at ISO100. That will give you the look that you want for your background. But now you need to create some light in your subject’s eyes and create a nice, pleasing light pattern on the face. That’s where high-speed sync flash comes into play. Your base exposure settings dictate the need to use a fast shutter speed, which forces you to use high-speed sync on your flash. That’s it. That’s really the only reason to engage high-speed sync.
And you even have two schools of thought on that: one school believes in using high-speed sync, and the other believes in using neutral density filters on the lens to bring the shutter speed back down to normal sync. In my exposure example I outlined earlier, a neutral density filter aficionado would use 4-stop neutral density filter on the lens to drop the exposure from 1/3200th of a second at f2.8 and ISO100 down to 1/200th of a second at f2.8 and ISO100. Then they could use the flash in normal mode and not worry about the power loss associated with high-speed sync. Which is a totally valid workflow for some. For me? I don’t like neutral density filters for the type of work I do. It slows me down, number one. Number two, I have had difficulty grabbing focus when using a 4-stop neutral density filter. And number three, I don’t like putting $150 filters on a two or three-thousand-dollar high-end lens. I prefer to flip on high-speed sync – actually, I just leave it enabled on my Godox XPro transmitter, so if my shutter speed goes past the x-sync speed it just kicks in. But that’s just me. You have to decide what works best for you.
To recap, what is high-speed sync good for? It is for when you want to shoot a wide aperture in strong ambient light and your base exposure requires a shutter speed that is faster than your x-sync speed. That’s it. End of story.
So do any flashes do a good job of freezing motion? Yes, many flashes can. The best use IGBT technology to control the flash output. All of the AD series of battery-powered flashes from Godox utilize IGBT and do a good job of freezing motion as long as the flash is the dominant light source, meaning you are shooting in the studio or in very low ambient light, making the flash the primary or only source of light hitting the subject. In that scenario, a quick flash duration (measured as a t0.1 time) will do a great job of freezing motion. The best flashes in the Godox lineup for freezing motion are the QT series of AC-powered studio strobes, as these are the only Godox AC-powered strobes that use IGBTs. I recommend t0.1 flash durations of 1/3000th of a second or faster if you are trying to freeze motion.
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