How to Use Multi-Flash to Capture Stroboscopic Action Photos

Have you ever been to a disco, performance, or another place where they used a strobe light? If so, you saw the interesting effect created by the rapid flashing. Smooth movement gets broken into a series of frozen-stepped motion, not unlike the frames of an old-time movie. 

Now, what if you could do that with your still camera – that is, create a series of images, all within one frame? If you have a portable flash or studio strobe capable of generating the stroboscopic effect, there’s a good chance you can do this. You can create images that are a great way of analyzing and showing motion, and this article will show you how!

What is an in-camera stroboscopic effect?

Different flash manufacturers may use different names for this capability. Canon, Godox, and Yongnuo call it Multi-Mode, while Nikon calls it the Repeating Flash Function. However you refer to it, multi-flash is the capability to fire multiple flashes during one camera exposure.

The best way to see if your flash is capable of this effect is to read your flash manual. If the flash you own does have this capability, a photo illustration will often accompany it showing the kind of images possible, like this one here:

If your flash unit supports it, there will be three constants you can control regardless of the make or model of your flash unit. They are:

1. Power output

This controls the intensity of the light output. Typically, output runs from 1/1 (full power) down in fractions of that, such as 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, or 1/128. The smaller the fraction, the less intense the flash output.

There are two other things to remember about the flash output:

1. The higher the output, the more battery power used, and the longer it will take to recycle the flash before it’s ready for another burst.
2. The duration of the flash is shorter as the output power gets lower.  As a result, lower power/shorter durations have more “stopping power” when it comes to freezing motion.

Just to give you an idea of different flash speeds at different power outputs, the chart below shows approximate flash durations for various power settings on a Canon 580EX Speedlite:

2. The number of flashes

This one is easy and is exactly what it sounds like: it’s the number of times the flash will fire during the exposure.

Set it for however many times you want the flash to fire in your image. That’s how many “steps” of the moving object you will see.

3. The flash frequency

This one can sometimes throw the new user as it uses a term not always familiar to everyone: hertz. In very simple terms, hertz refers to the number of cycles in one second. So, 1 Hz=1 flash per second, 10 Hz=10 flashes per second, etc.

How to determine your flash settings: the formula

Here’s how you put it all together.

Figure out how much power you need and set the power output first. Your distance from the flash to the subject will help you determine it, and so will how fast and how many flashes you expect to fire and how much “freezing action” you need.

Then think about the speed of the action you intend to capture and its duration. Finally, determine how many steps you want to see freezing the action.

The formula looks like this:

Number of flashes/Hz = Shutter speed

Let’s use an example. You want to take a strobed shot of a hammer swinging down and striking a nail. You can put the flash close to the action, so 1/32 power might be enough. If you use a slow swing, you can complete the action in one second. You’d like 6 steps of action in the shot.

Plug those numbers into the formula:

6 flashes/6 Hz (6 flashes per second) = 1 second shutter speed

Now say you want to capture something faster like a club hitting a golf ball off a tee. You can still get the flash close enough to use 1/32 power. You want 15 steps in your sequence and guess the action will take just 1/30th of a second to complete.

Here’s how the formula looks for that:

15 flashes/199 Hz = ~1/15s shutter speed

The formula is right, but perhaps the speedlight you’re using (in my case, a Canon 550EX) is only capable of a 199 Hz maximum. Even at that, the shutter speed would have to be about 1/15s, not the 1/30s you wanted. Could you live with just eight steps in your shot?

8 flashes/150 Hz = ~1/20s shutter speed

That’s closer! If you slow down your swing, it just might work.

You will find that at the higher hertz rates, the flash fires so fast that it seems like just one burst. However, when you check your shot, a fast-moving subject lit with a high flash-per-second (hertz) rate should show the individual steps in the movement.

Exposing for a stroboscopic image

You’ve used the formula to determine what numbers you want to enter into the flash, and that’s determined your minimum shutter speed. Here, however, the flash is firing within the scope of the shutter duration, and shutter speed isn’t really a factor in setting exposure.

Here are the factors that will affect your exposure:

Ambient light

You want the flash doing the work here. Also, you will typically be shooting at longer shutter speeds to capture the duration of the action.

Ambient light is not your friend as it will begin to force settings you may not want. You also want to eliminate distractions in the shot as the steps of the object in motion will create a busy enough image already. Your best bet is to work in a darkened room and use a black or very dark background.

Set everything up with a work light on, but switch it off before making the shot so the flash is the only source of illumination.

That leaves a few things you can do to adjust exposure:

Set the ISO

ISO adjustments can be helpful here as they allow you to have the aperture and shutter speed where you want them and adjust this third leg of the exposure triangle to get the exposure where you need it. As always, aim to keep the ISO as low as possible to limit noise – but also remember modern cameras have become far less noisy in recent times.  Know what your camera can do and at what point you will get too much noise.

Set the aperture

You will want to adjust your aperture based on the depth of field you need for your particular shot. Also, keep in mind that most lenses perform best between f/8 and f/16, so try to be in that range if you can. You can also adjust your aperture for exposure if you need to. However, I recommend making exposure adjustments using ISO and flash power first.

Set the flash power

Remember, this is one of the settings you enter into the flash. The flash output will very much control your exposure. The best rule of thumb here is to only use as much power as you need.

To recap, these are the advantages of lower flash power settings:

• You’ll use less battery. (If you have an external power source for your flash, use it.  Stroboscopic flash work drains batteries fast.)
• The flash will recycle faster.
• Lower power = shorter flash duration = more motion-stopping capabilities.

Increase the flash output if you need to, but remember that increasing the ISO might be a better move.

You may also find the flash will limit what other settings you can input, especially with higher power settings. To allow sufficient time to recycle between flashes and to prevent the flash from overheating, it may not allow many flashes or a higher hertz setting at higher power settings.

For example, my Canon 550EX can shoot 70 continuous flashes at 10 Hz if the power is turned down to 1/128 power. However, it can only shoot 2 consecutive flashes at that same 10 Hz rate if the flash is turned up to 1/4 power.

The Multi-Mode on my Canon flash will not work at all if the flash power is set to anything higher than 1/4 power. Full or 1/2 power in Multi-Mode on the 500EX? It simply cannot do it.

My flash manual has a chart showing how many sequential flashes are possible at various power and hertz settings. Also, the flash’s programming will not allow settings to be input that exceed its capabilities. Canon also warns: “To prevent overheating and deterioration of the flash head, do not use stroboscopic flash for more than 10 frames in rapid succession. After 10 frames, allow the 550EX to cool for at least 10 minutes.”

So whether you’re using a Canon flash or another make/model, know that stroboscopic flash works your unit hard and be aware of its limitations.

A longer shutter speed is (generally) okay

Here’s one more thing to think about when inputting the three parameters into the flash and calculating the shutter speed: 

When you click the shutter, the flash will immediately begin its strobed sequence.

If you input, say, 6 flashes at 6 hz at 1/32 power, per the formula, it will take 1 second for the flash to complete the programmed cycle. However, there’s no reason that the shutter speed couldn’t be longer, especially since in low ambient light conditions, little if any additional light will add to the exposure once the flash cycle completes.

So to amend the formula just slightly:

The number of flashes/Hz = The minimum shutter speed

With no additional flashes after the sequence completes, further action is not likely to be seen in the shot. So overestimating the shutter speed is usually not a problem. Underestimating the shutter speed, however, won’t allow the flash sequence to complete before the shutter closes.

Determining the perfect exposure settings

We’ve covered how to determine shutter speed, but what about setting the perfect aperture, ISO, and flash output power? I’ve offered some guidelines in the previous sections, but how can you ensure perfect exposure? There are a few ways to approach this:

1. Use an external light meter.  Fire the flash and take a reading as you normally would with an external meter. Use that to determine your camera setting at the predetermined shutter speed. Adjust the ISO, aperture, and/or flash output power to get a proper exposure. If you are familiar with using an external flash meter, you will know how to do this. But if you don’t have an external light meter, you might try the next method:
2. Do manual calculations. Look up the guide number of your flash, determine the distance to your subject, and then – using the formulas in your flash manual – calculate your settings. It can work! But if math is not your forte, or you don’t fancy crunching numbers, you could always try the third option:
3. The trial-and-error chimping method. Okay, that’s my name for it. But it simply involves starting at, say, an ISO of 100, an f-stop of about f/8, and a flash output of 1/32 power. Set the number of flashes and frequency that you think best. Shoot, take a look at the LCD playback (i.e., chimp), and if the image is too dark, increase the flash power or open the aperture. Test, chimp, and repeat as needed until you get it dialed in. Digital storage is cheap, and once you figure out your settings, unless you change the flash-to-subject distance, you should be set for the session.

How to get even better multi-flash photos

At this point, you should be familiar with the basics of stroboscopic photography. However, here are two other items to consider if you want to take your images to the next level:

1. Think about the color and brightness of the objects

You will very quickly find that because each step of the sequence in a shot only gets a portion of the total light during the entire exposure, darker objects in motion may not show up well. Also, because static objects in the shot will get the full sum of the light, they will be brighter.

You can learn from your mistakes, but why not learn from mine instead?

For the stroboscopic shot above, I used a dark, patterned photographer’s popup background, but I should have used a solid black background and placed it farther from the subject. Also, the golf club I used had a dark head and shaft – so while the white ball, golf tee, and reflective chrome parts of the club showed up reasonably well, other parts of the club disappeared. Finally, the patterned background got too much light; as a result, the pattern interfered with the shot.

Here are two lessons you can learn from this:

• Use a black, plain background and place it as far from the subject as you can such that little-to-no light illuminates it.
• Use bright objects so that the steps in your sequence show up well.

If you take a look at the multi-flash pepper photo toward the beginning of this article, you’ll see that the bright orange color of the bell pepper and a dark black background worked much better.

And I’ll note that a redo of the golf shot incorporating those exact principles resulted in a much better shot:

2. A remote trigger can help

Unless you have an assistant (or maybe three hands), trying to control the camera while performing the action you’re trying to capture and also nailing the timing is not impossible, but it makes things a lot more difficult. Therefore, a remote trigger that allows you to fire the camera as you start the action sequence can be a huge help. 

If you are mounting your flash off-camera, a means of triggering the flash will also be necessary. Use either a wired connection, a wireless radio trigger, or an infrared camera/flash system.

Another level of sophistication, if you want to add it, would be a flash trigger, perhaps activated by sound, breaking a laser beam, etc. I have used the MIOPS Smart Trigger on other photo projects with success. A real advantage it adds is precision and repeatability of a shot – something that you will otherwise be forced to leave up to luck and timing.

In a dark environment, use Bulb mode. Open the shutter, and when the action activates the flash trigger (i.e., breaks the laser beam or creates a sound), the flash fires its strobe sequence.

Good flash triggers aren’t cheap. However, if you do a lot of this kind of photography, they significantly speed up the work and permit the repeatability of a shot, which saves a lot of time and effort.

Practice makes perfect!

Like any photography, practice will improve your results with stroboscopic flash work. You will better learn how the three flash settings – flash power, number of flashes, and flash frequency – work together to create a multi-flash photo.

You will also learn the kind of action sequences that make for good shots, how to tune your composition, and camera settings, and how to edit your photo for the best results. Plus, you’ll find that making lots of shots, checking your work, fine-tuning, and repeating are key to getting that one really great keeper.

I hope you will take the time to try and learn this new flash trick and then share your results in the comments. If you have questions or other comments, please share those too!