This Simple Flash Mistake Is Ruining Your Images, Here Is How To Fix It

In the past ten or so years, one of the most common features flash photographers started using is HSS. However, HSS can ruin your images when it comes to freezing ability, consistency, and light output. In this article, I will show just how HSS can be a detriment to your images and what you should do instead.

Before HSS

Back in yesteryear, photographers did not have HSS, and still somehow managed to freeze motion perfectly with flash. Moreover, although modern remotes and packs have HSS support, I see very few photographers using this mode for their images. All of the lights I own have HSS support; however, I can’t remember the last time I used it, except for this article’s purposes. All of this is because the way HSS works with modern DSLR shutters is pretty inconsistent and simply put: bad.

Before going further, the differences I am about to discuss are most noticeable to the photographers who shoot high-resolution stills and then view them on huge monitors where every detail is noticeable. Additionally, these details will be very obvious to the photographers who take extra care to dial in flash power. There might be product photographers, where a tenth of a stop of light can make all the difference. Another genre of photography where this is important is sports photography. While you need to make do with what you have if you’re covering a sporting event, the same can’t be said about shooting a promo poster. Photographing athletes in motion for a poster, campaign, or just their social media allows you to control the light around you much better, and it is simply unacceptable to have HSS ruin your images.

I’m talking a lot about freezing motion here. Have you ever noticed that shutter speed doesn’t really change how your images look, assuming the only light you’re working with is flash? That’s right. This is because shutter speed affects the amount of constant light that’s hitting your sensor, but not flash. Assuming you’re in a perfectly dark room, you can theoretically leave the shutter open for as long as you want and still get the right exposure given the right flash power, aperture, and ISO. Let’s see why.

What Is a Flash and How Does It Work?

Flash works by releasing a huge amount of light in a short interval. This is done by some clever electric work, but in short, what we need to care about are three properties: flash duration, flash power, and color temperature. They are all interlinked. Let’s go one by one. Flash duration is the property that controls the freezing power of your light. Think of it as shutter speed, but in flash. The shorter the duration, the more chances you have to freeze motion. The light output is the amount of light you are throwing at the subject. Usually, the higher the output, the longer the flash duration, as the capacitors inside your flash unit need to release more power and follow some gas laws. Lastly, color temperature is the temperature of your light in Kelvin. Ideally, it stays the same throughout your power range; however, it can change with less expensive lights.

Now that we discussed how flash works in its most primitive form, let’s see how HSS works. High-speed sync happens when you exceed your typical flash sync speed. It is always below 1/250th of a second, however does vary from camera to camera. For this example, we will exclude cameras that have a leaf shutter or a global shutter. Essentially, most cameras on the market sport a curtain shutter, which opens fully until the sync speed of the flash; however, as you increase your shutter speed, the sensor is exposed with a slight delay. The two curtains form a slit which passes across the sensor. As such, if your flash does not support HSS, you will get a black strip on the side of your frame if you shoot past the sync speed.

How Does HSS Work? 

HSS comes in and allows you to shoot past your regular sync speed. There is a caveat though. If we think in tiny tiny intervals of time, fractions of a second, things look quite interesting. The time it takes for the flash to fire is much shorter than the time your shutter is open. While your shutter speed might be set to 1/8000th of a second, the time it takes to complete that exposure from top to bottom of the sensor is longer, as the curtain needs to travel from the top to bottom. As such, the flash duration is simply not long enough to adequately cover the entire frame. So, what to do? Well, extend the flash duration. This is done by pulsating the flash in incredibly high-frequency bursts. This way, the flash gives the output necessary long enough to cover the entire frame. You may have noticed that when you use your flash in HSS, the light output is decreased. This is because the flash can’t pulsate at full power. The light output should be at a level where the capacitors can recharge fast enough to enable the burst of light. Typically, this would be 3 or so stops under the maximum.

Freezing Motion

Having explained HSS and the principles of flash, let’s get to the meat. As you may have very well understood by now, HSS means shorter shutter speeds, but crucially, longer flash durations. As we know by now, the longer the flash duration, the worse the freezing power of the light. I demonstrate this by shooting a fan. Keeping the flash power the same: 9.0, I first shoot at a long shutter speed (1/25s), and then I take it to 1/2500th of a second. As you can see, the image which was shot with a longer shutter speed is sharper. To enhance the freezing power of the light, I used it at around 7.0-8.0 of power, as this is the sweet spot between output and flash duration. I would not recommend going beyond 9.0 to be able to adequately freeze motion. Instead, use a hard reflector, fresnel, or extra lights. This is why I used two heads both at power 7.0 with reflectors attached for the test. Visually, the light from a reflector also looks sharper.

Another problem with HSS is color temperature. While this isn’t as much of a problem with Profoto or any high-end flash, it can be quite a big issue in the consumer flash market. Because HSS means sending out pulses of flash, it brings up color shift issues. Each little pulse of light can have a slightly different color temperature. Each flash will vary drastically as well, meaning that your images will be all over the shop. If you need to composite afterward, well, you can forget about that as it will be a real pain in the back.

What Should You Do?

Well, there are many solutions. If you can avoid using HSS, do it. If it is unavoidable, set your flash to a lower power if you can and throw on a magnum reflector or a fresnel. Generally, I try to close down the aperture, lower the ISO, and, in the worst-case scenario, throw on an ND filter and add more light to the scene. Freezing motion is an expensive endeavor under the wrong conditions. The reason things such as Twin heads exist is that they can draw power from two generators instead of one, and as such, keep the duration short, while the light output is the same. For reasons such as these, I can strongly advise you not to worry about slight motion blurs in your images unless the client has specified they will need it to be razor-sharp. Unless you are shooting a global campaign on a 50MP+ resolution camera, small motion blur won’t be noticed.