HSS vs ND filters: Which is the best method for synchronizing flash? Both high-speed sync and Neutral density filters have advantages and disadvantages, and we’ll tell you all about them. Depending on the type of photography you do, one technique might be perfect for you.
First, what is flash synchronization? Basically, when you fire your camera’s shutter, the front curtain opens to begin the exposure, and the rear curtain follows it to end the exposure. The amount of time between those two things happening is your shutter speed. What happens if you want to add flash to your exposure? Well, for basic flashes, the flash can only “pop” after the shutter’s front curtain is completely open, but before the trailing curtain has begun to close. In other words, the sensor (or film) is completely unobscured.
If your shutter speed is fairly slow, then it’s easy for any flash to fire during the time your shutter is fully open. At very fast shutter speeds, though, what is actually happening is this: right after the front curtain begins to open, the rear curtain begins to close at the same time! Thus, at any given instant, only a portion of the sensor/film is being exposed. This makes it impossible for a flash to only fire once and be a part of the entire image.
Common Sync Limitation
In general, most cameras’ shutters have a shutter speed limit of around 1/200 sec before this high-speed mode begins. This is an important point when considering HSS vs ND filters. This may be slightly different for each camera, and is known as the camera’s flash sync speed.
Anything above that flash sync speed, and you run into flash synchronization problems. At a shutter speed of, say, 1/400, I will end up with a black bar in the frame where the flash was only able to fire during part of the whole exposure, and that black bar is actually the rear curtain blocking the flash!
Thankfully, some cameras will automatically limit you to 1/200 sec if they notice that a flash is attached, and if “High-Speed Sync” is not turned on.
HSS vs ND Filters | How A Mechanical Shutter Works
So, how do we resolve this issue? Let’s say, for example, you’re working outdoors in midday and you want to set your shutter speed to 1/200 sec in order for your flash to fire. This could mean you’re forced to use an aperture of f/8, f/11, or f/16 to get a correct ambient exposure and also have the shutter synchronize with your flash.
If you’re working with a beautiful background and you want the depth to field that shows the entire landscape while simply highlighting the subject with flash, that’s great! The problem is, what if you’re working in this midday situation and you want to blur the background?
If you want to create a shallow depth of field, and you lower (brighten) the aperture, you open up to, say, f/2.8 or f/4, then your shutter speed must go up to avoid blowing out all your highlights.
Reasons To Use HSS vs ND Filters | Shallow Depth Of Field
At f/2 or f/2.8, your shutter speed could be around 1/2000 sec or 1/4000 sec, and that’s far too quick for standard flash synchronization speed. What are our two options? The first is to use high-speed sync, a feature on some flashes which allows the flash to fire at a high shutter speed than 1/200 sec. (More on how, next!)
Or we can use a neutral density filter, which we’ll talk about shortly, as well. For example, the above image was captured using a 5-stop neutral density filter. It allowed the photograph to be captured at 1/100 sec instead of ~1/3000 sec, so that an ultra-shallow aperture of f/1.2 could be used.
High-Speed Sync (HSS) | How It Works
HSS (High-Speed Sync) is relatively simple to use: If your flash has the feature, then you simply turn it on! You can now raise your shutter speed as high as you want. Sometimes your camera will also need to have a menu option for HSS turned on too.
That sounds pretty fantastic and easy, right? Unfortunately, it’s a bit more complicated than that, (under the hood) and there is indeed a drawback. So, without getting too technical, here’s what’s actually happening; once your flash goes into HSS, it is no longer firing once when the shutter opens. What’s actually happening is, the flash is pulsing repeatedly.
It happens so fast that to human eyes it will still look like a single flash, and indeed, it only needs to pulse for the duration of that fast shutter speed, such as 1/1000 or 1/2000 sec.
High-Speed Sync | Drawbacks
On both sides of the battle between HSS vs ND filters, there are drawbacks. As you can imagine, if your HSS flash must fire 5, 10, 20, or however many times within that tiny fraction of a second. The downside is you lose some power overall because it cannot fire at full power that quickly. It has to recycle. If you have ever fired your flash manually at full power, you’ll know that it can take half a second, or sometimes a whole second or more, to fire again.
Needless to say, we have a tremendous amount of light loss if our shutter speed is all the way up at 1/8000 sec. How much light loss? In general, when you’re using high-speed sync, you’re going to be shooting at around 4-5 stops below full power at the highest shutter speeds, and at least 1-2 stops just for barely going up to, say, 1/250 or 1/400 sec.
Now, it still is usable, as long as you have enough flash power left to achieve the photo you need. There might be other compromises that you need to make, however, such as using a non-native ISO that potentially reduces image quality.
Using “LOW” ISO (50, 25) With Flash
For example, the previous image was shot at 1/2000 sec, f/2.8, and ISO 50. I couldn’t get any extra flash power, because my HSS-enabled flash was already at full power. (Remember, that’s not the same brightness as full power at 1/200 sec or less!) That means 1/4000 sec was out of the question for dropping my ambient exposure. F/4 was also undesirable because I wanted the background blur of f/2.8.
[Related: Exposure Triangle Definition]
So, in order to balance out the exposure I had to drop my ISO down to 50, which is not a native ISO for my Canon camera. This means I’m actually compromising a little bit of dynamic range there; I risked clipping my highlights.
It’s not a big deal in this case, because it turned out that the highlights were okay in the raw file. Still, I was cutting it pretty close! My light source, a big shoot-through reflector, was very close to the subject. If I were to step away from the subject and include more of the scene, I would have to also move the flash away, and its power would immediately become inadequate. Simply put: oftentimes there’s just not enough power there when I’m using high-speed sync.
Neutral Density (ND) Filters and Flash | How It Works
Now that you understand more about high-speed sync, aka HSS flash, you might be wondering about the alternative side of the HSS vs ND Filters showdown. Of course, I’m referring to neutral density filters. How do they solve the same problem of balancing your exposure, and allowing you to work with any aperture you want? Let’s dive in…
With the ND filter route, say for example a 5-stop ND, then in regular noon daylight, as bright as it’s going to get outside, we can shoot at f/1.2 or f/1.4, instead of f/8. If you only want to shoot at f/2.8 instead of f/1.4, then maybe a 3-stop ND filter is better. Alternatively, some photographers just buy the 5-stop ND; this allows them to work with both ultra-fast primes and fast zooms with all apertures wide-open.
Neutral Density Filters | Drawbacks
Is it that simple, or is there a catch like there is with HSS? Well, first let’s count the obvious benefits: You can run the flash at whatever power you want, and full power is full power. You also get the strobe’s optimal recycle speed, and you get to keep the ISO low, usually.
With that said, here are the cons: you do need to get a decent ND filter, otherwise your filter itself is going to be reducing your image quality. A cheap quality ND can cause colors to change, and even loss of sharpness. To get the best image quality, you’ll need to spend quite a bit on a good ND filter; usually $100-200.
Also, with the ND filter, keep in mind that you’re still darkening your whole image, not just the ambient exposure. This means that your flash is still technically not going to be as bright as if you had no ND filter AND kept your shutter speed below your sync speed. Unfortunately, your flash still needs to be powerful enough to “blast” pretty brightly for balancing out noon-day light, compared to an easy sunset shoot.
Oh, and with such dark filters as a 5-stop ND, you could see a serious hit to the reliability of your autofocus, depending on the camera and lens.
As you can see, there is no free lunch. With HSS vs ND filters, you’re going to be facing a challenge. The good news is, neither one is a truly inferior choice for everyone!
HSS vs ND Filters | Pros & Cons
So, let’s quickly compare the two options of HSS vs ND filters. With HSS, all you have to do is turn it on, and leave it on! HSS won’t negatively affect your images or flash power as long as you leave your shutter speed at or below your sync speed. However, as soon as you increase your shutter speed into HSS territory, you immediately start losing flash power. One thing we didn’t mention yet is, this can also kill your flash batteries sooner!
With a Neutral Density or ND filter, you don’t forfeit any flash power. Full power is full power, so, you potentially regain 3-5 stops of light. That’s huge! On the cons side, using an ND filter can be a hassle; you have to thread it onto the front of your lens, (or even unmount your lens to use a rear slot) and you have to deal with the potential loss of autofocus reliability. Last but not least, poor-quality ND filters can cause your images to have a faint color cast. The worst ND filters may even reduce the sharpness of your image!
High-Speed Sync vs ND Filters | Which is
Better Right For You?
So, what is the right choice between HSS vs ND filters? We recommend making this decision primarily based on your shooting style, first and foremost. Then, consider your budget. If you usually just need to add a “little bit of light” to your images, and if you’re more of an on-the-go, rapid-fire photographer, then HSS is just incredibly convenient.
However, if you’re often pushing the envelope with your flash power and creating dramatic, high-contrast images, I would recommend if you’re going to go the ND route. At least get a decent ND filter. Good ND’s start at $100, with $250-ish for a “very good” one. Likewise, a high end option, like a Singh-Ray, will cost around $300. Of course, sizes and styles (and prices) will vary based on your needs, not to mention your lens size or filter holding system.
Those are the pros and cons HSS vs ND filters! I’m going to prefer using neutral density filters because it’ll allow me to get the best image quality possible when using a high-quality ND filter. However, when you’re starting out and you’re learning, feel free to try out both!