It’s the age-old question. If I’m shooting outdoors in bright conditions and I want a shallow depth of field with flash, should I go high speed sync or just stick an ND filter over my lens?
This time, it’s Gavin Hoey’s turn to try to answer the question. In this video, Gavin shoots a series of identical images using both 3 & 5-stop ND filters as well as the Godox AD400Pro strobe to see how the methods compare and what the advantages and disadvantage of each are.
Gavin begins by taking a quick test shot without flash and without neutral density filters to explain what the problem is that needs solving in the first place. Essentially, when you’re shooting with a wide-open aperture in bright conditions. Basically, you’re usually going to blow out big chunks of your image. Without flash, normally you’d just speed up your shutter, but with flash, that takes us beyond the camera’s sync speed (the maximum speed at which it can utilise “normal” flash).
The first test is to add neutral density to reduce the amount of light entering the lens to be able to keep the shutter speed below the flash sync while still getting good exposure on the ambient light without portions of the scene blowing out. Gavin adds a 3-stop ND filter over the lens. Of course, now the subject is underexposed compared to the background, which is why we need to add flash. The final shot for this first setup is ISO200, 1/250th of a second at f/1.2 with the flash firing at 1/16th power.
Gavin repeats the test without the neutral density filter, using high speed sync, which allows the camera to use flash beyond sync speed. He brings the shutter speed up to 1/2000th of a second (3 stops darker than 1/250th of a second) to match the exposure of the previous shot with the ND filter applied. Interestingly, his flash power here is also 1/16th of a second to give a good exposure on the subject.
I say “interestingly”, because one of the big arguments against HSS is that you “lose power” more quickly when you go over the flash sync speed vs staying below sync speed and dropping it down with neutral density.
What’s particularly interesting, though, is Gavin’s other test, where he puts a 5-stop ND filter against high speed sync. Here, it looks like he actually used a much lower relative flash power vs the neutral density exposure.
The 5-stop ND filter exposure was 1/125th @ f/1.2 and ISO200. The HSS flash exposure was 1/8000th @ f/1.2 and ISO200 (bringing the background down a little more). That’s a whole stop darker than the neutral density filter offered (5 stops from 1/125th would be 1/4000th of a second). Yet the 1-stop darker HSS exposure required that the flash be at only 1/8th power, also a stop darker than the 1/4 power of the neutral density shot. That’s 2 whole stops of difference between the extra stop on the shutter speed and the lower flash power – the opposite of the expected result.
Of course, your mileage will vary. With some cameras, some flash systems, and some filters, you will more often run out of power using HSS before you will using neutral density if you’re at the limits. But as long as you’re not maxed out, does it really matter? If you lose a stop by shooting at 1/8000th of a second instead of at 1/250th with a 5 stop neutral density filter, why do you care if you’re only at 1/8th power anyway?
The drawbacks of using neutral density filters over HSS, though aren’t as bad as they once were. In the days when DSLRs reined supreme, if you put a 5-stop ND filter over your lens, seeing through the viewfinder will get quite challenging. But with mirrorless cameras, the EVF will allow you to compensate for the reduced light, letting you still see your scene perfectly.
As Gavin concludes, it really doesn’t matter which method you use as long as you’re getting the results you’re hoping for. Personally, though, I’m with Gavin. I’ve been using HSS for the last fifteen years. All my strobes support it now, and I’ve got triggers to use them with five different camera systems. It’s just less hassle than dealing with constantly swapping out filters as the light changes or you move around a location to different viewpoints.