Using a very shallow depth of field is highly popular for portrait photography, but when you combine that with flash, you can run into a tricky situation when you bump into your flash sync speed. There are two ways around this: high speed sync and ND filters. Which is right for your work? This excellent video tutorial will explain the differences as well as the pros and cons of each.
Coming to you from David Bergman with Adorama TV, this great video discusses using high speed sync versus ND filters. The need for these arises from the fact that most cameras can only shoot with flash up to a specific shutter speed, normally somewhere around 1/250 s. As such, whenever you want to shoot at wide apertures, you will often run the risk of overexposing your images, particularly if you are working outside. One way around this is to simply use an ND filter to cut down the exposure and then adjust the flash power to compensate, but this introduces issues like increased difficulty focusing because of the darker viewfinder. On the other hand, high speed sync changes the flash's firing pattern to approximate a constant light for the brief time the shutter is open, allowing you to use any shutter speed, but this introduces its own set of issues, such as reduced power range or the potential for slightly uneven exposures. Neither solution is perfect, and it is about finding the method that works best for your workflow. Check out the video above for the full rundown from Bergman.
I generally lean very heavily towards using an ND. You lose so much output from a Speedlight when going into HSS that it can make things much more difficult when competing with the sun. You also blow through batteries so fast if your speed lights are all at max power for the whole shoot.
Doesn't an ND filter do the same things? Increase the power requirement from the flash.
Yes, but the key is power output relative to ambient not power output relative to exposure. No matter how dense of an ND filter you put on your lens, your flash will generate the same amount of light relative to ambient when set to a given power so long as you stay out of HSS.
But once you go into HSS the amount of light generated by the flash decreases. This due to how HSS functions. It is no longer a single burst of light discharging the full charge of the flash. It is a rapid pulse.
As a result, the faster you ask the flash to HSS, the lower its max output relative to ambient. With most flashes, this can be pretty dramatic. My SB700s, for example, produce 2-3 stops less light at 1/800 than they do at 1/200. At 1/1600 it is even less.
By adding a neutral density filter you adjust overall exposure but it doesn't impact flash output relative to ambient at all. This makes things far easier.
If I had excess flash power. (Say I'm carrying around a set of big boy strobes) It doesn't matter so much but with small flash, I often can barely compete with ambient at full power once you punch it through a softbox. If I want to shoot at say f/1.8 in daylight. I need to be shooting at like 1/2000th. HSS at 1/2000th is pitifully weak coming from a speedlight.
Alternatively, I can toss on an ND and shoot at f/1.8 at 1/200th with my flash at full power.
Flashes and cameras with hugh speed sync tend to be more expensive and ND filters have othwr uses. They will allow you to use a slower shutter speed for thing like waterfalls (way iver.done theae days) or create a sense if movement in action shots. A polarizing filter can also be used as an ND (2.5 stops i think).
I believe most PL filters have a light reduction rating of 0.66-1 stops of light (dependent upon intensity of PL), at least that's what I've found in my usage...
Why is there no mentioning of H-Sync (or Hyper-Sync)? Please check out:
https://www.elinchrom.com/learn/hss-hs.html
The page explains both methods. In short: H-Sync outputs the flash over the period of time the shutter needs to move over the sensor. While HSS outputs many (thousands?) of flashes and therefore has a lower output of light, H-Sync delivers almost full power up until about 1/2000 second.