Trivia - Using a fixed ND filter like a variable ND filter

The 2 principles for maximum flash power outside are

1) Use an ND filter to avoid HSS when shooting wide (HSS is normally about 2.5 stops less efficient compare to ND)
2) Shoot at the highest shutter speed possible up to max flash sync speed for max flash efficiency

@puppetwarp What you did is correct! However, it is possible that the outside ambient is too bright to shoot wide open without going into HSS. Consider the example below with the max flash sync speed of 1/250s and a base ISO of 100.

Current settings are 1/6400s @ f/2.8 @ ISO 100 for a 1-stop underexposure for the ambient on a bright sunny day. However, your flash will certainly struggle at such HSS speed.

If you have a 5-stop ND filter, you can shoot below HSS for greater flash efficiency at

1/200s @ f/2.8 @ ISO 100 (with 5-stop ND filter)

But wait, if you want to squeeze in 1/3 more power for your flash, you can shoot at max flash sync speed of

1/250s @ f/2.8 @ ISO 125 (with 5-stop ND filter)

where your ISO is no longer at the lowest base ISO since you use the formula ND filter + ISO = variable ND filter where in this case your 5-stop ND filter functions like a 4.67 stop ND filter.

 

@puppetwarp For some people, they use HSS with a higher ISO with the intention to freeze motion using shutter speed. The reason is at higher shutter speed, the slit between the first and second curtains is much narrow, giving us a perception of less motion blur. In reality, the true stopping power for shutter speed (for the whole photo) is still the flash sync speed despite the higher shutter speed. We can see this clearly in "bend" plane propellers exhibiting weird distortion effects from the focal plane shutter using this method.

True stopping power comes from normal flash, not continuous flash like HSS. When the ambient light is not a factor (sufficiently low), we can freeze motion using flash (e.g. 1/40,000s using flash duration alone) even when using a slower shutter speed of 1/200s.

 

I always love Neil van Niekerk's writing. I touched on this indirectly in my last post. Flash has true stopping power when ambient light is not a factor. If the ambient light is a factor, then go ahead and increase your HSS speed if you are already shooting HSS already.

 

In your want to consider a reasonable price but high-quality variable ND filter, you should consider the Freewell Variable ND filter.

E.g.

Freewell 82mm Hard Stop Variable ND Filter All Day 2 to 5 Stop & 6 to 9 Stop - 2Pack for US$199.99.

* Optics: Pure High Definition Optical glass 18Layers Multicoated ensure filmmaking experience
* Built: Hard stop Variable ND eliminates any chance of cross-polarization & zero vignetting (16mm above) during the shoot
* Coating: Each side of the glass element is coated with Waterproof, Dustproof, Scratchproof, Oilproof making sure you are ready for all the adventures
* Function: 2-5 Stop equivalent to ND4-ND32, 6-9 Stop equivalent to ND64-ND512 is the ultimate all-day solution from cloudy days to a bright sunny day for all your videography needs
* What is included: Variable ND Filter, Hard Case, Lens Cover, Cleaning Cloth

It has good reviews on youtube. I suggested the 82mm since you can always use a step-up ring and it will be compatible with the rest of your lens.