ND Filters, often used to slow exposure times can also achieve great effects with flash.
Being a loyal Pentaxian, one of the biggest limitations to my setup is the flash sync speed of 1/180.
I often wonder where Pentax came up with this exposure and why they hadn’t developed a higher sync speed of 200 or 250 like many of their competitors.
Then I remember my film days and cameras that only had a sync of 125 and realize that even a shutter of 800 is slow compared to the speed of flash.
Flash or strobe work is really the balance of two exposures simultaneously.
Background or intent – exposed with ambient light, controlled by shutter, ISO and aperture plus Subject – exposed by flash, controlled by ISO and aperture.
Shutter speeds don’t really come into play during the flash part of the exposure, as most flash units fire much faster than the top sync speeds in cameras today.
Often, I find the strength of my studio strobes and non-ttl speedlights powerful enough on their lowest settings, that even with a low ISO 100 and the fastest sync speed of 180
I must still use an aperture of f11 or f16 to keep the highlights from blowing out beyond recovery.
When I have my ideal exposure setting for the flash, these small lens openings often cause an undesired effect,
they create a greater depth of field, taking focus away from the subject.
To balance exposure in the background part of the photo and increase my aperture openings for shallow DOF:
I can readjust and move the light source farther away (drastically changing the quality of light I’m looking for on my subject),
try to diffuse the power further with scrims, boxes, etc. (again chancing a change in the quality of light)
or simply adjust aperture with a neutral density filter to the opening I desire.
Two series were done setting exposure to flash at ISO 100 and sync 180. Detective Story (Sigma 18-50mm f2.8 Macro) & Time (Lensbaby Sweet35 & Sigma 70-200mm f2.8):
Detective Story – original aperture f8, bringing extreme detail to the black background cloth.
Both speedlights were set at 128th power (lowest possible), 1st-left side with box and inner diffusion panel only, 2nd-overhead with bounce shovel. A reflector was used from right front.
Using a 0.9 ND filter (3stop), I was able to bring the aperture up to f2.8 eliminating any background detail.
I then added a #1 soft focus filter and toned that photo for Black & White.
Time – original aperture f16, extreme detail to the middle grey background.
Three speedlights were used for the Black and White. Right & left side set at 128th power (lowest possible) +1/3ev, Both hard light with no diffusion. Third was from behind, 1/4 power no diffusion.
Using a 0.9 + 0.6 ND filter setup (5stop), I was able to bring the aperture up to f2.8.
The Lensbaby Sweet35 at f2.8 gave me the blur and shadow I was looking to create in the Black & White.