There are at least two distinctly different causes for flash-induced blurring:
- Impact by a sound wave that originates at or near the flash unit --- the "sound wave effect"
- Local heating of the subject and surrounding air caused by absorbing energy from the flash --- the "radiometer effect".
It's a safe bet that in some cases, flash-induced blurring is caused primarily by the radiometer effect. The one that comes to mind is Chris S.'s report of flexible glass fibers that either blurred or not, depending on whether they were black or clear.
But it is
not a safe bet to assume that the radiometer effect dominates all the time just because it does in specific cases.
In my own experiments, when I've taken care to reduce or eliminate the sound wave effect, the remaining amount of flash induced blurring has been negligible.
As you say, very fast flash pulses can be one key. If the light pulse stops before the sound wave arrives, then the sound wave won't matter.
Looking at Kurt's description, I notice that he says
Distance flash unit to white plastic drinking cup approx. 5mm.
Distance of white plastic drinking cups to the object (scales) approx. 20-30mm.
Noting that speed of sound in air is about 350 meters per second, and assuming 35 mm from flash to subject, this suggests that a flash duration of less than 1/10,000 second should eliminate all or most of the sound wave effect. At flash powers of 1/32 and less, I'm guessing that Kurt's Yongnuo units are working in that regime.
except that the washer must wreck the scales beneath them, no?
The scales themselves are pretty tough. I would worry more about dislodging them. That said, standard practice for mounting lepidoptera is to press the wings against a spreading board so that their position is fixed by friction. It's not clear to me that Kurt's washer, placed gently, would be much more damaging than that.
--Rik
