Macro the 1/f rule and flash duration

Started Sep 8, 2008 | Discussions thread
jpr2 Forum Pro • Posts: 15,554
re: several things to consider/clarify

JimH wrote:

It looks as though the modulation frequency is not rock-steady. But
based on the two shots that I posted above, it appears to be in the
range of 20-25 microseconds per cycle.

if I'm reading markings on your grid lines correctly, the frequency is about
twice as slow as each pulse width at it's 1/2, or in other words, there is
40 microseconds between each peak or valley - the usual definition
of frequency for repetitive patterns;

But the brightness never drops to zero. Instead, it appears that the
plasma in the tube doesn't extinguish instantaneously, but rather is
sort of maintained, but at a varying intensity. So you'd never
really get full "strobing" at these very high rates (40 to 50 Khz).

the question is than how much (if any) this residual glow is able to
illuminate a scene highly enough to be "recognized" by a sensor?

Also, keep in mind that when you shoot at a shutter speed above the
X-Sync speed, the entire frame is not exposed all at once. Instead,
we get a traveling slit which exposes any given location on the
sensor for 1/8000th of a second (in this example), but still takes
much longer to traverse the entire frame.

this is a valid concern, and [if we take the 1/8000 example] during it's
travel, a shutter slit will traverse (in time) figuratively speaking either
3.25 or 6.5 micropulses, depending of what we agree upon is THE
frequency of the modulation; so indeed each pulse might be a light
source to a slice of cells on a sensor;

So we'll still end up with quite a few of these cycles needed to
expose the entire frame. I don't really know how long it takes each
shutter curtain to traverse the frame.

some engineering of course would take place to have it as linear as
possible, but perhaps the actual traversing speed might be different at
the beginning and the end of each shot;

All of that adds up to a strange effect when we try to capture
fast-moving objects using the HSS mode of these flashes.

but then... how fast is FAST? we are not talking about a bullet hitting
an apple, are we?

The 580EX does provide a strobe mode, but the frequency only goes up
to 199 pulses per second which may not be fast enough to do what you
want.

I know, the 550ex does the same (and also from 2 to 199 strobes),
which is better than nothing, but even at 200 Hz it is very useful to
e.g. determine the real angular velocity of that revolving fan in my
tests, but not really useful to capture strobe effects of IIFs;

I think we'd need to use a different approach, perhaps building an
LED illuminator that we can drive with very fast pulses (nice
rectangular ones ;-).

my only hope would be if there is some source to offer it commercially,
but then it might be rather frightfully expensive, although all componetnt
parts are quite off-the-shelves and cheap[ish].

All this pushes are further and further towards a set of questions
concerning a real latency times of PCBs in our optical computers of the
Eos series, latency and reaction times of sensors, but also of the Digic
FW and HW as well - any sources of information on these? And I'm well
past the time when my knowledge here was approaching any kind of
reliability - out of my depth indeed, but sensing that these are of some
importance too,

jpr2
--
http://www.flickriver.com/photos/qmusaget/sets/72157600341377106/

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