P3D Re: Flash Cluelessness

[michaelk@xxxxxxxxxxx (Michael Kersenbrock)]

> Auto: Set  your ASA on the handy-dandy sliding scale, and use the f-stop
> it tells you.  For ASA 100, and "blue" it dictates f4, for "red" it's
> f2. (Never mind "red", can't imagine needing to light a subject 40 feet
> away.) So, for "blue" it tells me to set aperture to f4 and shoot...
> but, I don't wanna use f4, I want more depth of field!  Well, what if I
> change the ASA setting?  Note that I have ASA 100 film in the trusty
> Realist. If I set the flash to ASA 200, then the sliding scale dictates
> f5.6, if I set it at ASA 400, then f8, and so on up to ASA 1000 for f11.
> With ASA 100 in the Realist, if I set the flash to ASA 1000, and the
> camera to the indicated f11, will it work (expose properly)?  I don't
> think so.  I'm thinking the strobe puts out a set, consistent volume of
> light, regardless of what you set it on, and a faster film speed
> necessarily means you better use a smaller aperture. 

You're thinking is correct.  The only "adjustment" you make to the
flash is the picking of blue or red.  Off hand, with that flash
I think you need to forget Auto-mode.

> Manual: Set your ASA on the scale and the moving scale gives you f-stop
> setting per distance.  For example, ASA 100.  5 feet = f16, 7 feet =
> f11, 10 = f8, 15 = f5.6, 20 = f4, 30 = f2.8.  Now, imagine a near object
> is 10 feet away.  The scale tells me to use f8. What to do if I want to

Which gives a Guide number of 80  @ ASA-100 (10 ft x f/8).


> use f16 for DOF reasons?  (Assume ASA 100 in the Realist). If I set the
> flash on ASA 400, then the scale lines up 10 feet with f16.  Will this
> work?  

Yes if you mean using ASA 400 film to take with the flash with those
settings.  No otherwise.  The moving scale is a "calculator" for
your use, the flash isn't aware of the "adjustments".  In a
single "manual" mode, the flash will have a single brightness for 
a fixed period, so for a given distance (light falloff) and given
film speed, there is only one "proper" f/stop.

> Specific Question Number 1: Given my conclusions per the above, I can't
> figure out to use this Vivitar 2800 unless I use it at the recommended
> f-stop.. which I don't want to do, as the DOF limitations are
> unacceptable.  Is my flash wimpy, or does my brain fail me?

On the wimpy side, but bigger problem is the f/stops "picked" for it's
auto-ranges.  My ancient Vivitar 292  has continuous selection of 
any f/stop that I want when I have the remote sensor attached 
(and it can attach non-remotely) and the "range" just moves with
my f/stop selection.  Something quite handy.

> 
> General Question Number 1: Flash duration for this Vivitar 2800  is
> rated at 1/2,000 to 
> 1/30,000th sec.  What conditions dictate this speed?  What if I wanted

The auto-exposure mode adjusts exposure by changing the flash duration
rather than changing how bright the flash is.  The speed that it uses,
is therefore, dependant on how much flash is needed for the selected
exposure.  If you calculate the subject distance for manual mode (and
the f/stop being used) and use auto-mode for that distance or further,
you'll get the 1/2,000 flash duration ("the whole thing").  If you put
a white sheet directly in front of the sensor, you'll get the 1/30,000
duration (theoretically).

> to be Harold Edgerton in my 3D dreams, and I really wanted to get
> 1/30,000 out of this, how would I do it?

Put the subject REAL CLOSE (and possibly a white background just
behind it).


>
> General Question Number 2: It is reasonable to consider "hard wiring"
> two flash units together, so that the hot shoe (switch) completes two
> circuits in parallel, causing a main flash and a second (fill) flash to
> fire at the same time?  Yes, I know that I can buy slave sensors, but is
> hard wiring an option?  I know how to solder wires, but I'm not smart
> enough about flash to know if this is a knucklehead idea.

The 292 has connectors for connecting multiple flashes together.  If
you wire it yourself, it might work, but diode isolation may be needed
for same reasons as for electronic camera shutters.  Copy of the 
circuit would make that determination pretty easy.

> 
> General Question Number 3: I've seen some talk about either voltage or
> current levels in strobe units being high (I mean on the shoe...  of
> course inside the unit, it's dangerous).  What is the implication of a
> higher current in camera use?  I gotta imagine that, for example, inside
> the trusty Realist, it's just mechanical situation where widest open
> shutter equals closed contact points which lead to shoe).  Example: I
> have a flash from a Nishika (cheap, I know) but it's got a main light
> and a bounce light in the same unit,  nice idea, but I've heard they put
> out a lot of current.  So what?  Can't imagine it'll eat my Realist
> (worst case is arcing on internal contacts, leading to pitting, or weird
> light leaks from "internal flash"). Nor can I imagine it'll kill the
> strobe, as a shoe is an open point in a circuit asking for closure.
> What's the deal on high current across the shoe?

Some do seem to arc a bit if you manually short it out.  Not sure
how this compares with the bulb-firing current that the camera 
contacts were made for, although I have my personal guesses.

Mike K.



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