From: jacobson@cello.hpl.hp.com (David Jacobson)
Newsgroups: rec.photo
Subject: Re: Flash Photography
Date: 29 Jul 92 04:45:13 GMT
Distribution: na
Organization: Hewlett-Packard Laboratories

In article <1992Jul28.183104.14316@twisto.eng.hou.compaq.com> parikh@sirocco.eng.hou.compaq.com (Vijay Parikh) writes:
> I have a Minolta
>X-700 with manual shutter speed, exposure, and aperture control.
>
>In a hall (size 25x25 feet), what would be the best shutter speed and aperture
>combination?  I have been using F8 (aperture) and 60 (shutter speed) when I takepictures during night time (requiring flash).  Does my aperture have to be
>smaller if I want to take picture of something which is about 10-15 feet away?
>Can I simply use P mode (programmed mode) with F22 (minimum aperture) for the
>same scenario and achieve the same results?  Waht would be the maximum range
>of my flash with F22 selected?
>
>BTW, I use 100 ASA film.  Any help or tips would be really appreciated.
>
>Thanks in advance!

Ok, a brief tutorial on old fashioned manual flash photography.  

Basics: One characteristic of your flash unit is its "Guide Number",
which depends on the film speed.  If you look in the manual, you will
find some remark like "Guide Number 110 at ASA 100".  (This is
strictly hypothetical.)  When using ASA 100 film you compute the
f-stop by dividing the distance into the guide number.  So if you are
25 feet from the subject you would need to use 110/25 = 4.4.  So you
set your lens on f/4 (close enough) and you take pictures.  To a first
approximation the shutter speed doesn't matter, as long as it is not
faster then the "sync speed", ie the fastest speed at which the entire
frame is exposed at once.  This will be mentioned in your camera's
manual, and perhaps marked in red or with an X on your camera's
shutter speed dial.  Typical speeds for older 35mm cameras are 1/60th
or 1/125th second.

Calculation shortcut: The guide number varies with the square root of
the film speed, so it will be 220 for ASA 400 film.  But if you've
lost your manual, or just don't feel like calculating if you don't
have to, just look on your flash.  There is almost certainly a
calculator dial.  You set some pointer to the ASA reading of your
film, then find the distance and read the f-stop next to it.  (In all
sensible parts of the world different guide numbers based on measuring
distances in meters are used.)

(Hit n now.  This is too long.)

Balancing:  If you shoot at the sync speed, you get the max ratio
of flash illumination to ambient illumination.  Longer speeds let in
more ambient light, but the flash, being much shorter than the shutter
speed, is unaffected by shutter speed.  Thus you can control the
appearance by simply using the shutter speed.  Clearly you don't want
the shutter speed so slow that ambient light by itself overexposes the
picture by you can vary it between that point and the sync speed for
artistic effect.  

Auto flashes: Sometime in the last 20 years, someone invented
"automatic" flashes.  They are simultaneously much easier and a little
harder to use.  They have a sensor on the front that monitors the
light bouncing back from the subject.  When a selected amount of light
is reached, a fancy electronic circuit "quenches" the flash.  So
(naively) as long as you use a large enough aperture (small f number)
to cover to the longest distance you imagine you might need, you can
work without thinking about the flash---it will control the light
automatically.  

Auto flash complications: The part that makes it harder is that it
really can't cut the light down arbitrarily small, but rather there is
some minimum power, typically 1/16 to 1/64th of full power that it
will put out.  Check the manual for this.  So you really have to
figure a range of working distance you will likely be at, then select
an f-stop that will make the flash able to cope with that range.  And
even though for small changes in distance you don't have to diddle
with f-stops, you do have to remember the range and ask "Is the
subject still in the range?"  

Further complicating things is that you typically get the choice of 2
or 3 thresholds, and you have to choose which one you want.  The
choice will affect
	* Max and min working distance.  
	* f-stop and hence depth of field
	* Balance with ambient illumination
	* How fast the flash recycles
	* How much battery power consumed
	* How bright the flash is (important when photographing
		newborns in dimly lit nurseries).

Now you see why auto flash is both simpler and more difficult.  And
that wasn't all.  I didn't tell you about the problems with bright or
dark or small subjects fooling the sensor.

Sorry for getting carried away with more than you asked.  Just ignore
the bottom part.  Oh yes, and you asked about Program Mode.  Check
your manual, but I suspect sticking with manual mode will give you a
better chance of getting it right and you will better understand what
is going on.

  -- David Jacobson