Here, here, m'dear,
Never fear,
Dr. Chuck is here
To illuminate the way
of the flash by day
and by night,
why 'tis nothing but light
But always remember
when pressing the shutter
the ambient still counts
with the flash on the mount
If the shutter is long
then the flash ain't so strong
(well, uh, relatively speakin' of course)
and two exposures be here
so be of good cheer
and count each its own right
in the sum of all light.
And finally,
remember thee well the square root of two
lest the inverse square law
make a fool out of you
-----------------------------
That was an intro to my basic flash tutorial. I will give you one hint
that you may not find so clearly stated elsewhere but you must always
remember when shooting flash. When using flash there are *always* two
exposures. I repeat. When using flash there are *always* two
exposures. And again until it sinks in. When using flash there are
*always* two exposures.
One exposure is from the flash unit and the second exposure is from the
ambient light. Either one can be the dominant light source or they may
contribute fairly equally. If you're in a dark room the flash will be
the dominant exposure and the ambient exposure will be small or
non-existent. If the flash is much more powerful than the ambient
exposure you will end up with a look like a bright object in a dark
cave. Typical of lots of point and shoot picutures taken in dark rooms
with little, built-in flashes. On the other hand, if you're doing
daylight fill flash the ambient daylight is the dominant exposure and
the flash is adding a lesser exposure that serves to lighten and soften
the shadows cast by the brighter sunlight.
With the exception of FP mode on the FL50 (which simulates *F*ocal
*P*lane flash bulbs) flash exposure is always very much faster than your
fastest shutter speed. What that means is that you can't control the
flash exposure with the shutter. No matter how fast you set the shutter
the full power of the flash will get through because the shutter is a
mechanical slow poke compared to all those light speed electrons and
photons. Thinking that through for a second you can see that aperture,
ISO and flash power (stongly affected by distance) are the only ways to
control flash exposure. On the other hand, that second, ambient
exposure I mentioned at the start *is* affected by the shutter speed.
So, when using flash photography, you can use an effect called "dragging
the shutter" to enhance the photo by capturing not only the flash
exposure but the ambient light in the room as well. You do that by
using a longer shutter speed than for the flash alone to allow the
weaker ambient light a chance to build up.
This photo exhibits shutter drag since it was taken at 1/4 second to
capture not only the brief flash exposure but to allow the incandescent
lights lining the inside of the tent a chance to register.
<http://chucknorcutt.com/party.php>
If I had taken it at 1/180, for example (max flash sync speed for your
camera), the flash would have lit the scene but the incandescent lights
would have been almost invisible. If you look closely you can see some
subject motion due to the "shutter drag".
The final part of my little poem warns that the intensity of the light
from a flash falls off very rapidly with distance... and, like f/stops,
according to the square root of two. You can use f/stops in place of
distance to illustrate the effect. For example, let's pick on f/8, f/11
and f/16. Assume the subject is 11 feet away and the flash is set for
proper exposure at 11 feet. If there's something ahead of the subject
at 8 feet that subject will be one stop overexposed. If there's
something behind the subject at 16 feet that thing will be underexposed
by one stop. The lesson is that the light inensity is varying so fast
with distance that the proper exposure is only obtained within a narrow
range.
Now lets keep on with the f/stop analogy. Let's back up to 2.8, 4 and
5.6 as model distances. Now we're really in trouble. If proper
exposure is obtained at 4 feet we've only got to come foward 1.2 feet to
2.8 feet to be a whole stop overexposed. Likewise, we've only got to
add 1.6 feet to get to 5.6 feet and find that were already underexposed
by a full stop. Moral of the story? Don't get too close with flash.
Now if I said use 16, 22 and 32 as the model distance you'd be able to
fill in everything for yourself and see that distance aids exposure
latitude. Ah, and what happend when you bounce your flash? Why you add
distance of course. But all is not roses. Distance tends to make a
flash look more like a point source of light which causes harsh shadows
so you add a stronger need for diffusers to soften the light by making
it larger. Size counts in diffusers.
Ah. That was fun. Now what was your question? :-)
Chuck Norcutt
Candace Lemarr wrote:
> Second issue...I am not getting the lighting right with my FL50. It's
> driving me nuts. I don't understand much of what I find when I am
> surfing for answers, and the English manual is just awful....or I am
> very very unintelligent. Does anyone have online links to resources
> about the FL50? I am not finding anything useful yet. And all the things
> I do read seem so complicated....is there a way to uncomplicate external
> flash photography? I am just not-getting-it, and feeling very frustrated.
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