----- Original Message -----
From: "Hans Klemmer"
: Now, if you would like me to begin a discussion by asking a question,
: I will; how do the rest of the instructors in this forum teach the
: inverse square law to new students?
I start by telling them that light levels fall away the further you are
from the light source unless the light is colimated and travels parallel
(as such this is treated differently) then I check for the blank stares and
suggest they rmember the last time they used a torch in the dark Vs say a
laser pointer - the sighs and smiles indicate they've gathered the concept.
I then remind them that when we're shooting under daylight conditions this
usually isn't a problem because our big yellow keylight (he sun) is a
massive distance away and it really doesn't make any practical difference
whether we're standing at the bottom of a hill or at the top given the
relative difference in distance from the sun. they grin and say 'duh!'
I then tell them that our huge blue fill light (the sky) is similarly
unaffected given it's relative distance, again under normal circumstances.
they look confused and ask how the sky can possibly be a fill light so I
explain some rot about atmospherics and then demonstrate the practicalities
by holding a shade over an object thus blocking the 'keylight' and they all
see that the fill does indeed 'fill' - they smile again ..
then I remind them of the torch example and suggest it would be *really
nice* if we could work out some sort of relationship between the light
falling on our subject distance to the light source - they furrow their
respective brows and try to think of one..
Then I draw (really badly) an angle on the board , marking it off at
arbitrary but consistant intervals away from the apex like so (the actual
angle doesn't matter as I later demonstrate with a number of badly drawn
angles, I couldn't think how to make a ascii angle look good )
.
.
. <-- I can't fit 3 or 4 dots in here
. in ASCII and get the intervals right :-(
. .
.
.
.
.
and then I run a series of height dividing lines up the side (imagine the
dots are where I place the distance marker lines, and the dots are also
intersected by the actual lines of the drawn angle)
they easily see that the relationship is:
at a unit distance of '1', the light covers 1 unit
at a unit distance of '2', the light covers 2 units
at a unit distance of '3', the light covers 3 units
at a unit distance of '4', the light covers 4 units
they also easily understand that for a unit of light to travel distance '4'
and cover 4 units, the amount covering each individual area must be only
1/4 the amount
I then look wise and say "this simple fact is called 'the inverse square
law' and it's REALLY complicated and not many people understand it" - they
all smile wisely and look knowledgeable
..and I show them what inverse means and square means by way of equations
and stuff .
..and then I tell them it really only applies for a point source of light,
and their brows furrow again until I ask them "if you were using a
reflector behind say a light bulb, then the light would be semi-collimated,
wouldn't it?
and they all go 'oh yeah..' and realise that it would be a lot harder to
calculate a relationship like that, so I tell them not to bother for now.
Some smarty always puts up their hand and says 'yeah but thats just for a
simple angle in 2D and not a real 3D light' so I congratulate the smarty
and suggest they see if it works with a circle of light or a square of
light at the same distances and a few pull out calculators while others do
the math in their heads and they allo conclude that it's the same and
drawing cones of light initially would have been a bit confusing.
After that I find them applying this concept in the studio with softboxes
and the like and I see they've all got it :-)
k
