Penumbra the prettist part of the light or the edge of the light
example instead of turning a softbox towards our subject so that the face of the light panel is squarly facing our subject we turn the light so that it is pointing away from the subject BUT still the edge of the light is illuminating them.
In a way skimming the light.
Main Light is the light that determines the thickness of our negative or the base exposure iow this is what we meter and then set our camera’s aperature and shutter speed at
Fill Light Both Broad and difused iow a large light source that is Non specular
typicaly placed behind the camera
This light softens or adds detail to the shadow area of our images
Incident the light FALLING onto our subject
The MOST acurate way to measure light for portraitue is to use a hand held light meter and read the light falling on to our subject
This is known as an Incident Light Meter reading
it is more accurate than a reflected light meter reading because reflected light readings change based on what the scene looks like
(Note: For the following examples we are using Hot lights or continuos lights NOT flash because I want to illustrate using the cameras buit in light meter which is not a flash meter)
iow if we are photograping a Black Dog
and we meter using the light meter in our camera
we may read 1/60 at f 5.6
if we use the same lights and the subject is placed in the same area as the black dog was and this dog is WHITE we will read more light on our cameras meter EVEN though we have not changed our lights iow this time we read
1/60 at f 11
Now we decide to use a Hand Held light meter and measure the light falling onto our subject
iow we place the meter at the subjects location and point the meter back toward the camera and measure the light (This is an Incident light metering technique)
we read 1/60 at f 8.0
this reading we be the same unless we change the distance of the light from the subject
this reading is not affected at all by what the subjects are wearing
iow we get a consistant reading and this helps us tremendously !
If you shoot film you can stop reading 
If you shoot digital read on….
You find that if you meter with you brand new hand held light meter
the camera is always under exposing or maybe yours is over exposing
iow you meter the light falling on to your subject and then set the camera at that exposure
but the images are too bright
and you confirm this by looking at the Histogram in your camera
This is caused because you camera and your light meter are not Calibrated together iow the chip at the “film” plane in your camera is more sensitive to light than the hand held light meters device is.
So we need to either adjust the camera or the light meter so that when we read f 8 with the light meter we set the camera on f8 and all is good
if you only have One Camera and your hand held light meter allows it
then just adjust the light meter so that it is at the same sensitivity as the camera iow reads the same exposure
the way to do this accurately is to take a photograph
of a scene that has the highlights shadow area and mid tones all properly exposed
use a Zebra card or Curacio card
photograph this target by filling the frame of the view finder
iow do not allow other bright or dark areas that are not on the target into the image
look at your histogram and see if the scene is properly exposed
if it is not adjust the camera aperture / shutter speed and re shoot until you have a perfect histogram balance
what are the cameras setting for this propper exposure ?????
let’s say you had the camera at 1/60 at f 4.0
Now using your light meter take an incident light reading from where the Zebra Card is at
you find you reading f 2.8 at 1/60
so we need to adjust the calibration of the light meter
on most sekonics it is a simple matter of pressing and holding the Two ISO buttons while we turn the dial
we turn this tell we get the amount of change in exposure that is needed
release the buttons and you should have f 4.0
Now the Light meter is on the same page and the camera, Calibrated
Here is an excellent in depth article on the Histogram and it’s use……
the following link explains histograms and has diagrams
http://www.luminous-landscape.com/t…istograms.shtml
