A follow up flash lesson, with many, many references to the inverse square law.

1. Flash-a Review

2. Flash Freezes
• For most of our applications flash will
freeze any motion
• Flash is metered through the lens.
• When enough light has bounced off the
subject and hits the sensor the flash is
cut off

3. Flash doesn’t care much care
about the shutter speed
• There is a maximum shutter speed that the
camera will allow us to use with the flash
turned on
• Different cameras are different but it’s usually
about 1/200 sec.
• Speeds slower than that, such as 1/125, 1/60,
etc. will not effect the flash exposure

4. Modern flashes don’t much
care about aperture either
• The aperture will not make the
exposure from the flash more or less
(because the camera reads the amount
of light coming hitting the sensor)
• But…..

5. A little bit of physics
• Light like other electromagnetic radiation
obeys the inverse square law
• The intensity varies as inversely as a square
of the distance.
• That means that every time light goes twice
as far it has a quarter of the intensity
• Tell that to the folks with their cameras in the
back of the concert hall
• So…

6. The inverse square law
there’s trees back there but not enough light

7. What that means for us
• The through the lens (TTL) metering takes
care of the exposure until the flash is putting
out it’s maximum power and then
• “I’ve done all I can do and I can’t do no more,
open up that lens and let the glass do the
work”
• Actually do the math.
• If the camera is set to iso 400 and the flash is
putting out maximum power and at f 8 the
flash gives us correct exposure at 12 feet, at
f4 it will give us the correct exposure at 24
feet, but

8. Hey that means that
• Yes! at f 2 it will give us the correct
exposure at 48 feet.
• Now pump up the iso to say 1600 from
400 and f 2 will give us the correct
exposure at 96 feet. Hey we can use
flash from the back of the crowd but..

9. I don’t like this atmosphere
• If the air is smoky (used to be a big
problem but finally the politicians did
something for photographers), or dusty,
or there is mist or rain or snow

10. Or Mosquitoes

11. • Our flash will light that up. (Oh no! it’s the
inverse square law coming back at us and it’s
not friendly)
• That being said we can light up far away
things with out flashes.

12. In the Nice Clean Air

13. Why does flash look like flash?
• It lights up close things more than far things.
(It’s not just a good idea, it’s the law)
• It is close to a point source (very hard edged
shadows.)
• It is close to the axis of the lens so the light is
coming back at us and it’s not friendly.(If you
wonder why faces are shiny with flash it’s
because they are acting like mirrors)

14. What can we do?
• Live with it.
• Keep important things about the same
distance away from the flash
• If you can’t keep them the same
distance keep them as far away as
possible

15. Pretty Even

16. What else we can to
• Bounce it. The flash is going further so the
you-know-what isn’t so bad, and it’s no longer
a point source, and light is bouncing all over
the place lighting up the shadows, but
• Don’t get too close, especially to us old folks
because
• The light goes straight up and comes straight
down making all kinds of shadows under eyes
etc.

17. The ball don’t bounce if there
ain’t no wall
• If there is no ceiling or the ceiling is high
or painted a dark colour it won’t work
• If it’s painted a funny colour let’s hope
you subject looks good in that colour.

18. If you can’t beat it modify it
• There are many things you can stick on your
flash to make the light nicer
• A diffuser is still directional but it bounces
light all around the room to fill in the shadows.
Try a foam coffee cup.
• A piece of card or paper attatched to the top
of flash that is being bounced will reflect light
into the shadows that are created with the by
the light bouncing off the ceiling.