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[OM] Ambient and Flash Ratio Tutorial

Subject: [OM] Ambient and Flash Ratio Tutorial
From: AG Schnozz <agschnozz@xxxxxxxxx>
Date: Wed, 13 Feb 2002 22:30:14 -0800 (PST)
Just to clarify something here:  When I refer to the ambient
reading, think of this as the background.  Your flash will light
your subject and the ambient light will take care of the
background.  Usually the distances are such that your flash will
have negligable affect on the background.

So, the question you ask yourself is:  "How many stops below do
I want the background"?  With wide latitude, good handling films
like Portra, I typically go two stops.  This gives the
background a nice warm glow without going black.  Your subjects
will stand out from the background without looking like cave
dwellers.  Slide films usually need tighter ratios of 1:2-1:3.

When doing flash/ambient exposures we have only five controls:

1. Shutter Speed
2. Aperture
3. Film Speed
4. Flash Power
5. Flash-Subject Distance

Focal-Plane shutters in our OMs limit us to nearly exlusively
using a Shutter Speed of 1/60.  So scratch one "user-selectable"
setting off our list.  1/60 is a fixed, known quantity.  We have
our start point.  In lens-shutter cameras, we can start with the
aperture if we desire.

Imagine for a minute that our cameras could ONLY do 1/60 second
exposures.  If you are shooting Fuji Velvia (ISO 50) what is
your bright sunny day exposure going to be?  F16.  (sunny 16
rule).  A cloud rolls over, now what?  F11.  Unfortunately our
camera is stuck on 1/60 so I can only change the F-stops. 
Pretty limiting, huh?  It can be, but as always, we have ways to
get around it.  We go indoors and the meter now indicates that
my proper exposure is 1/60 and F1.4.

1/60 and F1.4.  Oops, Our lens only goes down to 2.8.  Now what?
 Let's look at our list of controls.  Assuming we aren't using a
flash yet, my only choice is to increase film speed.  Ok, I
install a roll of ISO 200 film in the camera and my meter
reading now says 1/60 and F2.8 is the correct exposure.  I can
work with that.

Now I want to mix flash with the ambient light.  If a correct
non-flash exposure is 1/60 at F2.8, then what would it be if it
is 2-stops under-exposed?  Again, we can't touch the
shutter-speed--Miatani's OM-.2n didn't have that ring installed.
 That's right, 1/60 at F5.6.

Ok, now we have locked in three of our five variables.  These
are now the "untouchables".  You can't do ANYTHING about them
now.  1/60 is glued in place because of the focal-plane shutter,
the film speed was selected to give us a range of usable
F-stops.  The F-stop was selected to give us a desired ambient
lighting exposure.

Now what?  Ah!  Flash power and flash-subject distance.  These
two work hand in hand.  If you have a manual flash (no auto
anything) you will take your F5.6 (as determined two paragraphs
above) and look at the calculator dial to see what the
flash-subject distance should be.  Since our flash is mounted on
the hot-shoe, the only way I can control that distance is with
my feet.  You move towards or away from the subject to the
indicated distance.  Compose (zoom or change lenses if
necessary) and shoot.  Result is a perfectly exposed picture.

So you have an auto flash or you are using OTF flash control.
Now what?  Look at the flash calculator dial to determine the
maximum distance the flash will be affective at the selected
F-stop.  Don't exceed this distance unless you like flat,
lifeless, grainy pictures.  You can shoot nearly any distance in
the range without worry.

Many flashes have multiple power levels. These give you the
ability to shoot from more than just one distance.  The Vivitar
283/285 are good examples of variable power flashes.  If I'm
using manual flash settings, and I'm shooting a subject at one
meter with an aperture of F4 I probably would need to adjust the
flash down to 1/16 power.  At two meters I would use 1/8 power,
four meters would be 1/4 power...  That's exactly what OTF or
auto flash does.  It is essentially varying the flash power to
compensate for variations in flash to subject distance.

One final note.  I've been referring to the "Flash to Subject"
distance, not "Camera to Subject" distance. This is because
exposure is NOT dependent on where the camera is, but where the
flash is.  If you seperate the flash from the camera and place
it at the "ideal" distance from the subject, you are completly
free to move around with the camera and shoot from any distance
and with any focal length.  Most pros still don't understand
this concept.  Every time they change camera locations they
re-meter the scene.  Even though the strobes and subjects
haven't budged.  I floored another wedding photographer when I
took the camera and shot from clear across the church without
changing ANY settings.

Executive summary (focal-plane shutters):
1. Shutter Speed of 1/60 is mechanically required by the camera.
2. Select Film Speed to get a usable range of apertures.
3. Use aperture to control background exposure.
4. If manual flash, use calculator dial to determine
flash-subject distance.
5. If auto flash or OTF, use calculator dial to determine
Maximum and Minimum flash-subject distances.

Ken N.

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