Pentax ZX-5N ZX-5n Manual - Page 77

vignetting

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SNOLLVklad0 030NVACIV Calculating the flash effective distance according to the camera-to-subject distance. Maximum flash distance = Guide Number ÷ Selected aperture Minimum flash distance = Maximum flash distance ÷ 5* When the distance to the subject is less than 0.7m (2.3ft), the flash cannot be used. If the flash is used within that distance, it causes vignetting in the picture corners, light is distributed unevenly and the picture may be overexposed. • The value 5 used in the formula above was obtained from the built-in flash. The guide number (GN) depends upon the film speed used as shown below. ISO25 --- GN5.5 ISO200 -> GN15.6 ISO50 --. GN7.8 ISO400 -> GN22 ISO100 -> GN11 Guide Number (11) f/2.8 = 3.9m 3.9 ÷ 5 = 0.8m Thus, the flash effective distance is from approx. 0.8m to 3.9m. Calculating the aperture according to the camera-tosubject distance Aperture = Guide Number ± Camera-to-subject distance If the calculated aperture value is different than an indicated f-stop on the aperture ring, for instance f/3, choose the next smallest aperture ring number (f2.8 in this case). Calculating the camera-to-subject distance in the Shutter-Priority AE Mode. The camera-to-subject distance can be calculated using the above mentioned formula. However, in the ShutterPriority AE Mode, the-camera-to-subject distance will change depending on what aperture is set. If an ISO100 film is used at an aperture of f/2.8, the flash effective distance is obtained as follows:

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Calculating
the
flash
effective
distance
according
to
the
camera
-to
-subject
distance.
Maximum
flash
distance
=
Guide
Number
÷
Selected
aperture
Minimum
flash
distance
=
Maximum
flash
distance
÷
5*
When
the
distance
to
the
subject
is
less
than
0.7m
(2.3ft),
the
flash
cannot
be
used.
If
the
flash
is
used
within
that
distance,
it
causes
vignetting
in
the
picture
corners,
light
is
distributed
unevenly
and
the
picture
may
be
overexposed.
•
The
value
5
used
in
the
formula
above
was
obtained
from
the
built-in
flash.
The
guide
number
(GN)
depends
upon
the
film
speed
used
as
shown
below.
ISO25
---
GN5.5
ISO200
->
GN15.6
ISO50
--.
GN7.8
ISO400
->
GN22
ISO100
->
GN11
If
an
ISO100
film
is
used
at
an
aperture
of
f/2.8,
the
flash
effective
distance
is
obtained
as
follows:
Guide
Number
(11)
f/2.8
=
3.9m
3.9
÷
5
=
0.8m
Thus,
the
flash
effective
distance
is
from
approx.
0.8m
to
3.9m.
Calculating
the
aperture
according
to
the
camera
-to
-
subject
distance
Aperture
=
Guide
Number
±
Camera
-to
-subject
dis-
tance
If
the
calculated
aperture
value
is
different
than
an
indi-
cated
f-stop
on
the
aperture
ring,
for
instance
f/3,
choose
the
next
smallest
aperture
ring
number
(f2.8
in
this
case).
Calculating
the
camera
-to
-subject
distance
in
the
Shutter
-Priority
AE
Mode.
The
camera
-to
-subject
distance
can
be
calculated
using
the
above
mentioned
formula.
However,
in
the
Shutter
-
Priority
AE
Mode,
the
-camera
-to
-subject
distance
will
change
depending
on
what
aperture
is
set.
SNOLLVklad0
030NVACIV