The Nikon N6006 (F601 outside the U.S.) has two program exposure
modes, P and Pm. This article will compare the two
and explain why you should probably never use the P
setting.
Program Mode P
The Canon A-1 (1977) was the first 35mm SLR to offer program
mode. This program took a very simplistic approach to choosing a
shutter speed and lens opening: starting at 1/1000 sec. and f/16,
it reduced the shutter speed by one step and opened the lens by one
stop in a linear fashion.
As other makers added program exposure to their lines, they
tended to follow Canon's lead. It wasn't until the Minolta X-700
(1981) that someone devised a better program.
What Was Wrong With Canon's Program?
The problem with Canon's program mode was low light photography.
One of the first rules of 35mm photography is that you should avoid
hand-holding exposures below the reciprocal of your focal length --
that would be roughly 1/60 sec. with a 50mm lens or 1/125 for a
135mm telephoto. In low light, that often meant shooting your
normal lens (most often f/1.4-1.8) wide open or close to it.
Canon's program, which Nikon essentially duplicates with the
N6006's P mode, doesn't do that. It's not intelligent; it won't
open an f/1.4 lens all the way until the shutter speed reaches 1/8
sec. Knowledgeable photographers would be shooting wide open by
1/30 or 1/60.
Put another way, at EV 7 (EV is a standardized light level) the
program would choose roughly 1/20 sec. at about f/2.4, which could
easily lead to camera shake showing up in the picture. A smart
photographer would shoot at 1/60 and f/1.4 if his lens was fast
enough, 1/45 and f/1.8 if that was all his lens allowed.
With program shift and several exposure modes to choose from, a
savvy photographer can override the default program settings. On
the other hand, the whole point of program mode is that the camera
is supposed to choose the best setting for you.
Until the Minolta X-700, program mode was counterproductive to
low light photography. Minolta gave the whole thing a bit more
thought and devised a program that (a) would shoot "wide open" by
1/60 sec. and (b) chose even higher shutter speeds with tele lenses
with sufficient light. Thus was born the multiprogram mode.
Nikon's P Program
The standard P mode in the N6006 and earlier Nikon SLRs was
based on Canon's model, but extended to 1/2000 sec. and f/22 on
cameras and lenses that allowed those settings. It's a simple
step-for-step, stop-for-stop program that's okay in bright light
but bad news in low light.
Program Mode Pm
The multiprogram mode (Pm) in the N6006 follows Minolta's lead
-- and then goes beyond it. In all cases, if a lens goes to f/22,
the Pm mode will set 1/2000 and f/22 at EV20. But after that, all
bets are off.
Nikon's Pm mode follows some rules that are based on the focal
length of the lens you are using and their probable
application.
- The lens will always be wide open at the reciprocal of the
focal length: 1/30 for a 28mm lens, 1/50 for a normal lens, 1/200
for an 80-200 zoom at 200mm. This guarantees the highest possible
shutter speed in low light conditions.
- If then lens is faster than f/2.8, the camera will use the
reciprocal speed between f/2.8 and wide open. This provides the
extra depth of field most trained photographers would choose in
that situation.
- Between f/2.8 (or the maximum aperture for slower lenses), Pm
mode closes down 2 stops for each step of shutter speed. In the
case of a 28mm f/2.8 lens, than means 1/60 @ f/5.6 (EV 11) and
1/125 @ f/11 (EV 14). In the same light, the 50/1.4 lens would be
set at about 1/90 @ f/4.8 and 1/180 @ f/9.5, respectively.
- In the case of the 28mm lens - and this probably applies to
other wide-angle lenses as well - once Pm mode reaches f/11, it
switches to two steps faster shutter speed for each stop. Reasons
for this probably include the luxurious depth of field and a desire
to avoid the smallest apertures, which tend to be less sharp than
medium ones. (For most lenses, best optical performance ranges from
about two stops from maximum to about f/8.)
Instead of a "one size fits all" program mode, Nikon engineered
a program mode based that accounts for focal length and the way
trained photographers typically work. Where Minolta's dual program
X-700 had been a real innovation, Nikon's Pm mode fine tunes it for
better performance with any focal length lens.
Visual Aids
As photographers, we generally believe a picture is worth a
thousand words. On the other hand, some people are intimidated by
graphs, so I wanted to explain in words just what these graphs
illustrate. Except for the 105/2.5 chart, these are copied from the
N6006 manual.
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This graph shows exposure settings with a 28mm f/2.8 lens. Note
that with this slow a lens, both the P (red line) and Pm (green)
programs are wide open at 1/30 sec. Also note that Pm provides more
depth of field when light permits.
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This graph shows exposure settings with a 50mm f/1.4 lens. Note
that the P program (red) begins to close down the aperture at 1/8
sec., while the Pm program (green) waits until 1/50 sec. to do so.
This will minimize camera shake. The Pm program then closes down
the lens without changing the shutter speed until it reaches f/2.8.
From there, it soon provides more depth of field than the P
program.
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This graph is extrapolated from the above rules and the chart in
the N6006 manual. It shows the exposure settings we can expect with
a 105mm f/2.5 lens. Where the P program (red) begins to close down
the f-stop at about 1/20 sec., the Pm program (red) keeps the lens
wide open until about 1/100 sec. to reduce camera shake. Except in
bright conditions, this program will use a wider aperture and
higher shutter speed than the P program.
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This graphs shows exposure settings with a 500mm f/4. The P mode
(red) would begin closing down the lens at 1/60 sec., which is
obviously unholdable using a 500mm lens. If I read the chart
correctly, the Pm program (green) waits until about 1/700 sec.
before it starts to close down the aperture, leading to
consistently higher, more hand-holdable shutter speeds.
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Using the rules above and the graphs in the N6006 manual, you
could probably create your own charts (as we did for the 105/2.5)
for such interesting lenses as the Sigma 20mm f/1.8, 300mm f/2.8,
and others you might own - even variable aperture zooms. If you
understand the dynamics of shutter speeds, f-stops, and
hand-holdability, I think you'll agree that the Pm program is
vastly superior to the P program for general use.
As I said above, you should probably never use the P
program.
Go to Nikon N6006/F601
page.
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