| The
autofocus
The
autofocus is one of the F5's really bright sides. The F5's
AF is very fast and precise, even in low-light situations
and even so without any help of nervously blinking IR LED's
on Nikon Speedlights.
The
module responsible for the magic is called "CAM1300",
where "1300" is roughly the number of CCD-elements
used in the module (there are actually over 1,300 CCD elements
in the CAM1300). The CAM1300 is split upon three separate
chips where all the CCD elements are divided upon five sensors
(small printed circuit boards, or PCB's).
| |
Christmas
market in the city of Villingen, southern Germany.
December 1999.
Nikon F5, Nikkor AF 50mm/1.4 on Fujichrome Provia
100F.
|
Let's
have a closer look at how this work
These
five sensors are located in a cross arrangement, where one,
the largest sensor is located in the center having four sensors
surrounding it. You could probably write a book about how
these sensors and the electronics behind work, but let's keep
it short.
The
sensors cover the central area of the view. This is then approx.
the circle you see if you look through the viewfinder using
the standard focusing screen EC-B, an area of some 16 x 7,1mm
(W x H). There are a total of 13 interchangeable screens available
for the F5 . The EC-B screen comes with a transparent LCD,
so actually the standard F5 has four LCD's: The main on top,
the rear, the one at the bottom of the viewfinder and the
transparent one in the screen. There are four small squares
located around the middle square on nearly all screens. This
is also the case on the EC-B screen. The squares then correspond
to the location of the focus areas (the five sensors).
The
sensors can be divided into the following three types:
Line
sensor. The two sensors responsible for the top and bottom
metering areas are of this type.
Small
double cross sensor. The left and right metering areas
each uses a sensor of this type.
Large
double cross sensor. The central metering area uses a
single sensor of this type.
The
top line sensor is slightly tilted from left to right. The
right and left double cross sensors are both tilted inwards.The
lower line sensor and the central double cross sensor are
not tilted. The sensor tilt was done to improve the measure-accuracy
independently of the viewed object's form.
The
three double cross sensors are really interesting. Each sensor
consists of two overlapping crosses arranged with a slight
offset. One cross has thin arms whereas the other cross has
wider arms. The "thin cross" is used for rapid focus
detection when there's lots of light available, whereas the
"wide cross" is used in low light situations. The
length of each arm used for detection is also depending on
the light conditions, i.e. the utilized number of sensors
in the cross arrangement is light dependent. In good light
conditions, a bit less than the full cross size is used, whereas
in low light conditions more, or all of the sensors in the
cross are used. Ignoring some of the sensors available improves
autofocus speed. All sensors are used in "Dynamic AF".
This
great system provides accurate autofocus detection in the
range of EV -1 to EV +19 at ISO 100. This means that it still
can focus correctly with no IR help even when your own eyes
have a hard time seeing the finer details through the viewfinder.
Combining
this smart arrangement of sensors with a fast, coreless DC
motor, and you got the AF you've been looking for.
The
AF modes
There are two basic AF modes in the F5:
1.
Single (S) servo AF
2. Continuous (C) servo AF
You
select the desired mode using the focus mode selector located
at the lower left front. You can of course select manual (M)
focus mode using this switch too, but if you have modern AF
lenses, I guess you won't use that mode much - if ever.
A
small note on using S or C AF modes: You shouldn't try to
manually adjust the focus using the focus ring on any lens
not being of AF-S type. You can move the focus ring
lens in these modes on a non AF-S lens, but I don't think
the coreless DC-motor driving the lens will be too happy about
it.
There
are three LED's in the viewfinder, indicating focus status:
two red colored, arrow formed LED's, indicating that the current
focus is either in front of or behind the object (i.e. the
servo still runs) and a green colored round LED which lights
up when the object is in focus. I've found out that I seldom
look at these LED's, but rather at the object itself. Since
the focusing is so fast, the two red colored LED's are of
no interest anyway.
Single
servo AF
First of all, in this mode the F5 treats stationary and moving
objects differently.
By
slightly pressing down the shutter release button, the AF
servo is activated and the F5 tries to find the correct focus.
You can also start the AF servo by pressing down one of the
two AF-ON buttons at the back of the F5. I never use these
buttons. You can disable AF start through the shutter release
button by activating CS #4. This would then be a real reason
for using one of the AF-ON buttons.
If
the object is stationary right about the time the F5
tries to achieve the focus lock, the focus is locked and the
F5 stays in Single servo AF mode. Keeping the shutter release
button pressed and the focus remains unchanged, even if you
move the camera. This is then the major difference between
Single (S) servo AF and Continuous (C) servo AF mode.
If
the object is detected as moving, before the focus
lock has been established, the F5 will temporarily switch
over to Continuous servo AF mode as long as the object is
moving. If the object stops, the focus locks and the C mode
is abandoned. You must then let go and repress the shutter
release button to start a new focus hunt. This behavior is
basically a good thing. There are times though when you might
wish the F5 wouldn't do this magic, i.e. temporarily switching
over to C mode, and this may be the case if you're using the
F5 handheld with a tele lens; rather than letting you get
a focus lock and shoot, the F5 is fooled to believe that your
slight lens movements equals a moving object and it nervously
hunts for a lock.
So,
the default behavior in S mode is not to allow you
to take a shot as long as the first targeted object is out
of focus. This is called Focus-Priority. You can change this
with CS #2. Activating this CS and you can shoot no matter
if the F5 still tries to detect the focus or not. This is
then called Release-Priority. I don't have the CS #2 activated,
i.e. I am using the S mode with Focus-Priority.
Continuous
servo AF
In this mode, the F5 doesn't lock the focus when you gently
push the shutter release button and keep it pressed, but rather
it hunts the focus. The focus hunt in C mode is quite delicate:
the F5 continuously calculates the estimated distance to the
object at the time of an eventual shutter release based upon:
a)
The current distance to the object,
b) the direction in which the object is moving and
c) the current speed of the object.
This
means that for the short moment when the F5 is blind, i.e.
when the mirror is up, the lens is forced to be in focus on
a location where the object should be based
upon the parameters a, b and c.
The
default behavior in C mode is to let you shoot while the F5
hunts the focus. This wouldn't be an F5 if it didn't allow
you to change this behavior too. CS #1 does the trick. Activate
CS #1 and you won't be able to shoot while being out of focus
in C mode. I have this CS activated, i.e. I am using the C
mode with Focus-Priority.
I
use both the S and C mode a lot. For stationary objects it's
the S and for moving objects it's the C mode (pretty smart
of me huh?). As I shoot a lot of landscape, it tends to be
a lot of S mode right now - landscapes are slow joggers you
know.
Single
area AF or dynamic AF mode
The F5 can either use a single or all five focus
areas for the AF. In single area AF, you have selected one
out of the five available areas (sensors), typically the one
in the middle, the center area. This is then the active AF
area. In dynamic AF mode, you select one primary area which
will be used for the first focus hunt, whereas the others
will be used if the object moves. The active area in dynamic
AF mode will change according to how the F5 sees the object
move.
No
matter if you use the F5 in single area AF or dynamic AF mode,
you might want to select the active/primary focus area.
Selecting
a focus area
I don't use this function, but it might be useful,
especially in combination with dynamic AF mode. There's a
fairly big, round push button, the "focus area selector"
located on the back at the right, made for being operated
by the thumb. It has four small arrows on it which you can
barely feel. This button can be used for selecting one of
the five focus areas (remember, there are 5 focus sensors
in the F5).
Typically,
the center focus area is selected (the primary area being
the center area), but you can use this button to select the
center, top, bottom, left or right area. The selected area
is displayed on the main LCD and on the EC-B screen in the
viewfinder. On the main LCD, the selected area is represented
by a single small square while in the viewfinder you'll see
one of the five squares being a bit darker (actually getting
fatter) - the fatter square represents the primary focus area
(sensor). Also, one out of five yellow colored arrow LED's
will blink once, indicating which sensor is the active one.
This ain't much of any use with the EC-B screen, since it
tells you directly which sensor is being the active one -
the fat square - but with some other screens, such as the
type C and type M (which don't have the "fattening focus
area squares"), the arrow LED's are the only indication
you will get on which sensor is currently being active.
The
big focus area selector is easy to change by accident, such
as with the thumb or with the palm of your hand. You typically
want to have this button locked. To lock it, you simple press
the [L] Lock button (you remember? That's the button hidden
by the small "door" at the bottom left back of the
camera) while you press the focus area selector once. A new
press having the [L]
Lock button pressed and the lock is removed. Locked focus
area is indicated in the main LCD with a small "LOCK"
being displayed over the currently active area.
The
dynamic AF mode
This
is a cool one. Basically,
the F5 will haunt around for the correct focus using one of
the five available autofocus sensors. You select this AF mode
by pressing down the AF Area mode [+] push button while dialing
the MCD one step. Active dynamic AF mode is indicated by five
small plus signs in the main (top) LC-Display and where the
primary AF area is represented by a squared plus sign. There
is no change in the viewfinder when you have the dynamic AF
mode activated and the primary area has the "fat square".
In
this mode, all five sensors are active. This mode is made
for capturing moving objects and where you have the AF in
continuous (C) mode and probably the film advance set to Ch
(continuous high speed, 8fps with the MN-30 accumulator pack).
If
the object in the viewfinder now moves from the primary area
to another focus area, the F5 will automatically adjust the
focus. You may not be able to detect that this has happened,
except for that the object is still in focus, since the primary
area is still marked with the fat square in the viewfinder,
even if another area may currently be the active one.
Since
the F5 continuously calculates the estimated location of the
object in this mode, it will activate a neighboring sensor
whenever the object is in the close vicinity of it.
The
F5's dynamic AF mode is one of the prides of Nikon. It's about
100% faster than the dynamic AF of the F4 and it is able to
track objects having a velocity of up to 20mm/s at the film
plane. This equals to an object with a velocity of 300km/h
at 20m distance if you're using a 300mm lens!
The
F5 is the action photography
tool. |
