| In my first tutorial
on photographing wild birds I said "The following is a list
of the most frequent errors that I have noticed that people make
when photographing birds" :
1) Subject too distant.
2) Subject not in focus
3) Blur due to subject movement - motion blur
4) Blur due to camera movement
5) Incorrect exposure
6) Little consideration given to lighting or composition
We covered the first issue in depth in the first
tutorial and digressed a little into equipment in the second tutorial.
So let's get back to moving on down the list to find counters
to the common errors.
2) Subject not in focus
Firstly we need to separate this from other causes
of image blur such as motion blur and camera shake. What we are
talking about here is failure to obtain critical focus either
by limitations in autofocus or our own failure to accurately focus
manually on the subject. When I talk about an image
being critically sharp or critically in focus, I mean that it
still looks sharp when viewed at 100% or "Actual pixels"
when viewed on the computer.
Failure to use correct focusing mode - single shot or AI servo.
I am often amused to see people trying to photograph
birds in flight in One Shot mode and wondering why everything
shot is out of focus.Let's look at these modes (and some others)
in more detail.
One shot focus.
In this mode, when the shutter button is pressed half way down
the autofocus focuses on the subject and the camera beeps a confirmation
(unless deactivated in custom functions). This is the mode generally
used for static subjects.
One Shot can be slightly more precise than AI Servo,
and is a bit more sensitive in low light, but it should not be
used for moving subjects. This is because if the subject moves
after locking focus you will get a blurred shot. If this happens
you would need to take your finger off of the shutter release
button, and then half-depress it again, to regain focus.
Many people use this mode to focus and recompose
the shot if their subject is off-centre in the frame. However,
if you are using a lens with a very wide aperture (such as f1.4)
or a telephoto lens with the subject large in the frame there
can be enough of a difference between the original point of focus
and the final composition for a focusing error to creep in and
soften the shot.
Off- centre subjects are much better handled by doing the reverse
of focus and recompose - that is to compose and move the autofocus
point over to the subject. Cameras with advanced autofocus systems
have more AF points to choose from (45 points plus) compared to
lesser models which typically have as few as 9 AF points.
If the tail of a bird goes out of focus but the eye is still sharp,
the human brain can accept that as being a natural thing. If the
eye is not sharp you might as well bin the image. So: When
photographing static birds I always move the autofocus point over
to the birds eye.
This takes a few moments to do but it is time well spent. If the
bird is large in the frame and you focus on the centre of the
bird with a fairly wide aperture, there may not be enough depth
of field to get the eye in focus. If I had used the centre point
on this puffin, the shoulder would have been sharp and the eye
soft. Note the 9 AF points of the Canon 40D that was used to take
this shot.
AI servo and AI focus (Continuous focus on Nikon)
This is the mode you should use for moving subjects for example
a bird in motion on the ground or in flight.
Focus and recompose does not work in AI servo mode
as the camera continually tries to refocus on a subject as it
is moving and tries to predict where it will be just before it
takes the next shot.
You have to select an autofocus point if the subject is off centre
otherwise the camera will focus on the wrong part of the subject.
With a bird that is large in the frame, the eye is usually off-centre,
so you will need to get used to moving AF points around. On most
cameras you have to press the Autofocus select button that looks
like this:
You then either scroll to the point you want to select with the
main and quick control dials or the multiselector button (joystick).On
some cameras you have to to assign this function to the multiselector
button in the custom functions. Usually a prod on the multiselector
returns you to the central AF point.
Although you must use AI servo for a moving subject, there is
no law that says you have to use one shot mode for static subjects.
Birds rarely keep still and you don't know when they are going
to give you some action to photograph, so
I always use AI servo when photographing birds regardless
of whether they are still or moving.
AI focus is available on some cameras and is meant to offer the
best of both worlds by acting like one shot mode but commencing
tracking when a subject begins to move. Unfortunately it is not
very reliable and most people, myself included, prefer to use
AI servo.
Automatic focus point selection (AFPS)
In this mode, which can usually be accessed by moving
the AF point selector right over to one side of the viewfinder
and beyond until all the outer AF points light up. You might have
expected all the AF points to light rather than just the outer
ones to indicate that all 45 points are active, so this mode is
often nicknamed the "ring of fire" which describes it
rather well.
The way to use the ring of fire (AFPS) is to focus
on the subject with the centre AF point and give it about a second
to lock on. You then try to keep the focus point on the target
and the camera is meant to hand-off the focus to adjacent points
to maintain focus. The trouble is, in my experience the system
does not work totally reliably and I find that often the camera
will focus on a high contrast part of the subject such as the
bird's wing rather than the intended bit - it's head. AFPS works
pretty well for birds flying against a clear sky, but often loses
focus if the background gets busy (trees and bushes).
Centre point vs point expansion
With some cameras you can expand the focus points to include a
cluster around the selected point. On a fast moving subject it
can be a lot easier to keep the cluster of focus points on the
target than by just using a single point. However, the central
focus point is usually the most sensitive one and if you are skilled
enough to keep the dot over the target it is often the most accurate
method of focusing and the one I tend to use most.
Breezebrowser Pro can show the active AF point superimposed onto
RAW images (but not TIFF or jpegs) Below is a cropped shot of
a gannet in flight with the central AF point bang smack on the
eye - just where you want it to be. The image was taken with a
1 series Canon camera - note the 45 AF points. If you get an out
of focus image it is worth checking to see if you did actually
nail the focus like this one. If the camera repeatedly misfocuses
despite the AF point being on target you may have a faulty camera.
In the next image, an off-centre AF point was selected for this
kingfisher. Notice that with even 45 points to choose from, there
was still not one that I could put exactly over the bird's eye.
In actual fact, the AF points are bigger than the boxes displayed
in the viewfinder, so the eye was still probably covered. This is
close enough - even if a wide aperture (shallow depth of field)
is employed.
Back button focus
This is an advanced AI servo technique in which you reassign functions
to the camera buttons, so the shutter release button no longer
causes the camera to autofocus but it still meters the scene and
takes the picture when you fully depress the button. Autofocus
is assigned to another button on the back of the camera such as
the star button (hence back-button focus) so that the photographer
needs to keep his/her thumb on the star button for AI servo to
work. Take your finger off the star button and the focus is locked
wherever it last focused. This emulates being in one shot mode
again. Latest cameras have an "AF ON" button on the
camera back and you can use this to either start or stop focus
according to the camera custom function set .I have my Canon 1DmkIII
set to Cfn IV,1,1 which makes the camera behave as usual but stops
AI servo if I press the AF ON button.
Back-button focus is very popular with sports photographers and
is very relevant to active birds that continually alternate between
moving around and standing still. Waders feeding on the sea shore
spring to mind. If you fancy reading more and trying it out, click
here.
Manual focus
Autofocus is wonderful but there are occasions when it
just won't work and you are better off switching it off. These
situations include very low light situations, very low contrast
subjects and when a bird is partially obscured by twigs or grass
(So the AF wants to focus on the twigs instead of the bird).
On these occasions it is best to switch the AF button on the lens
to OFF and focus the lens manually.
There are occasions when I like to manually focus.
The commonest occasion that I prefer to focus myself is when trying
to photograph a bird taking off or landing on a perch. By pre-focusing
on the bird or the perch and then switching AF off on the lens,
this can be far more reliable than the autofocus system which
simply cannot respond fast enough. I used this method to take
pictures of bee-eaters in Hungary when they repeatedly landed
on the same perch. I set the cameras to live view and focused
manually on a perched bird. Then, using a cable release I simply
watched for the bird to return again via the rear screen and machine-gunned
it with the motordrive on my 1DmkIII . This was the result:
Other causes of out-of focus images
If the image looks blurred through the viewfinder
and the camera is not focusing, you may have accidentally knocked
the lens Autofocus on/off switch into the off position. This is
not as daft as it sounds .I find that with certain lenses - particularly
my 70-200 f 2.8 L this happens pretty much every time I take it
out of my bag. This is a silly design fault. You often see pro
sport and photojournalist photographers with black tape on their
lenses for this reason!
Another reason for out of focus images on certain
lenses is that the focus range limiter switch has been set to
wrong range. If you have been photographing birds in flight with
the distance limiter switched to 16.2m to infinity and then you
try to photograph a bird on a post at 10m the camera can't achieve
focus. This is easily done - as I can testify !
|
Typical Canon lens switches - the top
one is the focus limiter. |
Also, if you try to focus on a bird's head when it is really close,
you may be a bit closer than the camera's minimum focus distance.
If you want to get closer than the normal focusing distance you
can achieve this by fitting an extension tube. (A tube with no
optical elements). The ring-billed gull below is a rare migrant
from USA that turned up in the UK in Kent. It was very confiding
and I was able to fit an extension tube to my 500mm lens to get
close enough for this head shot:
Canon 1DmkIII autofocus guide
This camera has had a checkered history with autofocus performance
in AI servo mode, but in its latest guise with all free updates
and latest software I find it autofocuses very well indeed.Les
Zigurski has written an extensive guide to setting up this camera's
custom functions that I won't attempt to emulate, so if you use
this camera and would like to read more, you will find this link
invaluable : Les
Zigurski
|
