second technical detail is focusing on the subject of the photograph. As
technology has marched forward focusing correctly has gotten harder.
Wait what? As much as sensor technology has advanced, focusing
capability has not progressed at the same speed. What worked in 2005
with 6 megapixel APS-C sensors does not today with 20+ megapixel full
frame sensors. Auto-focusing, self explanatory and commonly referred to
have either Contrast AF or Phase Detect AF. Although there are cameras
with both, as a general rule of thumb point & shoot and
cameras have Contrast AF while SLR cameras have Phase Detect AF.
Contrast AF is more accurate but slower, while Phase detect is faster,
and currently unparalleled for tracking an object.
most cameras there are two primary AF modes. The first mode is a closed
loop; it focuses once on the object under the AF sensor and stops until
the shutter button is released and pushed again. Nikon and Sony call
this AF-S short for for Auto Focus Single while Canon calls it One
Shot. The second mode is an open loop; it continues to focus on the
object under the AF sensor until the shutter button is released. Nikon
and Sony call this AF-C, short for for Auto Focus Continuous while
Canon calls it AI Servo because they are still stuck using nomenclature
from 1980. As one climbs camera hierarchy in any brand the options are
more complex and confusing, we'll get to a little of that later and
just stick with these two modes for now.
a contrast based AF system you're simply not tracking action. I suggest
that if you own a camera with contrast AF it's best to set the focus
to AF-S, focus on where the paddler will be in the frame and recompose,
taking the picture when the paddler moves to where the focus was set.
Then skip the rest of this section as it's pretty specific to Phase
Phase Detect AF is wonderful, frustrating and complicated. A lot goes
on "under the hood". Phase Detect AF needs straight lines to focus, and
there are two types of AF sensors; the most common are vertical line
sensors, which are good at focusing on horizontal lines. Cross type
sensors are two dimensional and more accurate. It's common for cheaper
cameras to only have one cross type point in the center, while a more
expensive camera like Nikon's D4 will have 15 cross type sensors. Now
things get a little complicated. The D4 has 51 AF sensors, but only 15
of those are cross type. Looking through the viewfinder they all look
the same. Don't worry, they are all in the center anyways. Despite all
the technological advances in the last ten years, they have not worked
out how to get accurate phase detect focus points to work towards the
edge of the frame. It just can't be summed up easily, even
the people who make it.
Do the research and see which phase detect sensors on your camera are
cross type, and try to use
only those. Why choosing to show only cross point AF points
an option is a sad oversight by all the camera makers.
Phase detect auto focusing is most accurate when the camera is set to
continuous auto focusing. Now it's impossible compose the shot
unless your subject falls under one of those limited cross type AF
points. Thankfully there is a solution, and it's known as AF-ON. Only
higher end cameras have a dedicated AF-ON button, but even consumer
models allow the Auto Exposure Lock (AEL) button to be reprogrammed to
AF-ON. Turning on AF-ON switches AF activation from the shutter button
to the dedicated button. Now it's possible to focus continually by
pressing the AF-ON button and stop focusing by releasing the button.
Then you can recompose and take a photo without the shutter button
activating the AF system.
I used the AF-ON button with focusing set to AF-C mode and the center
focus point, pointed at the lip of the falls. Then I recomposed the
shot where the lip of the falls is way out of the range of any AF
Nikon D600, Nikkor 24-85mm VR at 24mm
The focus and recompose method works well for wider angle shots,
barring any issues from field
curvature. But what about when
there is nothing to focus on before
of Field, (DOF) is the distance between the nearest and
objects in a scene that appear acceptably sharp in an image. It's good
to understand and wikipedia
has a tedious article on it.
There are situations where the focus and recompose method does not work
well. Big water is often one, where there is really nothing for the
camera to focus on. Another situation is when shooting with a shallow
depth of field. In these situations I will choose a cross focus point
and then track the kayaker by holding down the AF-ON button and keeping
said kayaker under the selected AF point. This sounds straightforward,
and it is, but things can go wrong. AF points are not exactly where
they are marked in the viewfinder of the camera. It's easy to have the
paddler go out from under where you think the AF point is. Even a brief
moment of this will cause the camera to start searching for something
else to focus on before getting back on track with the subject you
want. Higher end cameras allow you to set a delay in the AF system
before it will make large changes. It will continue to track small
changes like a subject ten feet away shifting few inches, but if for a
brief moment the AF sensor shifts off the subject onto a background
twenty feet away it won't instantly jump to that. Perfect for tracking
a kayaker, and I've found setting this option to the longest period
possible is best for whitewater.
with a shallow depth of field using the tracking method.
Nikon D600, Sigma 150mm f/2.8 @ f/2.8
There are other kinds of complicated focusing modes, but in reality
with kayaking we're tracking one object moving in a predictible path. Next Up:Lighting.