Canon Talk
Subject G2 Focusing- Some Solutions
Posted by nd [CLICK FOR PROFILE]
Date/Time 2:18:40 AM, Thursday, June 20, 2002
Like many people I’ve had a few problems with the G2 autofocus.
After moving from an EOS SLR to the G2, I was initially getting a
significant number of out of focus pictures. Having worked through
the issues and learnt to work differently when using the G2, I now
get reliable focusing and have very few out of focus pictures. It’s
made using the G2 a lot more enjoyable. Here are a few tips for
getting more consistent focusing from the G2.
1 The G2 focuses by racking the lens from infinity to 70cm and
selecting the point of highest contrast. It takes a few hundred
milliseconds to do this and it's best if the target is stationary
during this time. The LCD display freezes while the camera is
racking the lens.
If it does not find a point of high contrast after racking through
the complete focus range, the orange rectangle appears and it will
set the focus to 2m if at wide angle end of the zoom and 3m at the
tele end.
(Note that this is quite different to the auto focusing system used
in the EOS cameras. They use a phase detection method of
determining the correct focus distance and can drive the lens
directly to this distance.)
2 The G2 uses the horizontal readout from the image sensor for
focusing. Hence it requires a vertical edge with reasonable
contrast to focus on. As it uses the horizontal readout it will not
focus on horizontal edges. These run parallel to the sensor and
create little change in the output signal.
3 The G2 uses the luminance or green signal from the image sensor
for focusing. Hence it does not focus best on purely red or blue
objects. These have a relatively low contrast to the sensor.
(This is different from the autofocus sensors used in the EOS
cameras whose spectral response peaks in the red part of the
spectra.)
4 The focus assist light throws a beam that is offset slightly from
the central focusing area. As the lens optical axis and the axis of
the assist light are not parallel, the left edge of the
illumination beam will fall within the central focus area. This
gives a nice contrasty vertical edge that the G2 can focus on. This
feature enables the camera to focus on a plain white wall in
complete darkness. Though the power of the light limits the focus
range to around 2 - 2.5m depending on the colour of the subject.
Beyond that you need to go to manual focus.
5 The 550ex flash focus assist throws a red pattern of horizontal
bars. It is optimal for the multi point wide area focusing system
of the EOS3 and 1V cameras. But it's the wrong pattern and colour
for the G2's system. It would be nice if it would work and give you
lots of extra focusing range, but it doesn't. That's why the
designers have disabled it and unfortunately we’re stuck with the
built in light of the G2.
6 The above reasons mean that the G2 won't focus reliably on faces,
white T shirts, moving objects, items that contain no vertical
edges or are of low contrast. In these cases look for a vertical
edge at a similar distance you can focus on, or estimate the
distance and go to manual mode. If you are taking a landscape
photo, quite often the focus spot is covering a horizontal horizon,
or a relatively featureless part of the landscape. Don't be
surprised if you get a orange lamp and the focus defaults to 2m.
Use one of the other focus points or go to manual and set the focus
at infinity. Ie when taking a picture, always pay attention to the
focus indicators.
(Note: The autofocus system on the Elph and many other compact
cameras use the reflection of an infrared beam from an object to
measure its distance. These work best when they are pointing at
light objects such as faces or white T shirts.)
8 Know the depth of field for each aperture and focal length of the
camera. Know the hyper focal distance for each focal length. Have
this as a table stuck to the back of the camera. That way you can
very quickly set the focus manually without having to peer at the
tiny pixelated magnified view on the LCD display.
9 Always hold the camera steady and use a tripod if the camera
shake warning appears.
10 For optimum sharpness do not use f8. At f8 the lens aperture is
very small, around 1mm, and diffraction occurs which results in a
softening of the image. On the G2 the sharpest pictures occur
around f4 to f5.6.
11 To check the focus and exposure immediately after taking the
picture without entering replay mode, press the set then display
buttons to bring up the histogram, flick the zoom lever twice to
magnify the view and check the focus. If either focus or exposure
is off, reshoot the picture.
