Welcome to the world of Canon. If you think focus is tough, wait until you try to use an accessory flash.
The best information that I have found on dealing with G2 focus was in a posting from ND from Australia. I saved it as a local document. I wish I had his name so that I could give proper credit. I found his advice to be very helpful. I was focusing on faces until I read this. My out of focus shots have been greatly reduced just by focusing on something with a vertical edge. His posting follows:
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.
