Question - why are all camera AF points bunched around the middle. Even with a corner one selected I still end up doing some focus and readjustment. Why can't we have some points way oug in the corners? Is it not technically feasible?
It's not technically feasible. Since you asked... Canon EOS DSLR Autofocus Explained
The relevant part:
AF Point Coverage
For many people, this is a big issue in comparing cameras. While it would be wonderful to have AF points available over the entire extent of the frame, there are technical limitations on the spread of the AF points - at best, they can only occupy the middle area of the frame, because of simple geometry and optics. In a nutshell, there are four reasons for this limitation:
- Size of the secondary mirror. Light for AF passes through the semi-transparent part of the main mirror (most is reflected up to the viewfinder), then is reflected off the secondary mirror down to the AF sensor. There is limited space behind the main mirror, based on the necessary geometry (i.e. the main mirror has to be at a 45° angle to the incoming light, and the secondary mirror has to be behind the main mirror and at an angle of 90° to the main mirror, so it's length is limited by the distance between the main mirror and the image sensor).
- Distortion. With many lenses, the edges of the frame are subject to distortion (barrel/pincushion), and that reduces the accuracy of phase detect AF.
- Vingetting. The AF system needs a certain amount of light to work. Almost all lenses vignette to some degree, meaning there might not be enough light at the edges of the frame. For example, the EF 17-40mm f/4L has >2 stops of vignetting wide open at the wide end - that means at the edges of the frame, you're below f/5.6 and AF sensors would not have enough light to operate (i.e. in dim light you'd be below the EV sensitivity of the sensor).
- Temperature. Canon has stated that larger AF sensors are more susceptible to changes in temperature with the result that they change size, getting either larger or smaller as the temperature rises and falls. That reduces the accuracy of the AF system overall.
It's worth noting that none of these limitations apply to contrast detect AF, so using LiveView you can autofocus right out to the edge of the frame.
Cool thanks for the info. I had a feeling about the lens problems, that makes sense. And I deffo know about the 17-40 vignetting and its bad enough trying to focus with an outer AF point on a 5D 2 as it is!
Hmmm I didn't know about the liveview thing. I need to give it a try!