|
Forum: Troubleshooting and Beginner Help
04-06-2011, 11:23 AM
|
| |
Hi Macky,
I think you're experiencing a common situation with Pentax users. I'm assuming you're using the center focus point.
The center focus sensor area ("point") is actually about the size of of the ( ) brackets on your focusing screen, not the red square LED that lights up to indicate focus point. All AF sensors look for contrast borders within the focus sensor area. In many cases, it will pick the wrong border, because the contrast is stronger or the border aligns more to the vertical or horizontal planes.
Superimpose the ( ) brackets on each of your shots posted, and I think you'll be able to see an element within the area that would possibly cause an AF error. Realize that the center focus sensor is shaped like a + that fills the area of the ( ) brackets. It detects contrast borders that intersect the lines of the cross -- anywhere within this area. The closer this contrast border is to either vertical or horizontal, or the greater the contrast, the more likely the sensor will choose this as the thing to lock focus on. BTW, wide angle lenses could be more prone to these focus errors by the camera since the wider FOV probably includes more elements that might confuse the AF sensor.
#1 The stairs in the background -- there are both vertical and horizontal contrast borders that the camera might have mistakenly assumed to be the focus plane.
#2 The strongest contrast border in the focus sensor area is probably the inside of the thigh of the woman holding the child up.
#3 I'd say either the white fence or the intersection of the tree trunk on the white car between the couple is the culprit.
#4 The wedding party in the background is the highest contrast border in this one
#5 No problem -- either the highlights in the hair or the man's shoulder against the white background allowed the AF sensor to lock at close to the right distance.
Your job as the photographer is to understand the actual area of the AF sensor, try to make sure that part of your subject is the dominant contrast element in this area, and override the AF sensor when it makes a mistake -- assume it will make mistakes since it cannot know your intentions.
I shoot birds and wildlife, with lenses that have very narrow Depth of Field (DOF), so with them hiding behind tall grass and perching among a lot of branches, I have to make sure that the AF system picks the right thing to lock focus on. I make small adjustments in composing the scene, and sometimes have to make a couple of tries of the AF until I can see in the VF that the subject I want is what is actually in focus. At this point, I hold the AF lock, recompose the scene, then take the shot. the trick is to focus your attention on the ( ) brackets, make sure that the intended subject is in focus, lock, then open your attention to the entire scene to compose, then take the shot.
I hope I explained this adequately.
Here are a few examples -- not people shots, but you can see how they would cause problems. With the deer, the camera wanted to choose the grass. I had to make the top of the doe's head the dominant contrast border, then recompose the shot to include her whole head. With the bird, the camera wanted to focus on the intervening branches, so I had to move the VF so the main contrast border in the AF sensor area was the one between the bird's head and the background.
Scott
|
