Why did autofocus lock on the background at f/2, and how can I keep nearby subjects sharp?
Asked 11/19/2015
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I photographed two children about 1 meter away at ISO 400, 1/1700s, and f/2. The kids came out blurred, while the trees behind them look sharper. I was using autofocus and think the camera focused near the middle of the frame, between the children. Does shooting at f/2 make the camera focus on distant objects, or was this simply autofocus missing the subjects? What should I do differently next time to make sure the children are in focus?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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Your question seems to reflect the idea that a lower aperture number is for closer objects and a higher aperture number is for distant objects. This is not the case. Either aperture may be used to take photos of subjects whether they are near or far.
Besides controlling how much light is allowed through a lens, the aperture setting also determines how far things can be from the point of focus before they begin to appear blurry to our eyes when we look at a photo displayed at a particular size and distance.
The problem in your image is that the camera focused on the middle of the frame, which happened to be occupied by the trees in the background, rather than on the boys that were to the left and right of the gap in the middle. Depending on the capabilities of your particular camera there are several different ways to correct this. Basically you need to let the camera know what it is in the frame that you wish to be in focus. You can usually lock focus with the active AF sensor aligned over one of the two subjects and then recompose while keeping the focus locked or you can tell the camera to focus at a point other than the center of the frame.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
10y ago
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This was mainly a focus-point issue, not because f/2 is “for distant objects.” Aperture does not decide whether the camera focuses near or far; it controls depth of field and light.
At f/2, depth of field is quite shallow at about 1 meter, so if the camera focuses on the background instead of the children, the children will look soft. Your autofocus likely chose the trees because they had stronger contrast, especially if the camera was set to automatic AF-point selection.
For next time:
- Use a single AF point, often the center point.
- Place that point on one child’s face or clothing with clear contrast.
- Half-press to focus, then recompose if needed.
- If your camera allows it, move the AF point left or right onto a child instead of leaving it in the middle.
- AF lock or focus-and-recompose can also help.
- If you want more of the scene in focus, stop down to a smaller aperture like f/4 or f/5.6.
So: it was mostly missed focus, and the wide aperture made the mistake more visible.
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