Simply bad focus or too big aperture?

Question

I have made a mistake with this image:

ISO 400 | 1/1700s | f/2

I used manual Aperture mode and set it to F/2 (now I believe this is too much). Kids were one meter away. It was light cloudy morning.

However, they came out blurred - and the trees behind appear in focus. I was using auto-focus mode, and I think the focus was in the middle of the photo - but between the kids :(

How come my camera auto-focused the trees behind with f/2 (i was expecting this aperture works for close objects?). Was this only a miss-focus, or my aperture was wrong as well? What should I do next time?

Answer 1

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.

Answer 2

I have to assume that you are using a focus mode that employs all available focus points and chooses one itself. In this mode the camera will try to focus on the nearest object. But in order to focus it needs good contrast; ideally horizontal or vertical lines. In this case it looks like the trees behind had better contrast, the AF didn't get enough contrast from the boys and so the AF didn't see them, it chose what it thought was the closest subject: the trees. You are also hindered by the fact that the centre AF point is usually the most sensitive, so is more likely to pick up a "subject" than the other AF points.

The easiest way to deal with this is to:

• set your camera to use only the single centre AF point
• put this over the subject
• half-press the shutter
• whilst keeping the shutter button pressed, recompose the shot before fully pressing the button to take the shot.

This is the Focus and Recompose technique.

People like to worry about focus errors caused by focus-recompose, but if you are shooting at normal apertures, using normal cameras, it's fine. I've met top (Canon Explorer) pros who use focus-recompose exclusively. You do need to worry when you have long fast lenses wide open on full frame bodies, like a 85mm f/1.2, and you're are trying to get the eyes sharp, and little else. You'll know when it's time to worry.

Answer 3

The auto-focus (AF) focused on the trees in the background. In future, there are a couple of solutions around this problem .. if the camera has AF lock, you can focus on one of the kids then shift the camera to the center. AF is locked would keep the kids in focus. If you don't have AF lock, you can focus on a kid then turn AF off. The other thing depends on whether you can manually set a AF point. In my camera, I can set it to the left/right of center; perhaps you can too ... now the camera locks on a kid without moving the camera.

A wide aperture, like f2, is going to create a shallow depth of field. You'll have to experiment with your lens to determine the depth of field at various aperture settings. One idea is to photograph with a tape measure where you can see individual mm's or cm's marked out. Pull the tape measure out 3-4m and photograph along it at various distances and aperture settings. You can tell by the blurriness of the lines where the depth of field will be by how sharp the markings are on the tape measure.

You'll want to test the the focusing distance at different aperture settings because different lenses, especially those that have a minimum focus distance depending on zoom, will frequently also have different minimum focusing distances too.

The only way to know is to practice/experiment with the lens and get comfortable with how it operates.

Answer 4

You don't say what kind of a camera you're using, but if it's an SLR, then manual focus may also be worth trying.

Shooting with manual focus does take a bit of practice, but it has the big advantage of giving you full control over where you want the focus to be, instead of having to rely on the camera guessing what you want to focus on.

This is especially true at such wide apertures as f/2, where even small changes in focus depth can be noticeable. For example, in the scene you showed in your question above, you might want the sharpest focus on the kids' faces, but even with focus-and-recompose the camera might decide to focus on their shoulders (which are slightly closer) instead. With manual focus, you can tweak the focus to be just the way you want it before taking the shot.