Why does my DSLR focus on the background instead of my subject when taking shallow DoF photos?

Question

I am an amateur photographer and have a Nikon D7000 and I want to get the blurry background effect (shallow DOF). I have a 35mm lens and I have set the aperture to be 1.8.

While taking a picture I put the small box on the person's face and then semi press the shutter button and after it focuses, I take the picture. However, when I see these pictures on my computer, I see that the background is sometimes in focus and the person's face is blurred.

I suspect there is a problem with the autofocus, any ideas how can I fix this? I can’t make out the difference in the viewfinder, so its difficult to tell how the picture will appear enlarged.

Answer 1

Pretty much all recent DSLRs with multiple focus points such as your D7000 have many configurable options when it comes to setting up the way your camera autofocuses under varying conditions. This is great in that it allows a high degree of control over the AF system when shooting. The flip side of that, though, is that just as with any type of fairly advanced tool, one must be well versed in how it operates in order to use such a tool well.

Here are some things you can check to see what is causing the inconsistency in how you're telling the camera where to focus:

• If you are focusing (half-pressing the shutter button until the camera indicates it has focused) and then recomposing make sure you are in AF-S (Single) focus mode. This will lock the point of focus in once focus has been achieved. If you are using AF-C (Continuous) focus mode then the camera will refocus on whatever is at the selected AF point after you half press the shutter button and then move the camera to recompose.
• Learn the true areas of sensitivity for each of your camera's AF points. Hint: each AF point is sensitive over a much larger area than the little indicator square in your camera's viewfinder. Your AF system will attempt to focus on the area of highest contrast within the active AF point(s). Even if the area of highest contrast is on the extreme edge of the area(s) of sensitivity. There is no "center weighted average" with modern multipoint AF systems.
• These areas are normally much larger than the little square for each one that you see in the viewfinder! Several times larger in many cases.
• Some "points" can overlap each and share lines on the AF sensor array. Each camera has a specific coverage map.

Here is the map for the Canon 7D. The AF system of your D7000 is very similar, as are most multi point AF systems with large numbers of AF points. For a full explanation of the data on this chart, please see https://photo.stackexchange.com/a/41179/15871

For a look at how this works out practically when shooting, see this entry from Andre's Blog.
For a look at how AF accuracy can vary from shot to shot, see this entry from Roger Cicala's blog at lensrentals.com. With the shallower depth of field (DoF) obtained when using wider apertures, there is less room for error and often the standard deviation of an AF system will exceed the DoF for a given focal length, aperture, and subject distance.

It goes without saying that if your camera offers Auto Focus Micro Adjustment (AFMA) you should calibrate your body to each of your lenses. How to properly do that is covered at Which offers better results: FoCal or LensAlign Pro? and What is the best way to micro-adjust a camera body to a particular lens?.

With APS-C cameras the AF system can't be as accurate as a well designed AF system in a Full Frame camera because the smaller sensor and mirror dictate a narrower baseline for the AF sensor. When using an APS-C body I've gotten to the point that I will often shoot in high speed burst mode and take 2-3 frames of each pose to try and insure that one is reasonably close to properly focused.

If you are shooting in low light, you might also consider using the AF assist illuminator built into your D7000.

Answer 2

Since you have a DSLR, you should see things as they happen. A 35mm F/1.8 has a shallow DOF, so when you look through the viewfinder, you should see some depth in focus and the rest of the image gets gradually more blurry.

When you half-press the shutter-release, you should see the focus happening and it will be noticeable that it is on your subject or the background. You really shouldn't have to wait until you view images on a computer to know that there was a focus error.

There are two possibilities:

• One is that you half-press the shutter and nothing happens: Check that your camera is not set to MF, the switch on the lens is on AF and that the lens is capable of AF on the body you are using. It may be that your lens is MF only (I've seen people who bought MF only lenses without knowing). The AF motor in the lens or camera may have failed, in which case you would need to take it for repair.
• The other is that the focuses changes but moved to the background: I that case you AF-points may not be aligned where they appear. You may have to learn and adjust your point-placement. The point is that, at F/1.8, the focus plane should be clearly noticeable and it will change when autofocus is performed.