When Auto Focus Does Not Work


When Auto Focus Does Not Work

You have the moment, the expression is perfect, but the lens just spins in and out, trying to focus. Then the moment is gone; maybe even your subject is gone. The shot is lost . . . but you had it, right there.

What happened?

Here is an insight as to why it happened and how you can fix it.

Autofocus technology is a study of contrasts. When the focus point on your camera “sees” contrast it can focus. The contrast it is looking for is an area the size of the sensor point where there is a clear difference of dark and light, preferably a sharp line.

The darker the area you are focusing on, the harder it is for the sensor to find a contrast point. Another way the camera focuses is by texture: if there is no texture or weak texture it’s harder to focus. This is really where the advantage of newer and more expensive cameras, along with the better lenses come into play.

You will find that the newer cameras with their ability to go into insanely high ISO settings can now focus in nearly non existent light.

I remember a friend a few years back had bought a cheap older model autofocus pocket camera. Great price. But, in any soft indoor light situation the sensor could never find enough contrast to focus on. He never got a shot off at the party.

If your camera struggles to find a focus, move the focus point sensor into a high contrast area of the image, focus then recompose. If the light level is low or very flat, you might need to introduce a light to help out.

If you leave your focus on automatic, where the camera has all sensors active to find a focus point, you might be frustrated by what is in focus. When in automatic autofocus each focus sensor is looking for the contrast point to focus on. The first point that does wins the focus race. But that may not be what you wanted in critical focus.

I had a student a while back who showed me a photo of a blurry woman’s face in front of a razor sharp background tree. He was puzzled as to why. The model’s skin was smooth while the tree trunk was full of contrast points. The tree sensor won the race. This is why I always recommend taking control of your focus points in the camera. You are the photographer; you should decide the critical point that is in focus.

It is useful to know that all the sensor points don’t search the same way. Different camera models have different approaches. On my Canon 1DX the middle sensors look for a contrast vertically and horizontally. The outer points have some looking for vertical contrast lines and others look for horizontal contrast lines.

This comes into play when my subject is off to the side so I have the outer edge sensors in play. As I position the square sensor over the focus point if the contrast lines run vertical like her eyelashes while my sensor is looking for horizontal contrast lines it will struggle. Knowing how the sensor works, I would look for horizontal contrast lines for the sensor to focus on.

It’s good to know when you are focusing on a baby or young girl with wonderfully smooth skin in even light. You need to search for a contrast area for the focus to work.

This knowledge might be just the reason you need to upgrade your camera or lenses. The speed of getting in focus in tough light or subject situations might be the difference of getting an amazing, breathtaking image or one “that got away”.

Knowing how your sensor works, how it approaches finding focus, arms you in giving the sensor just what it is looking for. After all, the faster the camera is focused, the quicker you are able to get that prizewinning shot.


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