# Ghostbuster

Or rather, misconception-buster. Let me just remove a nasty misconception that crops up again, and again, and again. In people who ought to know better. Namely, focus.

Let’s start at the beginning. When we say “Focus” we mean:

• (noun): the focus distance, or
• (verb:) the action of setting the focus distance.

“The focus distance” means “the distance at which the picture is sharpest”. Not “what is my subject”, or “what else is sharp”, or “are we free of motion blur”, or anything else. It just means “at what distance from the sensor is the image sharpest”. If I set my focus distance to one meter, that means the picture is only really, really sharp at a distance of one meter. Objects that are 90cm, or 1.10 meters, away from the camera will be less sharp. Objects at 80cm, or 1.20m, less sharp still. Even when you have a great depth of field (i.e. the unsharpness away from the focus distance only sets in gradually), there is still one distance at which objects are sharpest.

```CAM--//-----VVF-----VF-----F-----LF----SH----LF-----F-----VF-----VVF
0          0.6    0.7    0.8   0.9   1.0m   1.1   1.2   1.3 metres
|==========| DOF```
`(SH=sharp, LF= A little fuzzy, F=fuzzy, VF=Very Fuzzy, etc)`

Between the two “LF” markers, the image is acceptably sharp; we call that the “depth of field”.

Select a higher “F-number” and you get greater depth of field. True, but the principle remains the same: there’s only one distance at which the image is totally sharp. If the schematic above is for a certain lens at f/4, then the same lens at f/8 might look like this:

```CAM--//--------VF---------LF-----------SH-----------LF---------VF--
0          0.6    0.7    0.8   0.9   1.0m   1.1   1.2   1.3 metres
|=========================| DOF```

We have more depth of field (sharp range). But the focus distance is unchanged.

OK, so now we know exactly what focus is. Now to the misconception.

There are two separate, unrelated focus system settings. One has to do with HOW to autofocus (ONE STOP vs AI-SERVO, or AF-s vs AF-C). That is not what this post is about.

The other setting, the one we are talking about now, is about WHERE to autofocus.

You can use the “where to autofocus” system in two ways:

1. You choose where the camera autofocuses, or
2. The camera chooses where it autofocuses.

If you set up your camera to use one focus point, for instance the one in the centre, or one of the others, then you are choosing what the camera focuses to choose on. So it’s still autofocus, but you choose where. (That’s the mode I usually use).

If on the other hand you set up the camera to use all focus points, so that when you press down the shutter, any combination of points might light up, that means that the camera is going to choose what it autofocuses on. It looks at all the focus points, and chooses the one(s) that have the closest object behind them; it lights those points up, and it focuses on those (close-by, equidistant) objects.

Let’s have that again. If you set up the camera to use all focus points, it looks at all the focus points, and chooses the one that has the closest object behind it; it lights up that point, and it focuses on the (close-by) object behind that point. If two or three focus points have a close by object at the same distance, it will simply light up all three of those points.

So, to the misconception…

NO: using all focus points does not give you “more focus”. It does not give you greater depth of field, or anything else. It just allows the camera to choose where to focus (namely, on the closest object it sees), instead of you choosing. If you see multiple focus points, that simply means there were multiple objects at the same close-by distance.

So, misconception solved. I hope. Finally.

Next time: Should you always format your memory cards? (Hint: the answer is “yes”).