Playing around with depth of field and I think I finally get it?

Started 4 months ago | Discussions thread
Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 16,810
A way of thinking about it

LillyRoseAvalos wrote:

One thing I observed is that the more distance you have behind your focusing point the more blur you get. Why is that? I get that if you use a telephoto lens you can get more background blur behind something because of compression but this is all at 12mm. hmmm is it because the object in focus is closer to the foreground?

A way of thinking about it is to imagine a cone that extends from your lens to the focus distance.

The base of the cone is the aperture: it will be bigger when the f/stop value is small, and smaller when the f/stop value is large.

The height of the cone is the focus distance. The cone is wider at the lens, and it comes to a point at the point of focus, and then the cone diverges again, at the same angle, beyond the focus distance.

The width of the cone at any given distance is a measure of how much out-of-focus blur will be generated at that point: lots of it right next to the lens, and none at the exact focus distance. Anything that is smaller in size than the cone width at a given distance will be blurred beyond recognition, while if you want to see any detail, the size of the object needs to be wider than the cone width. For example, I can put a couple of fingers in front of my 85 mm f/1.4 lens, when it is opened up all of the way, and I can't see them *at all*; all they do is darken the image slightly.

The apparent diameter of the aperture (or technically, the entrance pupil diameter) can be calculated by taking the actual (and *not* equivalent) focal length and dividing it by the f/stop value. It's for this reason that you won't get much out-of-focus blur at all from a smartphone lens, since it has a tiny aperture diameter: a finger in front of the lens will be well-resolved, and consequently block the light completely. With any two lenses of the same f/stop, the longer focal length will have a proportionally wider aperture width.

The cone comes to a point at the focus distance, and then it diverges again beyond it, at the same angle, and so as you go farther beyond the focus distance, the blurriness increases.

If you focus closely, the cone will become squatter, and so the depth of field becomes very shallow, and if you focus farther away, the cone becomes long and skinny, until you focus at infinity, and the cone becomes a cylinder. There is a clever trick about infinity focus: nothing smaller than your aperture opening size will be resolved, but if nothing important in your scene is smaller than this, then everything will be resolved, from here to infinity.

We define the "depth of field" by assuming there is a certain amount of blur that's going to be too small to be visible, and usually that's given to be a blur width that is somewhere between 1/1000th and 1/2000th of an image width, although it depends on how big the image is, how close you look, and how good your eyes are. It's possible to estimate the depth of field by knowing the width of the blur cone at any given distance, along with the angle of view of the lens and camera.

There are some caveats to this. First of all, 'bokeh' is usually defined as the "quality" of out-of-focus blur, and not its "quantity". But it does turn out that the quality of the blur matters: harsh bokeh tends to ruin the out of focus visual experience.

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