Depth of Field

"Depth of field" is a difficult term, but to put it simply, could we say it refers to the breadth of the range that is in focus?
In landscape photography, there are many photos with a deep depth of field (a wide focus range) where the entire photo, from the foreground to the distant view, is in focus. In portrait or flower photography, there are many photos with a very shallow depth of field (a narrow focus range) where only the area around one's eyes or the pistil of a flower is in focus.
Photos surprisingly do not capture things exactly as the eye sees them, so we organize surfaces and lines through composition, control perspective by utilizing the effects of telephoto or wide-angle lenses, and direct what to show and how to show it through the way we present it. "Depth of field" is also one of those techniques for direction.

Humans unconsciously organize the images they see with their eyes in their minds before perceiving them.
We can lift things we are interested in, things that move, things we know, and things we like out of the entire field of view in three dimensions and clearly recognize them.
For example, among many children playing, you can clearly see only your own child. The other children are also in focus, but because you are not focusing on them, it feels like you are unconsciously omitting them from your view, and you are not clearly recognizing them.
However, because photos capture things as they are, if you take a photo with a "deep" depth of field where the entire park is in focus, both the children and the parents are clearly depicted in the image, which can sometimes make it difficult to understand what was photographed or what the photo is trying to show.

When you want to make one thing stand out in a scene where there are many things, I think it is good to make the depth of field shallow so that only that object is in focus. Depending on the quality of the bokeh, it becomes very dramatic.

In the photo above, I wanted to express spring by utilizing both the snow patterns (melting snow shapes) on the mountains with remaining snow and the cherry blossoms in the village, so I made the depth of field very deep, focusing on both the cherry blossoms right in front and the mountains far away.
Depending on what you want to photograph and how you want to show it, being a little conscious of the depth of field will broaden the range of your photographic expression.

The discussion of "what changes the depth of field?" is a bit complicated, so I will save it for another time.
To write it simply, when the aperture value (f-stop) becomes smaller, it becomes shallower (the focus range becomes narrower). When the f-stop becomes larger, it becomes deeper (the focus range becomes wider).
Other things also have an effect, such as the lens focal length—that 35mm, 50mm, or 135mm thing—and the distance between the subject and the camera.
See, it sounds complicated, doesn't it? (^O^)
But there is no doubt that it is one piece of photography knowledge that is useful to know! ❣️