Depth of field. Many ways to achieve a sense of depth

The most usual means of separation in depth is to place one thing
 behind another with obvious spacial separation
 and receding perspective. Near things are larger.

Unlike our eyes which are mounted side by side ( binocular vision) and give us good depth perception, the camera is monocular, has no natural depth perception, so we must adjust our subject matter to make up for this deficiency. We may do this automatically as we arrange the parts of our 'reality', but using depth of field, useful as it is, is not the only way to go.

There has been a lot of discussion around this subject. The beautiful bokeh in the blurred background etc. are an important part of the vocabulary of photographers who use a large aperture and close subject to achieve a dramatic separation of frontal subject and softened background. One might think sometimes that that was the only way to achieve such a separation. But in practice we use several. 

The tree trunk is obviously in front of the cedar branches
 and is lighter in tone and different in texture and shape.

Colour or tonal contrasts are obvious ones: red leaves set against a green mossy bank will provide such separation even if the tonal differences may be slight, and a dark shadowy background will set off a lighter, closer, subject. Differences in texture, in shapes, in focus, in directional lines and so on are all available to the photographer who looks through his viewfinder and concentrates on what his photograph will look like rather than settling for a simple capture of an attractive subject and the devil take the details.

    

In  colour and texture there
 is little separation except for the grey trunk...

...especially in monochrome. With our eyes
 we can see the real depth 
that is present but not in the
 photograph. 
The tree trunk adds needed form
 to this busy  image.

Sharp, red and white rails set
 off the soft receding background.
 Allowing the background
 to soften in the distance supplies
 a  natural sense of depth. 
Atmospheric perspective.

In monochrome the close values tend
 to loose the separation.

Colour difference gives separation.

As it does in monochrome as 
well.
 Establishing the appropriate
 sharpness in a snowstorm 
is the tricky part here.

Even with all the green we can separate the shape
 of the lighter tree from background