A lens can only focus at one distance. In front of and behind this distance the subject is out of focus.

BUT - depending on the aperture of the lens - the degree to which things are out of focus varies. There is a distance range - dependent upon and controlled by the aperture - that appears to be in sharp focus. This is what gets referred to as 'Depth of Field'.

The smaller the aperture (i.e. the larger the f number) the wider this area of apparent sharp focus is.

Poser tries to replicate this physical phenomenon, using a 35mm camera as a basis.

You can read up on all the physics and maths and come up with a formula to achieve what you want, or you can use this 'cheat':

Determine the distances from the camera of both A and B. Set the focus distance about one third of the way between A and B. (so if camera to A = 1 metre and camera to B = 1.3M, set focus distance to 1.1M).

Set the aperture to around f11 or f16.

This is based on real-world experience. If it doesn't quite work first time in Poser-land, increase the f-number rather than change focus. And note that if you set the f-number high enough, C will start to come into focus too.

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Windows 10 x64 Pro - Intel Xeon E5450 @ 3.00GHz (x2)

PoserPro 11 - Units: Metres

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