terminology - What exactly determines depth of field?

There are several questions here about the definition of depth of field, about focal length, and about subject distance. And of course there's the basic how does aperture affect my photographs. And plenty of how do I get super-shallow d.o.f questions. There's related questions like this one. But there's no be-all-end-all question asking:

What exactly determines depth of field in a photograph?

Is it just a property of the lens? Can lenses be designed to give more depth of field for the same aperture and focal length? Does it change with camera sensor size? Does it change with print size? How do those last two relate?

Answer

Ok for a change I'm going to dispense with the formulas, photos of rulers and definitions of "magnification" and go with what you actually experience in practice. The major factors that actually matter to shooting are:

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  Aperture. Wide aperture lenses give you a shallower depth of field. This is probably the least controversial factor! This is important as some lenses have much larger apertures e.g. 18-55 f/3.5-5.6 vs. 50 f/1.8

  
  


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  Subject distance. This is a really important consideration. Depth of field gets drastically shallower when you start to get really close. This is important as at macro focussing distances DoF is a major problem. It also means you can get shallow DoF regardless of aperture if you get close enough, and that if you want deep DoF in low light compose to focus further away.

  
  


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  Focal length. This does affect depth of field, but only in certain ranges, when maintaining subject size. Wide lenses have very deep depth of field at most subject distances. Once you get past a certain point, DoF changes very little with focal length. This is important again because if you want to increase / decrease DoF you can use focal length to do this whilst still filling the frame with your subject.

  
  


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  Sensor size. This affects DoF when you maintain the same subject distance and field of view between sensor sizes. The bigger the sensor the shallower the depth of field. DSLRs have much bigger sensors than compacts, and so for the same FoV and f-ratio they have shallower DoF. This is important because by the same token cropping images increases DoF when maintaining the same final output size, as it's akin to using a smaller sensor.