Resolution of Sensor AFFECTS Lens DOF

[harddrive]

A lens has the exact same DOF on an 8 mp camera as it does on a 36mp camera, unless this topic is about a non-technical perception-related subject that I don't understand.

What you are overlooking is the *precise* DEFINITION of depth of field in the technical sense.

It goes with the previous comments made re circle of confusion, that the limits of depth of field are DEFINED according:

Precise focus is possible at only one distance; at that distance, a point object will produce a point image.[1] At any other distance, a point object is defocused, and will produce a blur spot ......When this circular spot is sufficiently small, it is indistinguishable from a point, and appears to be in focus; it is rendered as "acceptably sharp". The diameter of the circle increases with distance from the point of focus; the largest circle that is indistinguishable from a point is known as the acceptable circle of confusion, or informally, simply as the circle of confusion. The acceptable circle of confusion is influenced by .........

My emphasis. The question then is what is the value of the diameter of such a circle? With conventional photography it's not so easy to define, as it will depend on degree of enlargement etc to be sensible.

But for digital video it's easy. It's equivalent to a single pixel of the system standard. If the CofC is less than a single pixel, it gets resolved as such. If it is bigger, it will be resolved over more than one pixel. In other words, that's when it becomes possible to start to see the focus is less than perfect.

Now change the system resolution, and for all else equal a "single pixel" will be smaller. Hence the CofC doesn't have to get so large before it affects more than one pixel. Which is why in pure technical terms - by definition - depth of field is dependent on system resolution. For a native 36 Mp sensor, there will be one set of values if the camera is used for HD, another for 4K, and another if used for full resolution stills.

This is the strict technical definition.

What I think may be confusing is what the image looks like in practical usage. Imagining a simple (plane) foreground in perfect focus, and a defocused background, the only apparent difference between (say) the HD and 4K images from the sensor above will be that the foreground is (hardly surprisingly) sharper in the 4K image than in HD. The background will look the same. Defocused - but by the same amount in the 4K and HD images.

But this is *NOT* what is meant by the term "depth of field". Not in the strict sense, anyway. It may be correct to say something like the "differential focus" is independent of system or sensor resolution - but that is not the same as depth of field.