It has been mentioned at a few places that a larger sensor results in images with a shallower depth of field. Example image:
APS-C Canon 30D left, FF Canon 5D right, same lens, same composition, both f/2.8
I understand how sensor size would relate to, for example, field of view, but the relationship with depth of field does not seem straightforward.
It actually seems contradictory - I have more wells on my sensor, and I am able to focus on less number of points.
What is the reason for this effect?
Answer
Firstly, the number of wells on the sensor has no bearing on where you can focus or 'how much' you can focus on. Out of focus light is still light hitting the sensor.
Simply put, you have shallower depth of field with a full-frame sensor because you have to get closer to the subject (or zoom in) in order to fill the frame. Note that in your example it says same composition: the camera has been moved or the focal length changed in order to fill the frame with the bottle.
To put it another way, if you fix a tripod in place and take a shot with an APS-C camera with the bottle filling the frame, then take the same shot in exactly the same position with a full-frame camera, you will have more space around the bottle. So you will either need to zoom in or move the camera closer to achieve the same shot where the bottle fills the frame, either of which will alter the depth of field,
See this excellent summary of sensor sizes and their effects.
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