"The 'crop factor' does affect the apparent depth of field of the taken image: with the magnification of 2, the DOF is effectively doubled (again, it is important to note that this perceived effect is due to the FOV 'crop'; the same depth of field is present, but you are viewing it at twice the magnification).
It is important to note that neither the light gathering ability of the lens, nor any of its traits are actually affected: a 50mm f2 lens on a 4/3rds body has the same f-stop and depth of field as a 50mm lens on a 35mm camera, but the 4/3rds sensor only records the equivalent field of view of a 100mm lens. As a general rule of thumb the depth of field of a lens is halved as its focal length is doubled (if the aperture remains constant). The end result is therefore that a 4/3rds camera has twice the depth of field as a 35mm camera with a lens covering the same field of view. "
This is bullshit. very common bullshit that many people spout off when talking about depth of field and lenses. Cropping or using a smaller frame size affects DOF. Think about it: if you halved the sensor size again would you again double the DOF with a lens giving the equivalent field of view? how about 2 times or more. or how about going from 35mm to 6x6 or large format would you halve the DOF? if it did, normal f2.8 and f4 lenses on medium format cameras would produce super shallow DOF images like a 50mm 1.4 on a 24mmx36mm frame, and they just don't
- A MF camera with a 'normal' lens - e.g. 70mm f2.8, focused at an object 10 feet away would give you 1.56 feet of DOF.
- A 35mm camera with a 'normal lens' - e.g. 55mm f2.8, focused at an object 10 feet away would give you 1.69 feet of DOF.
- A 4/3rds camera with a 'normal' lens - e.g. 25mm f2.8, focused at an object 10 feet away would give you 2.58 feet of DOF.
It is not dependent just on the aperture, but on the equivalent field of view and focal length as well. It is very simple when you think about it; I tried to make it as simple as possible in the article. If you take an image with shallow depth of field, and then crop the edges so you are left with the central 25%, you will obviously have (assuming the focus was in the centre...) an image where much more of the subject appears to be in focus.
Have the lens properties changed? No, but your field of view has, apparently changing the depth of field the lens is producing. It is not too complicated; it just depends on how much of the total imaging area of the lens (i.e. what percentage of the total depth of field) you are viewing.