Yes, a larger-sensor camera is a lot easier to get the sharp people/blurred backdrop that you want to achieve. A medium-format, 120 rollfilm camera shooting 6x6 cm negs is dead-easy to get shallow DOF images with a 110 to 150mm f/2.8 lens--but you're not shooting that kind of a rig.
The main reason the 85mm f/1.8 lens does not throw the backgrounds as out of focus as you would LIKE them to be is because you're using it on a crop-frame camera, with a small sensor, and you're almost assuredly finding the need to physically move the camera back, far away, to fit your people into the frame.
Depth of field increases (becomes deeper) not in a linear way, but at a terrifically increased rate as the lens is focused closer and closer to the hyperfocal distance. Bob Atkins has the best article on the web to help understand how DOF increases VERY rapidly on APS-C sensor sized cameras; the gist of it is that once you move into the 10,11,12 foot range, and APS-C sensor camera begins to build DOF rapidly; by 30 feet, depth of field increases at a staggering rate with most normal lens focal lengths.
THe lens length, in relation to format size, and the focused distance are factors. Beyond just a few feet, FOCUS DISTANCE is a huuuuuge factor; it is the primary factor with a lens like an 85mm lens. With APS-C and FX cameras, the camera-to-subject focus distance is the primary variable on longer lenses.
The human brain is remarkably proficient at recognizing and identifying things in the background of photos. The classic prime lens lengths are all optimized, and were developed, for 24x36mm capture, since the 1930's. To shoot a 2-person portrait, full length,a man and wife side by side, with an 85mm lens on an APS-C Nikon you're at 34.5 feet. With a Full-Frame or FX Nikon or Canon, same exact 85mm lens, the SAME field height and width is from only 20.0 feet away. Bottom line...at f/4 or so, the APS-C camera's background is quite clearly "decodable" by the human brain--because the smaller sensor camera made you stand muuuuuch farther away, and with the size of the sensor, it already has moved into the zone where DOF behind the
34.5 foot focused distance is quite deep. I've been relating this example here for half a decade because the 85mm f/1.8 is the first prime lens most people buy...and it really does NOT work the same way on FX as it does on DX.
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This is the article that explains so,so much about DOF and bokeh. Be careful of 'learning' from web articles, many of which have erroneous or misleading info.
Depth of Field, Digital Photography and Crop Sensor Cameras - Bob Atkins Photography[/QUOTE
Thank you that was very informative abs helpful