I always wondered – “who died and made 35mm the reference size?” It’s popular for sure, but it seems a bit strange to call 35mm sensors “full frame” when there are plenty of bigger ones and even bigger slabs of film available. So I did a bit of research. “Full frame” does not mean 35mm film sized sensor. In fact it has nothing to do with the size of the sensor. “Full frame” refers to a type of CCD sensor that does not have “shift registers” interleaved between each photo-site to shift the values off the photo-diode. The space this frees up can be occupied by more photo-diode (around 70% surface area is light sensitive in a typical full frame sensor) making them more sensitive to light. They are called “full frame” as they shift the full frame out of the sensor to the storage array at a time. This also makes them subject to light smear as they are still collecting as the data clocks out line by line to the storage array. Without interline shift registers, full frame sensors are cheap to make, but require a mechanical shutter to stop and start light collection. Because of this they cannot do video, or live view.
Almost all digital cameras today are not full frame. They have Interline Transfer CCDs that are equipped with shift registers interleaved between the lines of photo-diodes to transfer the data off the photo-diode and onto an accumulation register (which can then be clocked out to the storage array while the light sensitive area is collecting photons for the next frame – allowing electronic control of the image start and stop and doing away with the need for a mechanical shutter (we then have to ask why manufacturers add mechanical shutters to their cameras – which cause all kinds of problems with vibration and for high speed flash – I don’t know why they do this and can’t find any explanation). Because they have shift registers next to the photo-diode, only around 30% of the photo-site is light sensing material. To compensate, these sensors have an array of “micro-lenses” on them to collect light from the full area of the photo-site and concentrate it down into that 30% area.
So, by this definition, if your camera, like mine, does video or live-view: it ain’t “full frame” – no matter how big the sensor is.
Another meaning of “full frame” originates in the movie industry. 35mm movie cameras using the full size film gate were called “full frame” or “full gate” https://en.wikipedia.org/wiki/Full_frame Smaller gates could be used to save film (which would run through the smaller gate slower).
This is also meaningless to 35mm digital cameras unless (as on some Nikon 35mm cameras) you have the option shoot with less than the full frame. Even then though, this makes no assumption about the actually size of the “full frame” – it could be anything.