Fujifilm X Series APS C sensor camera

To know what a diaphragm aperture on APSC sensor FUJI X corresponds on 24x36, simply multiply by 1.5, as for the focal length.

So :

- 1.4 X 35mm equivalent in 53mm FF to FF 2.1 for depth of field

- 1.2 X 56mm equivalent in 84mm FF to FF 1.8 for depth of field

It is not at all negligible, since this corresponds to a variation of an aperture full stop and when you consider the price gap between a f1.4 versus a 2.0 or f4.0 zoom compared to f2.8 zoom.

The Fuji XF55/140 f2.8 is more equivalent for this point of view  in  full frame more to a 70/200 F4.0 zoom than  a 70/200 F2.8 zoom
But the luminosity is maintained, which permit to "win" in ISO or speed.

At the risk of simplifying, source Wikipedia:

Dof= = 2 N c D² / f²

with:

Dof: Depth of Field

N: aperturec: value in mm circle of confusion.

D: Distance to the focus plane i.e. distance to the subject

f: focal length in mm

Finding: The sensor size is not involved. We should not be surprised with a "small" sensor it is as if we took the photograph with a full frame and that was cut (crop).

As regards the field of view, the "crop factor" k intervenes directly and for  the focal distance, simplifying again:fk = f / kis f = k fk, the known formula on a FUJI X APSC 35mm behaves like a 50mm 24x36, more precisely as a 53mm = 35 *  1.5.

In terms of the value of the circle of confusion, there recess ck = c / k
So DoFk = 2 N c / k D² / (f / k) ² = k DoF

It follows that for the opening of the diaphragm the DoF equivalent is

N = k  Nk.