Magnification will in fact never change from only a Resolution Change (at least not on this camera), if Lens, Zoom (if there is any), Focus and Distance to object isnt Changed.

Given A Fixed Magnification Ratio, for example 1:1 for simplicity (which would remain the Same at any time, like asumed above, as nothing is changed), at an Sensor Size of 19,2x10,8mm Of the Chronos 2.1 at full Resolution, an Object or Area of 19,2x10,8mm will always fill the Frame (or more correctly is projected onto the Sensor to fill the Frame, so if Mag. Ratio is exactly 1:1, this is How much you will see.).

This means, that the Area that is "covered" by an single Pixel will allways stay the same, at 1:1 Magnification Ratio and an Pixel size of 10µm on the 2.1 an single Pixel will allways Cover 10µm (0,01mm) Area of your Object or Area in Frame.

.

Now, when you change Resolution, This Optical Process of projecting your Object *at whatever Magnification Ratio you are at* onto the Sensor at the Same Size at all times will still happen, the only thing that changes is that you now use Less pixels, meaning also less Sensor Width and/or Height. So if you would actually overlay the Smaller Resolution shot onto the Higher Resolution shot Pixel over Pixel (not Streched to fit the Same canvas!), actuall Magnification would allways stay the same (you wont get any more detail out of lower Resolution Shots, as one Pixel will allways cover the Same Space, limiting the Resolution). What changes is Field of view as some people call it or "Region of Interrest". What that means is that just not all of the Image is used, resulting in an more narrow field of view. If now a lower Resolution Shot is Streched to fit the Same final Size as an Full Resolution shot on this Camera, obviously the Object or Area Shown will seem Bigger. But this is about the Same as cropping the Frame on a PC in editing, wont really change the actuall Magnification or give you any more detail on a Pixel by Pixel base.

.

To calculate the Actuall Optical Magnification (if that is what you wanted to know), Figure out whatever effective Sensor size you are using (it is easiest to calculate at full resolution, as the Nominal full Sensor size is known (see Datasheet), while the effective Used Sensor Area needs to be calculated depending on used resolution), and shoot some Ruler or anything else of known dimensions, and Either get an Reading of the Object size/ Exact distance that will fill Height or width of the Frame, or get an Reading of how many pixels something of known dimensions will fill. Depending on that, as Pixel size (is known)/ Sensor Size (is known for Full Resolution / Can be calculated for Smaller Res.), an Ratio can be Calculated by dividing one by the other Lenght, giving you an (optical) Magnification Ratio.

.

To Calculate the Field of view at given resolution (basically the Same kind of calculation needed as when trying to figure out you effective Sensor size/ diagonal) you first need a reference point, for example a known Magnification Ratio or known Field of view/ Distance, if you want your result to be in Absolute Numbers (if you want to know how much mm of effective Frame Diagonal is Shown at an given Resolution), if you dont have a reference point, your result will allways be an Ratio (as the Full sensor size is the Only thing known, if nothing else is given/ known. Result will basically be like: At x Resolution the frame will seem y times bigger than at full resolution if stretched to fill the Same space (depending on whatever you are After, you will need to calculate that Ratio/field of view based on Horizontal / Vertical /diagonal Base, as they can result in very different values in some Cases, especially if your Aspect Ratio (=relation of Image Height to width) is Changed dramatically between the Two)).

So for example to Calculate the Horizontal Field of view/ Sensor Size for 1280px Image Width on the 2.1 Compared to Full Sensor Size, you need to either Divide the Full Resolution (1920px) by 1280px, to get an Crop Factor or increase of object size when final image Is Stretched to same Dimensions for that Axis of the Image. This would result in an Factor of 1,5x (Which in the Case of 1280x720 would be the Same for Horizontal, Vertical AND Diagonal Dimension, as Aspect ratio is the Same as 1920x1080, both are 16:9). Now this Horizontal Crop factor / Ratio of Field of View/ "additional Magnification" would be the Same for 1280x1080, 1280x720, 1280x640, 1280x240 or any other Vertical Resolution, if lens and Camera /object Position are untouched.

Vertical and Diagonal Field of view will obviously change quite a lot between all the Settings above but can be Calculated the Same Way by simply dividing one by the Other and Applying the resulting Ratio to whatever Known Dimension you have (Sensor Size, some object in Frame, ....) or using the Ratio Straight away.

.

As far as Calculating Diagonal Sizes of any kind, no matter if Sensor Size, Resolution, Object Size or Field of view; there is allways a^2+b^2=c^2 (a Squared + b Squared is c Squared, PythagorasTheorem) where "a" and "b" are horizontal and Vertical Dimensions and "c" would be your diagonal Dimension. Works for Absolute Numbers as well as Ratios.