FHD (Full HD) Zoom

A way to compare the magnification capability of camera lenses.

To evaluate the ability of a camera to produce a closer image, and thus magnified, the optical zoom is not a sufficient measure. Firstly because the zoom index that is given in the specifications from the manufacturer is based on the starting focal length and varies from one device to another.
For example, the Canon SX 70 has a minimum focal length of 21mm while the Nikon P900 has a value of 24mm. The division of the longest focal value by the shortest establishes the zoom value: 1365mm / 21mm = 65 for the SX70 and 2000mm / 24mm = 83 for the P900.

The largest focal length indicates the real magnification

But the P900 offers a magnification of the image compared to the SX70 which is more than the difference between 83x and 65x.
83x is 27% better than 65x but 2000mm is 46% more than 1365mm!
If we want to compare the magnification of a scene by the two devices, it is therefore necessary to calculate the zoom value compared to a reference focal length, and 24mm is the most common short focal length, and is near what see the naked eye with zero magnification (estimated between 22 and 24mm by bhphotovideo).

But this is not enough yet, because another parameter must be taken into account, the size of the image in number of pixels. Comparing images that have a different resolution is not right because in the image that has the highest resolution, I can cut an image of the same resolution as the other and this results in further enlarging the image that is taken by the camera.

The resolution also counts

The Canon SX70 takes a photo of 20 MP while the maximum size of a photo taken by the Nikon P900 is 16MP. If in the photo of the Canon I cut an image of 16MP I get a magnification of 20%. This adds to the 65x zoom and results in a cumulative magnification of 78x.

Taking the resolution of 1920x1080 pixels, or Full HD, as a reference, that is the maximum definition of an image displayed on most current TVs. This corresponds to a size of 2 million pixels.
If my camera has a resolution of 16 megapixels, ie 16 million pixels, I can cut into it a smaller image in Full HD resolution and compared to a shot taken directly in FHD resolution that equates to a magnification of 16/2 = 8x.
This will give me exactly the same picture as if I set my camera to take 1920x1080 pixels and I zoom in eight times.

I use this reference to evaluate the magnification capacity of a scene for a camera by taking into account both the optical zoom, and the possible zoom inside the photo. I call this the FHD Zoom and it lets me establish a more accurate comparison between various devices.

Formula:

Finally, to produce a Full HD image, the Canon SX70 has a FHD zoom of 1365/24 * 20.3 / 2 = 577x while the Nikon P900 has a FHD zoom of 2000/24 ​​* 16.1 / 2 = 671x and the advantage of Nikon is only 16%, less than the difference between the zoom powers displayed (28%).