If you do photogrammetry using a camera that doesn’t have a full frame sensor (35 mm) pay attention to use the REAL focal length for calculating the Ground Sampling Distance (GSD).
If you chose the 35mm equivalent focal length you would underestimate the GSD.
And this will lead you to think that your pictures are good enough for your purposes.
Actually they’re not!
35mm equivalent focal length helps you to better understand how wide or short is your lens.
It’s a kind of common language that everybody can refer to in the jungle of sensor’s size.
There’s a simple formula that allows you to calculate the real focal length, starting from the 35mm equivalent one.
But you need to know a very special number: the crop factor.
Crop factor tells you how longer is the diagonal of a 35mm (FX) sensor compared to the one of your camera.
Here’s a nice image from the web.
Ok, how can i get to the real focal length.
You just need to take the 35mm equivalent focal length and divide it by the crop factor.
But let me share a couple of examples that are far better than thousand words.
DJI Phantom 4 Pro camera has got a 1” sensor.
Its crop factor is 2.7.
The 35mm equivalent focal length of the lens, integrated into the camera, is 24mm.
So the real focal length (f) is f = 24 / 2.7 = 8.8 mm
DJI Spark camera has got a 1/2.3” sensor.
Crop factor is 5.62.
35mm equivalent focal length of the lens is 25 mm.
Real focal length is f = 25 / 5.62 = 4.4 mm.
Old Nikon D300 has a APS-C sensor.
Crop factor is 1.52.
If the lens attached to the body says 50 mm equivalent focal length, then the real one is f = 35 / 1.52 = 23 mm
And so on…
Some camera manufacturers specify real and equivalent focal length, so there are no doubts about them.
But sometimes you find just the 35mm equivalent f.
That’s especially true for small sensors: some drones, point and shoot cameras, smartphones …
If you’re just taking nice pictures that’s fine.
But if you wish to do photogrammetry you need to switch to real focal length.