I am using a Blue-Wave spectrometer with an optic fiber that has a field of view (FOV) of 30 degrees to measure the reflectance of the crop canopy.
If I hold the fiber optic cable approximately at 1.25 m above the canopy for reflectance measurements, I have a ground footprint of 0.27 m in diameter.
What happens with the footprint diameter if I attach to the optic fiber a fore optic: a collimating lens (LENSQ-COL, StellarNet) that has FOV 3 degrees, an 5 mm diameter?
What field of view you will get depends on the core size of your fiber. 3 degrees is a capability of the lens, not an indication of what you will get. What they are saying is that the lens will focus well onto the focal plane at field angles of up to 3 degrees off axis. This means it can support fiber core diameters up to 2*tan(3 deg)*focal length. Anything bigger than that and you MIGHT lose some light (it doesn't necessarily stop working at 3 degrees. It might just starts to get blurry. However it might also start vignetting.) Assuming your core is smaller than that limit, the full-angle field of view you will actually get is 2*arctan(fiber core diameter/(2*focal length))
The flip side which is probably equally important to you is that the reduced field of view comes with a larger collection area (and so sensitivity to signal). The aperture diameter is increased from the core size in the case of the bare fiber to 2*tan(arcsin(fiber NA))*focal length
This all assumes a multi-mode fiber which is almost certainly your case. It also assumes the lens' numerical aperture (NA) is larger than the fibers. Any amount larger is fine for multi-mode fibers which have fairly sharp angle cut-off.
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