all of the non contact temp guns use infrared, and it is a surface reflection measurement 
so the better reflection you can get, which would be a straight shot coming off your object, the better the reading will be. if you are shooting at a sharp angle, you can imagine how a majority of the signal is not being returned, so the device has a harder time reading.
as for distance, the infrared comes out in a "cone" shape, so the further you are away from the object, the wider the cone, the wider the area is that you are measuring.
i shoot most reptile stuff at 8-12 inches, and more precise stuff like incubating eggs at just an inch or two, so i can get the smallest spot reading possible 
AWESOME TOOL though, unblieveable feedback on setups, animals, and temps 
thanks for the feedback!
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robyn@proexotics.com
Pro Exotics Reptiles