Any optical tachometer can be "spoofed". You do need good light and a contrasting background because the tack counts the "flickers" it sees when the prop goes by. Trying to tach a black propeller when the background is black in shade or cloudy conditions can cause a problem. Trying to tach under a tree where the shade varies and moves around will fool the tach.

The best place to tach the engine is from behind the prop next to the cylinder. This gets it a few inches behind and in the outer 1/2 of the propeller arc. You may have to angle the tach slightly to get a stable reading.

In direct sunlight, there should be no problems getting a reading, but most optical tachs don't work well if they're more than 4-6 inches away from the propeller.