When equipped, it can be peered through to reveal a cone-shaped portion of the map in the direction the player clicks with Right Mouse Button (RMB).
The range Spyglass will reveal changes by the time of Day, farthest being during day time. The range is also reduced on boats, and during Hurricane Season.
The Super Spyglass is a better version of the Spyglass with doubled map reveal range. It can be crafted using a Spyglass, an Eye of the Tiger Shark and a Gold Nugget. It requires a Science Machine to prototype and can be blown away by Strong Winds.
The range reduction while on a boat doesn't apply to it, but the range is still affected by the time of day.
Either version can be found in X Marks the Spot treasure chests.
The distance revealed by the Spyglass can be sailed in a short amount of time with boats, so it is recommended to use it to peek towards other directions that the player does not travel into.
Bugs
On console editions, having a Boat Lantern equipped prevents the use of Spyglasses while at sea. Removing the lantern will allow normal use of the spyglass. This bug also affects other hand-held equipment that have a cursor (such as the Volcano Staff) from being used while on a boat.
Trivia
Old Super Spyglass icon.
The art for the Super Spyglass was changed in the Eye of the Tiger Shark update to reflect it being craftable with an Eye of the Tiger Shark.
Gallery
Range Comparison (Normal Spyglass on the left and Super Spyglass on the right)
Wilson holding a Spyglass.
Wilson holding a Super Spyglass.
Wilson using a Spyglass.
Wilson using a Super Spyglass.
Walani using a Super Spyglass, as seen in the Shipwrecked trailer.