Observation device

An observation device for observing an object, the observation device comprising:

• an objective that forms an image with light from the object, that includes a lens group, and that has a magnification equal to or higher than 4× and equal to or lower than 20×, the lens group having a positive refractive power and including, in order starting from an object side, a single lens having a concave surface on an object side and a variable aperture diaphragm configured to change a numerical aperture on an exit side of the objective;
• an image pickup device that converts an image of the object into an image signal, the image of the object being formed by the objective;
• an observation scope changing device that performs electronic scaling on the image signal obtained by the image pickup device and thereby performs an observation scope changing process of changing an observation scope, the electronic scaling changing the observation scope by changing an amount of the image signal that is subject to electronic extraction;
• and
• a control device that controls the variable aperture diaphragm in accordance with the observation scope changing process,
• wherein the control device, when narrowing the observation scope or setting an electronic scaling ratio to an enlargement scaling ratio, increases a diaphragm diameter of the variable aperture diaphragm,
• wherein the control device, when enlarging the observation scope or setting the electronic scaling ratio to a reduction scaling ratio, reduces the diaphragm diameter of the variable aperture diaphragm,
• wherein, during the observation scope changing process, each lens included in the objective remains fixed in its position along an optical axis direction of the objective, and
• wherein the single lens is a front lens included in the objective that is arranged closest to the object, and the objective is configured so that the following conditional expression is satisfied:
• 3≤|fa/fGS|≤10
• where fa is a focal length of the single lens, and fGS is a focal length of a lens group from an object plane to the variable aperture diaphragm.