An illumination system includes an excitation light source, a first variable focus lens, a second variable focus lens, and a control unit. The excitation light source is adapted to provide an excitation light beam. The first variable focus lens is disposed on a transmission path of the excitation light beam. The first variable focus lens is located between the excitation light source and the second variable focus lens. The second variable focus lens is adapted to receive the excitation light beam exited from the first variable focus lens. The control unit is electrically connected to the first variable focus lens and the second variable focus lens, and is adapted to synchronously adjust focal lengths of the first variable focus lens and the second variable focus lens. A projection device including the above-mentioned illumination system of the invention is further provided.

There is provided an objective optical system, an image pickup apparatus, and an endoscope that are small in size and easy to assemble. The objective optical system includes, in order from the object side to the image side, a negative front lens group, an aperture stop, and a positive rear lens group. The front lens group includes a negative first lens disposed closest to the object, a meniscus second lens having a convex surface facing toward the image side, and a view-direction changing element. The objective optical system satisfies the following conditional expressions (1) to (4):

9<FL2/FL<15 (1)

0.3<FL1/FLf <0.64 (2)

95<(r2f+r2r)/(r2f-r2r)<−18 (3)

2.8<FLr/FL<4 (4).

There is provided an objective optical system, an image pickup apparatus, and an endoscope that are small in size and easy to assemble. The objective optical system includes, in order from the object side to the image side, a negative front lens group, an aperture stop, and a positive rear lens group. The front lens group includes a negative first lens disposed closest to the object, a meniscus second lens having a convex surface facing toward the image side, and a view-direction changing element. The objective optical system satisfies the following conditional expressions (1) to (4):

9<FL2/FL<15 (1)

0.3<FL1/FLf <0.64 (2)

95<(r2f+r2r)/(r2f-r2r)<−18 (3)

2.8<FLr/FL<4 (4).

The imaging lens consists of, in order from the object side, a front group, an aperture stop, and a rear group. The front group includes a diffractive optical element having a positive lens and a negative lens in order from the object side. A diffractive surface is provided between an object side surface of the positive lens and an image side surface of the negative lens. Assuming that a distance on an optical axis from the diffractive surface to the aperture stop in a state in which an object at infinity is in focus is Ddoe, and a focal length of the whole system in a state in which the object at infinity is in focus is f, the imaging lens satisfies Conditional Expression (1): 0.02<Ddoe/f<0.11.

The imaging lens consists of, in order from the object side, a front group, an aperture stop, and a rear group. The front group includes a diffractive optical element having a positive lens and a negative lens in order from the object side. A diffractive surface is provided between an object side surface of the positive lens and an image side surface of the negative lens. Assuming that a distance on an optical axis from the diffractive surface to the aperture stop in a state in which an object at infinity is in focus is Ddoe, and a focal length of the whole system in a state in which the object at infinity is in focus is f, the imaging lens satisfies Conditional Expression (1): 0.02<Ddoe/f<0.11.

An optical lens (10), a camera module (100), and an electronic device (1000), where light that enters the optical lens (10) is axially folded by using a reflex optical element in the optical lens (10), so that the optical lens (10) can have a relatively long focal length to achieve a long-range photographing effect, and the optical lens (10) has a relatively short total track length. Therefore, when the optical lens (10) is used in the electronic device (1000), thinning of the electronic device (1000) is not affected. In addition, no prism is required to implement optical path folding in the optical lens (10).

An optical lens (10), a camera module (100), and an electronic device (1000), where light that enters the optical lens (10) is axially folded by using a reflex optical element in the optical lens (10), so that the optical lens (10) can have a relatively long focal length to achieve a long-range photographing effect, and the optical lens (10) has a relatively short total track length. Therefore, when the optical lens (10) is used in the electronic device (1000), thinning of the electronic device (1000) is not affected. In addition, no prism is required to implement optical path folding in the optical lens (10).

An objective optical system for an endoscope consisting of, in order from an object side toward an image side: a front group having negative focal power; an aperture stop; and a rear group having positive focal power, wherein: the front group includes only three lenses, which consist of, in order from the object side toward the image side: a first lens having negative focal power; a second lens having negative focal power; and a third lens having positive focal power, the rear group includes only three lenses, which consist of, in order from the object side toward the image side: a fourth lens having positive focal power; a fifth lens having positive focal power; and a sixth lens having negative focal power, the second lens and the third lens are cemented to each other, and the fifth lens and the sixth lens are cemented to each other.

An objective optical system for an endoscope consisting of, in order from an object side toward an image side: a front group having negative focal power; an aperture stop; and a rear group having positive focal power, wherein: the front group includes only three lenses, which consist of, in order from the object side toward the image side: a first lens having negative focal power; a second lens having negative focal power; and a third lens having positive focal power, the rear group includes only three lenses, which consist of, in order from the object side toward the image side: a fourth lens having positive focal power; a fifth lens having positive focal power; and a sixth lens having negative focal power, the second lens and the third lens are cemented to each other, and the fifth lens and the sixth lens are cemented to each other.

An optical system includes a front lens group consisting of at least one lens unit that moves during focusing and having a positive refractive power, and a rear lens group consisting of a lens unit that is immobile during focusing and having a negative refractive power. The rear lens group includes at least two negative lenses and one positive lens. The front lens group and the rear lens group satisfy a predetermined inequality.