Zoom lens and imaging apparatus

1. An optical system, comprising:

a light folding element configured to reflect received light onto an optical axis;

at least three lens groups, each lens group comprising one or more respective lens elements, each lens group positioned along the optical axis so as to receive and refract at least a portion of the light directed along the optical axis; and

at least three actuators, each actuator coupled with a respective one of the lens groups, each of the actuators configured to:

receive a corresponding signal indicating a corresponding direction of displacement along the optical axis for the respective lens group; and

in response to receipt of the corresponding signal, move the respective lens group along the optical axis by a corresponding displacement in the corresponding direction of displacement,

wherein to increase a focal length of the optical system, the at least three actuators are configured to:

move a particular one of the at least three lens groups along the optical axis in a direction toward an image plane toward which the received light is directed; and

move two others of the at least three lens groups along the optical axis in another direction that is away from the image plane;

wherein an f-number of the optical system is less than f/3.0.

2. The optical system of claim 1 , wherein a first one of the at least three actuators is configured to displace one of the lens groups in one direction along the optical axis while two others of the at least three actuators displace two others of the at least three lens groups in an opposite direction along the optical axis.

3. The optical system of claim 1 , wherein to decrease a focal length of the optical system, the at least three actuators are configured to:

move a particular one of the at least three lens groups along the optical axis in a direction away an image plane toward which the received light is directed; and

move two others of the at least three lens groups along the optical axis in another direction that is toward the image plane.

4. The optical system of claim 1 , wherein the light folding element comprises a mirror, or a prismatic element having a refractive index greater than or equal to 1.7.

5. The optical system of claim 1 , wherein the optical system has a focal length C at a maximum focal length state associated with the optical system, and one of the lens groups that is closer to the light folding element along the optical axis than others of the lens groups, has a negative refractive power and a focal length B and satisfies a relationship that a ratio of the focal length B to the focal length C, (B/C), is between −0.6 and −0.1.

6. The optical system of claim 1 , wherein the optical system has a 35 mm equivalent focal length of at least 78 millimeters and less than or equal to 130 millimeters.

7. The optical system of claim 1 , the at least three lens groups comprising, in order along the optical axis:

a first lens group comprising a fixed lens having a positive refractive power and fixed in position on the optical axis;

a second lens group comprising one or more lenses and having a negative refractive power and movable along the optical axis;

a third lens group comprising one or more lenses and having a positive refractive power, wherein the third lens group is fixed in position along the optical axis;

a fourth lens group comprising one or more lenses and having positive refractive power, wherein the fourth lens group is movable along the optical axis;

a fifth lens group comprising one or more lenses and having negative refractive power, wherein the fifth lens group is movable along the optical axis; and

a sixth lens group comprising one or more lenses and having a positive refractive power, wherein the sixth lens group is fixed in location along the optical axis.

8. The optical system of claim 1 , wherein the optical system is configured to provide a 35 mm equivalent focal length between 78 mm and 130 mm.

9. The optical system of claim 1 , wherein the f-number of the optical system is between f/2.3 and f/2.9.

10. A method, comprising:

receiving, by a controller, a request to change a focal length associated with an optical system comprising a light folding element and a plurality of lens groups positioned serially on an optical axis; and

responsive to the request, sending respective signals from the controller to each of at least three actuators, each of the at least three actuators configured to move a respective lens group of the optical system along the optical axis responsive to receipt of the respective signal that results in changing the focal length associated with the optical system, sending the respective signals comprising:

sending a first signal to a first actuator of the at least three actuators to move the respective lens group for the first actuator in a first direction along the optical axis;

sending a second signal to a second actuator of the at least three actuators to move the respective lens group for the second actuator in a second direction that is opposite to the first direction along the optical axis; and

sending a third signal to a third actuator of the at least three actuators to move the respective lens group for the third actuator in the second direction along the optical axis,

wherein an f-number of the optical system is less than f/3.0.

11. The method of claim 10 , further comprising:

receiving a request at the controller to focus an output image onto an image plane resulting from light refracted by the optical system; and

responsive to the request, sending a corresponding focus signal to one of the at least three actuators to change a position of the respective lens group that is closer to the image plane of the at least three lens groups configured to be moved by a respective actuator.

12. A camera, comprising:

an image sensor configured to sense light impinging on a surface of the image sensor that intersects an optical axis;

an optical system comprising:

a light-folding element configured to receive light at an object side of the lens system along an input axis, and to reflect the received light along the optical axis; and

at least three lens groups located along the optical axis between the light-folding element and the image sensor and configured to refract the received light that has been reflected along the optical axis for an image,

wherein the optical system has a focal length C at a maximum magnification, one of the lens groups has a focal length B, and wherein a ratio of the focal length B to the focal length C, (B/C), is within a range of values from −0.6 to −0.1;

wherein an f-number of the optical system is less than f/3.0;

at least three actuators each configured to move a respective one of the at least three lens groups; and

a controller configured to receive a request to change a focal length of the optical system from a current focal length to a requested focal length, and in response to the request, send a plurality of different commands, each command sent to a corresponding actuator of the at least three actuators;

wherein each actuator of the at least three actuators is configured to receive a respective one of the plurality of commands and responsive to receipt of the respective command:

move the respective one of the lens groups along the optical axis in a corresponding direction of movement indicated in the corresponding command.

13. The camera of claim 12 , wherein the requested focal length is greater than the current focal length and the controller, in response to the received request, is configured to:

send a corresponding signal to one of the actuators, to move the respective lens group along the optical axis in a particular direction that is toward an image plane toward which the received light is directed; and

send corresponding signals to each of two others of the actuators to move respective ones of the lens groups in an opposite direction that opposite to the particular direction.

14. The camera of claim 12 , wherein the requested focal length is less than the current focal length and the controller, in response to the received request, is configured to:

send a corresponding signal to one of the actuators to move the respective lens group along the optical axis in a particular direction that is away from an image plane toward which the received light is directed; and

send corresponding signals to each of two others of the actuators to move respective ones of the lens groups in an opposite direction that is opposite to the particular direction.

15. The camera of claim 12 , wherein one of the lens groups comprises a lens with positive refractive power and that has an Abbe number that is greater than 60.

16. The camera of claim 12 , wherein a height of the optical system, measured on an axis perpendicular to the optical axis, is less than or equal to 6 millimeters.

17. The camera of claim 12 , the at least three actuators configured to move the at least three lens groups along the optical axis that causes a variation in 35 mm equivalent focal length between 78 mm and 130 mm.