Optical Instruments and Human Vision
Optical Tools
Lenses and mirrors are used to build optical instruments. Some are well-known and commonly used: cameras, binoculars, or a magnifying glass. Others have great scientific interest, such as microscopes or telescopes. In both, the visual tool for our purpose, optical systems, is a friend because this is our natural optical system. Therefore, it is important to understand how our vision works.
Human Eye
The aim of objects and images of eyes is to be estigmatic without creating defects. Back-spherical shaped eyes have a wide visual field, an average of 180°, and are able to change focus very quickly, providing good resolution and high-power operation.
Retina
Cornea is a resistant membrane that surrounds the eye, called the sclera. A section is transparent, where light enters.
Crystalline lens is a convergent shape of the soft body.
Retina is a membrane that covers the inside of the eye. The projected image of the object influences light-sensitive cells composed of several layers.
Image
The size of the organized image on the retina (real and smaller) depends on the size of the objects and how much closer they are. The closer the object, the larger the image; the further the object, the smaller the image.
Our optical system, like the eye, consists of the following elements: a diopter, a spherical cornea, a lens, and a crystal. Typically, the focus is infinite, so the infinite distant point is what we see. To focus on near objects, the ciliary muscles contract, thus adapting the crystalline form, making it more spherical, changing the curvature and focal length to a smaller radius.
This adaptation is called focusing capacity. As a result, the human eye can clearly see objects from 25 cm, which is called the near point of the eye.
Photo-camera key elements are as follows:
- Body: The camera is essentially a dark box. An inverted real image of the object is formed at the back, where the light-sensitive photographic film or plate is placed. A photochemical process captures the image on the plate.
- Objective: The reflected light from objects is collected with a convergent system, the simplest being a convergent lens.
- Objective should create a good image without errors and should have deep focus. Depth of focus allows the objective to focus on both near and far distances simultaneously.
- Viewer: Frames the desired part of the image on the film.
- Exposure: Controlled by time, the duration of light reaching the film.
- Diaphragm: A metal ring in the objective that regulates the amount of light reaching the film.
- Shutter: Opens for a specific time during exposure, allowing external light to impress the film.
Camera and human eye functions: The crystalline lens is similar to the camera’s objective, the retina to the sensitive film. However, the photographic camera’s aperture is fixed-focus, while the eye has a larger visual field.
Focusing is adjusting the distance between the lens and the film to get a clear image of the object.
The amount of light entering the camera is regulated by the diaphragm and exposure time, selected according to the film’s sensitivity.
A Simple Magnifying Glass or Microscope
A magnifying glass, also called a microscope lens or a simple magnifier, is a convergent lens that allows seeing an object larger than itself.
To see the details of a small object, we usually bring it closer to our eyes, making the retinal image larger. However, there is a limit to how close we can bring an object and still see it clearly. Therefore, we use a magnifying glass to place the object closer than the near point.
When an object is placed within the focal length of a convergent lens, a virtual, upright, and magnified image (A2B2) is obtained. The lens is placed close to the eye to see the object.
The angular magnification of a magnifying glass is the ratio between the visual angle with the magnifying glass and the visual angle without it.
For focused vision, the object is at infinity, meaning no adaptation is needed. Thus: