​‌Role of Lens in Light Capture

A device that captures the light is Camera. Shutter, lens and film are the essential component in a camera. In digital camera film is replaced by CCD or CMOS sensor array. Shutter decides the quantum of light falling on the film.  Top layer of film is made up light sensitive granules. When film is exposed to light, granules get oxidized with respect to intensity of light. The exposed film is chemically washed to make negative film. The negatives are used to create photos. The sensitivity of film is described using ISO number. Higher the number results in more sensitivity towards light.

Lens plays a dominant role in determining the cost and quality of camera. Lion share of camera cost is attributed to lens. Professional photographers will have lenses ranging from 35 mm, 50 mm, 85 mm up to 200 mm. But, one can take photos without lens. The statement may evoke surprise but it is a fact. A camera without lens is called 'Pin Hole Camera'. The construction of the camera is simple. Take a rectangular metal or wooden box. Place a film in one side of the box and close it. Make a very small hole on the other side. This hole is technically called as aperture.  Via aperture, light from outside scene (say tree) falls on the film and an inverted image is formed. After getting sufficient exposure to light the aperture has to be closed with a shutter. Otherwise due to over exposure film will be washed out.

Pin hole camera exists over for centuries. Even now the concept of pin hole camera is extensively used in Computer Graphics [1].  The drawbacks of the camera are listed as follows;

1.  It requires few minutes to a photo.  So, taking sporting event photo is unthinkable.
2. Contrast of the photo will be very low as the light comes through a small hole. Film contrast can be increased by exposing the film with more light. Make the size of the aperture big so as to allow more light. Due to this, contrast will improve but crispness of the image will come down (image will be blur). Lens is primarily employed to have crisp picture with high contrast.
3. We are able to see distant objects as well as nearer objects. Zooming facility is unthinkable in Pin hole camera. Using lens, zooming becomes a reality.
4. In pinhole camera viewing angle is fixed. With lens only viewing angle can be increased or decreased.

Knowledge on geometric optics is sufficient to understand the light capture by camera and functioning of lens. Geometric optics introduces 'ray model’ which assumes light is composed of several non-coherent rays and each ray is emanating from a point source.  Geometric optics introduces reflection, refraction and absorption mechanisms to explain the interaction between light and matter. In a material with glossy or polished surface, light ray is just bounce back with 180 degree phase shift. This is called 'specular reflection'.  If the surface is irregular then diffused reflection takes place. Here incident light ray hits the surface and emerges as multiple rays that permeates in all directions. Incident ray's power or field strength is equally divided among the outgoing rays. That is why rough surface looks dull. Some material or medium may allow the light to pass (say glass or water) through it. The angle of arrival of light ray and refractive index of the medium may play role in bending the light ray. This is called 'refraction'. A light ray may pass from one medium to another but light ray never emerge from the second medium. This is called absorption.

Figure 1. Pin Hole Camera with large aperture

Light capture by a pin hole camera with large aperture is depicted in the figure 1. The sun rays falls on top arrow (intended to draw a tree) and the mildly diffused reflected rays pass through the pin hole camera's aperture and reach the light sensitive film. Thus an inverted image with low contrast and slight blur is formed. The sun ray that reaches top of the arrow is named as A and bottom side is called Z. After bounced on the subject, rays A to Z get diffusely reflected and produce A1, A2 ...A9 to Z1, Z2 ...Z9 rays. Only A1, A2 to Z1, Z2 pass through the aperture and reach the film. Other rays (A3, A4 ...A9 to Z3, Z4 ...Z9) rays are blocked by the box and they are not shown in the figure 1. Assume A1 to Z1 reaches the right place. Then A2 to Z2 will fall in the place adjacent to A1 to Z1. Thus B2 may fall on A1 ray. This will alter the light amplitude at the location A1. Due to this all the points will have the influence of neighbouring rays. Thus finding distinct edges in the image is very difficult. Next amplitude of each point will be nearly equal to neighbouring points. Due to absence of distinct edges and near equal amplitude make the image look blurred. In the pinhole camera only A1 to Z1 will be permitted to enter. So, possibility of blur is minimized.

Let a convex lens be placed on the aperture of pin hole camera. Convex lens has the ability to converge the parallel rays that emanates from a single diffused reflection into a single point. Thus insertion of lens will remove the blurness of the image. As multiple rays from same point sources are combined, every point in the inverted image looks brighter than earlier. This helps to increase the contrast of the image. Lens is invariably made of glass and convex lens shape is tapered in the end and thicker in the centre. This structure helps the parallel light rays to bend and get focussed on a spot.

Insertion of lens into pin hole camera rectifies all the earlier stated drawbacks. But, lens introduces new problems like 'Out of focus' and 'aberration'. Image blur will be the outcome of out of focus. Let us see this in detail in the next post.

Reference

[1] 3D Viewing: the Pinhole Camera Model (How a pinhole camera works (part 1)) | http://www.scratchapixel.com/lessons/3d-basic-rendering/3d-viewing-pinhole-camera