Looking into the Lens

Back in the 50’s and 60’s families relied on their cameras to capture special moments. The popular camera of choice was not today’s ever-present smartphone, or the sleek digital camera of today. It was the Kodak Brownie and it held a place of honor at family gatherings and was not to be left home at vacation time.

Everyone would line up to be arranged by Mom so Dad could look through the viewfinder and make sure the scene was picture perfect. At the say cheese prompt, smiles spread across everyone’s faces and all eyes would look into the camera’s big glass eye.

My childhood vision of a lens as the camera’s eye was not far off base. In the center of the eye is the pupil which dilates to admit light on the retina, allowing us to focus. In the center of the lens is the aperture which opens to admit light, in the day of the brownie, to the film plane.


The ensuing technology has taken film out of the picture. No longer is there the horror of coming upon the photo shot of a lifetime only to discover we’ve run out of film.

 Today’s cameras have a life time supply of “built-in film - image sensors.But it is still falls to the lens to perform that first and crucial step of directing the path of light rays to project the image on the digital sensor.
In discussing lenses, there are several terms to be defined. The first is…..

Field of View

Field of view is the area we are covering and the degree of detail we desire. Since it depends on two factors, the focal length of the lens and the physical size of sensor, the field of view is not a fixed characteristic of a lens and can only be stated if the size of the sensor, with which it is used, is known.
Focal Length
The focal length is expressed in millimeters (mm) and determines the magnification of the image projected onto the sensor; the higher the number the higher the magnification. Thus we would select a longer focal length for a subject farther away. The focal length is also linked to the angle of view. Using a higher focal length would narrow the field of view. Conversely, for objects closer to the camera and needing less magnification we would choose a shorter focal length giving us a wider angle of view.

Angle of View
Lenses are also identified by their angle of view. Taking into account the sensor size and the length of its diagonal, a lens is:

·         Normal – if the angle of view is about 50 degrees and its focal length equal to the diagonal measure of the sensor

·         Wide-angle – if the angle of view is wider than 60 degrees and the focal length is shorter than normal. Used for close-up objects

·         Long-focus – if it’s focal length is longer than the diagonal measure of the sensor. Used for objects farther away

·         Telephoto – the most common type of long focus lens, using special optical configurations to make the lens shorter than its focal length

F-number
Another term we use in selecting a lens is the f-number. This refers to the maximum aperture or opening of a lens and determines how much light can pass through the lens, thus affecting the intensity of the light that will hit the image sensor. The f-number is also known as the focal ratio and is calculated by dividing the focal length by the aperture.
A smaller f-number correlates with a wider aperture and since a wider aperture admits more light, it also allows a faster shutter speed.  Lens with a smaller f-number would be the best choice in an area of low light. Conversely, a larger f-number pertains to a narrower aperture, letting in less light, thus needing a slower shutter speed. A higher f-number will increase the depth of field.

Lens Footnotes
We cannot leave the topic of lenses without mentioning a couple of other factors that will be explored in more detail in a future post:

Image Sensor Size
As stated above, the relationship of the sensor size to the focal length of the lens is one of the determinants of the field of view. If replacing a lens on a camera, we must make sure the new lens is compatible with the camera’s image sensor. If a lens is made for a smaller sensor the resulting image will have black corners; and if the lens is made for a larger sensor the resulting field of view will shrink since part of the information will be “lost” outside the sensor.

Megapixel Lens
 If we are working with a megapixel IP camera, we want to take advantage of its high resolution so choosing a high quality lens is vital.  Megapixel sensors have smaller pixels so if we want frames with   crisp images from edge to edge, the resolution of the lens must match the resolution of the camera.

One More Lens Footnote
Interestingly enough (according to Wikipedia) lens derives its name from lentil since a double convex lens is the same shape as a lentil.

        

If you have any questions about IP cameras and the best type of lens for your application, Kintronics sales engineers will be glad to assist you. Call us at 800-431-1658 or use our contact form to request information.