Still Photography Camera Guide
Optical Lens or Lens.
The optical lens or lens in short is the most important item in both digital and film camera as the lens is the eye of the camera the prime image forming component. The Lens is at the front line of the image creating process before any electronic or chemical process can take place in forming the final images.
A good quality lens can cost several times more than the price of the camera body. Brand name does not warrant quality as many big time camera makers routinely out-source their manufacturing to OEM (Original Equipement Manufacturer) lens or OEM camera makers for their low end cameras.
Good lenses are made of optical glass and not plastic. Those who wear spectacles will know that plastic lenses are easily scratched and don't have the same clarity as glass lenses. Plastic lenses are more sensitive to temperature variations.
On the other hand, plastic lenses have actually improved the quality of many low-end cameras due to the fact that plastic lenses are cheap, easy to work, mould and manufacture.
In fact, special lens design that was impossible to manufacture inexpensively for low-end cameras has actually benefited from the properties of this optical plastic material. As a result many low-end cameras take beautiful pictures when they are new but as for durability of these lenses only time will tell.
Camera lenses are complex lens meaning that they are composed of many glass elements combined to form a single lens. A good complex lens has several lens element combined in a group to form a single better lens. A medium quality/cost lens usually has 3 to 4 lens elements and a better lens can have up to 6 to 8 lens elements.
In the construction of the modern complex lens every lens element is carefully positioned and set in a fixed point. If there is a plastic lens element in the complex lens with a slight variation due to temperature or humidity change it will result in a variation on the image quality. However if your needs are just 4R (10cm X 15cm or 4in X 6in) or 6R (15cm X 20cm or 6in X 8in) photo you may not notice the variation in image quality with those lenses. Purists and many professional photographers are not keen in zoom lenses for that same reason -- variation in image quality.
Cross section of a f1.8, 50mm prime lens
So far we are just talking about fixed focal lengths lenses. Fixed focal lengths lenses or prime lens as they are often called are lenses that have a single focal length versus zoom lenses that have many focal lengths within their zoom range.
Prime lens offer superior optical quality over zoom lens because the lens design only need to optimize for single focal length. Prime lens usually have larger aperture are also much lighter and smaller than zoom lens. These qualities of prime lens make them ideal for low light and available light photography.
Cross section of a 28 to 200mm zoom lens
Zoom lens is more complex than fixed focus lens and more expensive and difficult to design and build. Zoom lens can have as many as 18 lens elements but too many lens elements is bad for image quality due to light refraction and formation of flare between the many lens elements. So most manufacturers will try to limit the design to between 14 to 16 elements. The extra lens elements plus the movement of the lens elements within the zoom lens make it less preferable in image quality than the prime lens.
Zoom lens is convenient to use since a single zoom can replace several fixed focal length lenses. Also its advantages and convenience out-weigh the quality issue since most camera users do not need pin sharp photo as demanded by that of the purist or signboard maker.
Focal length of a lens in a camera is the distance measured from the optical center of the lens to the image sensor.
Since we have mentioned the term "focal length", let's take a look at what is a focal length? Focal length is the distance measured from the optical centre of a lens to the point where the light focuses. Often this is the point where the image sensor or the film surface in film camera is. Focal length is measured in millimetres and is directly proportional to the magnification of the images.
For digital camera with full-frame sensor or 35mm film camera the standard focal length lens or standard lens ranges from 35mm to 55mm. Telephoto lens for 35mm camera has focal length above 85mm extending as far as 2000mm. It creates an illusion of bringing distant subject nearer. Focal length for wide-angle lens of 35mm camera is below 32mm and it takes in a wider view than normally seen by the human eye.
The standard lens focal length is roughly equal to the diagonal of the sensor's surface or the diagonal of the film format. For a 35mm film format the diagonal is 42mm and the standard lens focal length falls roughly within +/-20% of that measurement. For a Four Thirds digital SLR camera the only open standard in digital camera which has a diagonal excatly half of the 35mm film format the standard lens is roughly 21mm.
Graphical illustration of focal length Vs image size
The lens focal length of any film camera is easy to work out or measure but not for point-and-shot consumer digital camera - you can't pyhically measure the diagonal of image sensor inside the camera. The outdated convention used to describe this type of sensor size such as 1/2.5-in or 1/1.8-in is misleading and tells nothing about the actual demension of the sensor
35mm Equivalent Focal Length
The size of the image sensor varies from model to model even from the same manufacturer. So if you see a figure like 7.2mm lens mentioned on a digital camera you couldn't really tell if that is a standard lens or wide-angle lens. Unless the demension of the image sensor is also mentioned then you can use the method above to figure it out.
The easier way is to look at the specification sheet or ask your dealer what is the lens 35mm format Equivalent Focal Length or EFL. The reason that many camera makers state the 35mm lens equivalent focal length on their specification sheet is because 35mm is the defacto standard. Today the 35mm format is also referred to as full-frame format in professional digital SLR and many photographers and camera vendors are familiar with it.
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