STsite's Guide to Still Photography Camera

Lens -- The Camera's Eye.

The optical lens or lens in short is the most important item in both digital and film camera. For the lens is the eye of the camera the main image forming component. The Lens is at the front line of the image creating process before any electronic or chemical process can take place.

A good quality lens can cost several times more than the price of the camera body. German lenses are traditionally the finest. However, with the help of high tech computer, modern manufacturing techniques and many more years of R & D, many modern Japanese lens makers such as Canon, Olympus and Nikon etc., are getting very close to the German quality if not equal. Brand name does not warrant quality as many big time camera makers routinely out-source their manufacturing to OEM lens or camera makers for their low end cameras.

Cross section of a f1.8, 50mm stabdard lens

Good lenses are made of optical glass and not plastic. Those of you 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. Camera lenses are complex lens meaning that they are composed of many glass elements combined to form a single lens.

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 or 6R photo you may not notice the variation in image quality with the lenses. Purists and many professional photographers are not keen in zoom lenses for that same reason -- variation in image quality.

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 with 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.

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. Those cheap single use disposable cameras normally have one or two plastic lens element. So far we are just talking about fixed focal lengths lenses. Fixed focal lengths lenses are lenses that have a single focal length versus zoom lenses that can have many focal lengths within their zoom range.

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 element 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 zoom lenses inferior in image quality compared to the fixed focal length lenses.

Zoom lenses are convenient to use since a single zoom can replace several fixed focal length lenses. Also its advantages and convenience out weigh the quality issue as most camera users do not need the pin sharp photo demanded by that of the purist or signboard maker.

Since we have mentioned the term "focal length", let's take a look at what is a focal length. Focal length is the distance from the centre of a lens to the point which the light focuses. Often this is the film surface or the image sensor or CCD in digital camera, is usually measured in millimetres. The focal length is directly proportional to the magnification of the images.

Graphical illustration of focal length Vs image size

For 35mm 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 negative or slide of that film format. For a 35mm film format the diagonal is Å (35² + 24²) = 42mm and the standard lens focal length falls roughly within +/-20% of that measurement. For a 6 X 7cm Medium format camera the standard lens is roughly 90mm. The lens focal length of any film camera is easy to work out or measure but not for digital camera - you can't measure the diagonal of image sensor or the CCD inside the camera.

The size of the image sensor varies from model to model even from the same manufacturer. So if you see a figure like 5.2mm lens mentioned on a digital camera you couldn't really tell if that is a standard lens or Telephoto lens. Unless the image sensor or CCD size 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 equivalent 35mm format focal length. The reason that many camera makers state the 35mm lens equivalent focal length on their specification sheet is because 35mm is the defacto standard and many photographers and camera vendors are familiar with it.