Digital Camera Accessories: Focal Focus and Lens Converters,

The focal length of a lens expresses it's convergence and the size of the images it forms. Since the larger or smaller convergence of a lens is the result of the sum of converging and diverging elements, we'd have to ask ourselves if it's possible to add new elements to a lens to obtain a different focal length. The focal converters aren't properly lenses, but optic groups that are added to a lens to alter it's focal length.

Additional Lenses (Non focal accessories)

When the lens is integrated in the camera's body (direct viewfinder cameras and digitals), the only way to alter it's focal length is by adding a lens or a group of them to the front part. The non focal devices are very cheap, don't reduce the luminance and can give access to greater or lower focal lengths than that of the used lens. Nonetheless, in a direct viewfinder camera, there is a certain margin of doubt on the new frame, focus and perspective of the image. The image's quality will be somewhat worse, although it is a good compromise between simplicity and increase of the cameras possibilities.

Proper Converters

Properly called focal converters are designed for reflex cameras and are interposed between the lens and the camera's body. The most used converters are the focal multipliers or teleconverters , through which the lens' focal length is multiplied by the conversion factor associated with the accessory. The conversion factor is fixed and, according to the models, produce a linear increase of the size of the image of 1 ½, 2, 2 or more times. The most widely used teleconverter is the one with a factor equal to 2, and that is frequently called a duplicator .

Teleconverters increase the range of lenses we have at our disposal but are associated with a loss of luminance in the number of exposure values equal to it's factor. This reduces the scene in the viewfinder's brightness, which can make focusing difficult, and forces us to use larger apertures and/or longer exposure times. The quality of the images, depending on the model and brand of the accessory, will be slightly inferior or practically the same as the correspondents to a fixed focal lens.

Approximation Lenses

The eye's "focus mechanism" is slightly different to the one we have seen for lenses. The muscles that regulate focus on the retina don't move closer and farther away to the lens, they vary it's curvature, and consequently it's convergence . Therefore, there is a slight variation in the lenses focal length. The variation of the focal length is measured by an alternative system but similar to the one we know: the strength of a lens, which expresses convergence in dioptries. The measurement in dioptries is nothing but the inverse of the focal length and is expressed in meters . Therefore, a lens of a focal length of 1 m is of one dioptre, a lens of .5 m focal length is of two dioptries, etc. When the convergence of the lens varies, we can consider we have added or subtracted from it's convergence a number of dioptries. Focus at short distances on an object can be done without varying it's convergence, which requires moving the lens away from the film, or also adding to the front part a converging lens of a determined number of dioptries. Approximation lenses let us focus at short distances without the need of separating the lens from the film. The loss of luminance, which is noteworthy when we separate the lens from the viewfinder is insignificant when we add one of these lenses. The convergence increase and, therefore, the reduction of the minimal focus distance, is proportional to the power of the lens.