Format Sizes - Large or Small?

Cameras come in a range of format sizes. In general, the larger the format, the higher the price of the camera, and the more storage is required for the video signal files in post-production.

Movie film formats are generally large and expressed as the width of the film stock itself, while video formats are expressed in terms of the equivalent outer diameter of a scanning tube on the end of which the light-sensitive target is placed. Widescreen variants of the film formats always fill the widths given, but the height may be reduced, or kept the same if an anamorphic lens is used.

Large Format Lenses

Clearly, the smaller the imager size, the greater are the demands on the lens for that format. For example, to achieve 1920 pixel resolution on 35mm film, the lens must deliver resolution up to (but neither including nor beyond, because of the threat of aliasing) 1920/2/22.05=43.5 cycles per mm (or line-pairs per mm). But, to achieve the same resolution on a 1"/3 video camera, the lens must deliver 1920/2/4.8=200 cycles per mm. Nevertheless, such lenses do exist and can perform well, but this shows that it is generally a poor idea to use a small format camera and fit a large format lens to it, the resolution will probably be poor, although the Depth of Field will be that of the larger format lens provided the lens adaptor forms an image within itself, relayed into the camera to fill its imager. If the lens is merely fitted with a mounting adaptor (no optical parts), then the resolution of the lens at its larger format will be delivered to the smaller video camera imager, and the pictures will be soft.

Zone Plate

Lens assessment is best performed using a Zone Plate test chart, since it shows not only the MTF of the lens and camera, but also identifies any aliasing problems caused by either the lack of a bi-refringent spatial filter in the camera, or the filter not being matched to the performance of the lens.

Contributed by Alan Roberts