Lenses Overview

In addition to digital video cameras and DSLRs, some smaller cameras now also offer the option to change lenses.

However, whilst this flexibility may be attractive, there is a lot to consider when selecting both the lens and the camera.

Focal Length “f” and Crop Factor

The focal length of an optical system describes how strongly the lens converges incoming light.
It is thus related to the Field Of View of a lens for a given sensor: the shorter the focal length, the wider the field of view. For a given image size – i.e. that of a standard 35mm film frame, and for a focal length of a lens this will give us a known field of view/photographic image.

If a lens with the same focal length is used with an image sensor smaller than the traditional 35mm the field of view is reduced and the lens appears to increase in its focal length. This is known as the Crop Factor.
For APS-C cameras the sensors “multiplier” for the focal length ranges from 1.6 to 1.5, for 4/3s system cameras it is a factor of 2, and for a Blackmagic Cinema Camera it is a factor of 2.28. i.e. a 35mm lens appears to work as an 80mm lens.

Aperture f/N and T-stop

The f-number/f-stop/focal ratio must not be confused with the focal length. The f-number is a ratio calculated from the focal length and the effective aperture of a lens. It describes how much light the lens can acquire.  Ignoring losses in the lens, for a given subject, a larger f-number will produce darker images.

A lens will normally have an iris to reduce the amount of light entering the imaging system for bright conditions. This “stops” the light. The iris is typically marked in discrete steps. Each step/stop is 50% of the adjacent light intensity and because the ratio is relative to the area of the aperture, the sequence is geometric. i.e. f/2.8, f/4, f/5.6, f/8, etc.
The T-stop is the transfer function of the light through the lens system including losses. Some lenses are measured in T stops and if you use different lenses in the same scene and want to avoid jumps in brightness, emphasis should be placed on the T-stop value.

A major difference with lenses built for television cameras and motion picture/stills cameras is in the operation of the iris. Motion picture/stills camera lenses normally have fixed iris points and click between f-stop values. In a television lens the iris has continuous adjustment. This brings a disadvantage to stills-type cameras when used in video recording mode. Changing the aperture during recording will cause unattractive jumps in brightness. It is best to stop recording between aperture changes.

Depth of Field

The Depth of Field is the distance between the nearest and farthest objects of a scene that appear acceptably in focus. When the iris is “stopped down” in the lens to reduce the amount of light passing through, this will increase the “focus range” / “depth of field”. A short depth of field may be the desired editorial effect.

With ever more sensitive sensor units an alternative method of light reduction is needed if one wants to keep a desired short depth of field. ND (Neutral Density) filters can be used to reduce the amount of light entering the lens without otherwise varying the image. However, a note of caution: using very heavy ND filters, i.e. one that reduces the amount of light by three apertures or more, can create very disturbing effects. If the camera does not use an Infra-Red cut filter, this leads to visible IR pollution. Colours behave unnaturally and black turns purple or red. It is essential to check and avoid this with ND-IR-blocking filters.

Flange-to-Film Distance (FFD) / Flange Focal Length (FFL)

The “flange” distance is one of the most important variables for correct focusing. It describes the distance between the lens mount-bearing surface and the sensor plane. The camera body flange to film/sensor distance must match that of the lens’s flange focal length. If this distance is mismatched the lens may not be able to achieve focus for objects at infinity.

Typical flange distances are: - Canon EF Mount is 44mm, PL Mount is 52mm, Nikon F-Mount is 46.5mm and Micro Four Thirds is 19.25mm. Camera manufacturers normally supply lenses to match their camera bodies, however, providing one is working in the correct direction (i.e. a positive air gap), flange adaptors can be inserted between a lens and an alternate manufacturer´s camera body.

Image Circle and Vignette

The image circle is the diameter of the area that is illuminated by a lens. If the image circle is smaller than the sensor, it creates a vignette. If you want to produce special optical effects by shifting or tilting the lens, the image circle should be much larger than the sensor, otherwise this will also create a vignette.