The sensitivity of the camera will usually be reference to 4 interlinking element by camera manufacturers.
1.A subject with peak white reflectivity.
2. Scene illumination.
3. F number.
4. Signal-to-noise ratio for a stated signal.
For example: A peak white subject that has 89.9% reflectance lit by 2000 lux quoting the signal or noise ratio will be expressed as the resulting F number when it is exposed.
From that example: most of the current digital cameras will be F8 or a better with a signal or noise ratio of 60dB. This does not indicate how much light the camera should use but instead, it allows different camera sensitivity.
Greater amplification of weak signals could increase the sensitivity of the camera but it may degrade the picture by adding noise which would be generated by the camera circuits. Contour or gamma correction will not necessarily be needed when measuring the signal or noise ratio. Manufacturers may vary in the way that they will state camera sensitivity, and because of this, comparing the differences between the models will require a conversion of the specification figures. If the camera has the same F number, then the higher the signal or noise ration and the lower the scene illuminance or lux, the more sensitive the camera will be.
If there is insufficient light that exposes the picture, then the gain of the head amplifiers may need to be increased and any additional gain will be calibrated in dBs. An example of this would be adding +6dBs which would double the amount of light that is available to the sensors. The amount of switch gain that is available would usually depend on the camera. Some cameras however, will automatically increase gain if the light were to decrease when a specific F number is selected. This could increase noise to an unacceptable level without the cameraman being aware. Cameras may also have a negative gain setting which would reduce noise and control depth of field without the use of filters.
GAIN AND STOP COMPARISON
+3dB will be equivalent to opening up 0.5 stop
+6dB will be equivalent to opening up 1 stop
+9dB will be equivalent to opening up 1.5 stops
+12dB will be equivalent to opening up 2 stops
+18dB will be equivalent to opening up 3 stops
+24dB will be equivalent to opening up 4 stops
Any extra gain in the amplification is a corresponding decrease in the signal to noise ratio and will result in an increase in noise in the picture.
CALCULATING THE ASA EQUIVALENT FOR A VIDEO CAMERA
A video broadcast camera will have a reliable light meter if it has a good auto-exposure system. Most cameras will use a combination of three different exposures:
1. Manual exposure
2. Instant auto-exposure
3. Zebra exposure
Some cameramen with a film background sometimes feel more comfortable when a using light meter to check exposure level; they would be able to do this by an equivalent ASA rating which would be logarithmic. There are several methods to determine the rating:
- The sensitivity of the video camera uses a stated light level, signal to noise level, a surface, with a known reflectance value and with the shutter set at 1/50.
- Japanese camera manufacturers use a standard reflectance of 89.9% as peak white while UK television practice is to use a 60% reflectance value as peak white therefore an illuminance level of 3000 lux must be used when transposing a rating of 2000 lux with 89.9% reflectance to 60% peak white working.
Camera sensitivity has increased over the years and in the last few, cameras have begun to include a negative gain setting.
To boost sensitivity, image intensifiers are fitted between camera and lens while shooting is occurring in low light levels. The end picture may lack contrast and colour but it will produce recognizable images.