Limitations of an Automatic Digital Camera Exposing and Measuring Systems

An automatic exposure system can be a tool almost as precise as a manual measurement and also provides a measurement and adjusts the exposure value infinitely faster. Nevertheless, in determined conditions, the system can have difficulties or limitations. Even though we are "automatic", before we shoot we have to supervise the scene's conditions and the decisions the system adjusts to our technical needs.

•  Exposure value selection : Automatic systems are susceptible to error when confronted with contrasted scenes (2.2.2). In these cases we know that the average brightness doesn't guarantee a correct exposure of the zone we consider most important. While with a manual camera we can measure and adjust the zone's value, with an automatic camera this isn't possible because after measuring the main zone, when we go back to the frame we want to use it will readjust for the new scene, the system automatically switches to an erroneous EV. This situation can be avoided in many ways, depending on what the system offers:

•  With a EV memory mechanism, as in manual exposure, lets us frame the main zone, set the EV and reframe as originally desired.

•  Alter the EV in a degree we consider necessary if the system is equipped with an automatic exposure modification control. We can order the photometer to expose one, two or even three exposure values higher or under the true measurement. If this control doesn't exist, we can obtain the same effect by placing the sensitivity ring in the according position. If, for example, we see that the automatic system gives us an underexposure of one diaphragm we can "trick" it by reducing the sensitivity on the selector in half, and doubling it if the error is one diaphragm of over exposure. We shouldn't forget to set the selector back to the real value of the sensitivity of the film in the camera. Obviously this resource isn't possible if we are using a digital camera. Some cameras simply have a button with which the photographer warns the camera that the frame is against the light and modifies the exposure value in a set quantity.

•  Passing to manual mode: If the camera doesn't offer the any of these ways out, we will have to confront the error (which we can evaluate) or limit ourselves to framing less contrasted scenes.

•  Selecting the aperture/speed combination: In the systems with a priority we have access to any possible EV combination. In a PROGRAM system, on the other hand, we cant get different speeds and apertures besides the one it considers correct; if we need a different aperture or speed we can't get it. In aperture priority cameras, the adaptation of speed and other variables can, by accident, cause many moved picture. If we forget to change the aperture and pass from a bright zone to a darker, the shutter speed can reach a value lower than recommended in 2.3.