Camcorder Masterclass - Index
Manual exposure

Eric is currently making a short video depicting Autumn and he asked us whether his photographic approach was along the right lines. We noted a marked improvement over previous camerawork but thought, that he needed to work upon exposure. For instance, foliage of autumnal gold was pictorially represented as a darkish mass, almost silhouette. Eric wondered why and what could be done to correct the situation. The answer was simple - he had placed too much reliance on the camcorder's automatic exposure facility.

Whilst most auto-exposure systems are very good and will usually provide a faithful on-screen representation, there are limitations. We'll spare you from too many technical details on how auto-exposure systems work. Sufficient to say that the intensity of light coming through the lens, governs the aperture setting (iris) and hence the electrical signals relating to colour, exposure and contrast, which are transmitted from the coupled-charge-device (CCD chip) onto the recording head. When the light is strong the iris closes down, perhaps to F.16, whilst a poorer light will open it up, maybe to Fl.2. There are various ways in which auto-exposure systems measure the light intensity in the area being framed but basically the result is a compromise or average for that area. If the lighting on the principal subject is reasonably similar to the compromise/average then the exposure on that subject should be good. But if otherwise, then the subject will either be under or over-exposed, depending upon the circumstances. In Eric's shoot, the overwhelming light (backlight) from too much sky in the picture caused the iris to close, with consequential under-exposure of the subject. Similar results can occur when shooting snow scenes, or against other bright backgrounds.

Bright Backgrounds

There are several ways of overcoming the problem of an over-bright background:

• The obvious is to cut down the extent of backlighting by either moving closer to the subject and/or reframing so that the subject matter dominates. The auto-focus reading will then be more appropriate for that subject.

• If the above is impractical or is not wanted then, provided the camcorder has a manual exposure facility, the auto-exposure can be over-ridden and the iris opened wider. The appropriate setting is achieved by manipulating the exposure control until the detail on the prime subject becomes most visible through the viewfinder. Where possible frame the most important part of the subject, such as a person's face, etc and then select exposure. Next reframe as required. There may have to be a trade-off in that the sky or other background might be over-exposed but it's up to you to decide the priority.

  Unfortunately manual adjustment doesn't stop the auto-exposure system from functioning; it just adjusts its assessment of the light level by the extent of the manual alteration. Thus, if the iris was manually opened up from F.11 to F.5.6 (two stops) and then the light changes, F.5.6 becomes something else. Exposure fluctuations can be disconcerting, particularly if they occur during the course of a shot; more likely a zoom or maybe a pan. People or other things moving across the frame, when fairly close to the camcorder, can also cause fluctuations. If you possess a camcorder, such as the JVC GR-S707, which allows the selected exposure to be locked, then such alterations can be avoided, even though you might have subsequently changed position or reframed the picture. However, herein lies the snag. Insofar as we are aware no subsequent camcorder has the facility - a deplorable omission. For the fortunate few we recommend use of the lock even when the auto-reading is what you want. It is also very handy when transferring cine film to video.

• A further means of adjusting the auto-exposure is to use the backlight compensator (BLC) which adorns many camcorders. The BLC opens up the iris by one or two stops but there is no control over the extent; it's take it or leave it. Sod's Law dictates that the BLC rarely gets it right.

Another thought on under-exposure. Many camcorders have an Automatic Gain Control (AGC) which, if switched on, will automatically electronically enhance the picture when illumination is very low and the iris cannot open up any further. There is a price, increased graininess - so only use as a final resort.

The dark background problem

Let's deal now with over-exposure caused by the auto-exposure facility being fooled when the subject matter is shot against a predominately dark background. Because the iris will have opened up too widely the subject will have washed-out colours and form. In extreme circumstances it may almost disappear. The BLC is of no use in such instances; it will either have no effect or will make matters worse. The remedy therefore lies in trying to avoid or to minimise the dark background by moving the camcorder and/or reframing. The manual exposure system can also be used, this time closing down the iris to the required degree.

There are other circumstances where manual exposure (preferably locked) can be useful. For example I recently shot a scene where the only illumination was from a candle included within the picture area. The camcorder's CCD didn't like this and a thick white vertical streak, running from the top of the frame, through the flame and down to the bottom, was portrayed on the screen. The solution was to switch on an overhead 60 watt bulb and then, because this overlit the scene, use manual exposure to close the iris until the required atmosphere was achieved. The bar was also converted to an acceptable ray of light.

Finally, don't forget to disengage the manual or BLC Override immediately the requirement ceases, otherwise you will encounter unnecessary problems on other shots.