Taking a hand-held meter reading

Incident reading

For an incident reading it is important to place the white plastic dome supplied with the meter over the light-sensitive cell. This is called an ‘invercone’. The purpose of the invercone is to diffuse the light falling on the subject from a wide angle of acceptance (180°) and transmit 18% of that light. The sensitivity or ISO of the digital image sensor must then be calibrated into the meter. The light meter is generally taken to the subject and the light-sensitive cell is directed back towards the camera. The reading may default to EV (exposure value) which can be interpreted or changed to an aperture/shutter speed combination. On modern digital meters the photographer is able to pre-select a particular shutter speed or aperture and have the meter indicate the corresponding value to obtain correct exposure.

Creating an incident reading using the camera’s TTL meter

You can place a diffuser such as the ‘expodisc’ over a camera lens in order to create an incident light meter reading using the camera’s own TTL meter. This type of product is also excellent at creating a custom white balance setting or can be used to capture a reference image that can be used to create a white balance setting in Adobe Camera Raw.

Activity 4

Take an incident light reading of a subject in a constant light source. Note the f-stop at an exposure time of 1/8 second. Increase the number of the aperture by three f-stops. Note the change in exposure time. What would the result be if the duration of time had been increased by a factor of three instead of the aperture?

For a reflected reading it is important to remove the invercone. The meter’s cell has an angle of acceptance equal to a standard lens (with a spot meter attachment the angle can be reduced to five degrees or less for precise measurements). The ISO setting is calibrated into the meter. The meter is pointed towards the subject. The exposure the meter recommends is an average of the reflected light from the light and dark tones present. When light and dark tones are of equal distribution within the field of view this average reading is suitable for exposure. It must be remembered the meter measures only the level of light. It does not distinguish between dark or light tones. If a reading is taken from a single tone the light meter will always indicate an exposure suitable to render this tone as a mid-tone. If the subject is wearing a gray flannel suit a reflected reading from the camera would give an average for correct exposure. However, if the subject is wearing a white shirt and black jeans a reflected reading of the shirtwould give an exposure that would make the shirt appear gray and a reflected reading of the jeans would also make them appear gray. When light or dark tones dominate the photographer must increase or decrease exposure accordingly.

General reflected reading If the reading is taken from the camera position a general reading results. This general reading is an average between the reflected light from the light and dark tones present. When light and dark tones are of equal distribution within the frame this average light reading is suitable for exposing the subject. When light or dark tones predominate in the image area they overly influence the meter reading and the photographer must increase or decrease exposure accordingly.

Specific reflected reading A more accurate reading can be taken when light or dark tones dominate the scene by moving the light meter closer to a mid-tone. This avoids the meter being overly influenced by the shadow and highlight tones. Care must be taken not to cast your own shadow or that of the meter when taking a reading from a close surface. If the photographer is unable to approach the subject being photographed, it is possible to take a meter reading from a tone close to the camera. A useful technique is to take a reading from a gray card angled to reflect the same light as the subject or of Caucasian skin (approximately one stop brighter than a mid-tone).

Charanjeet Wadhawa

There are many ways of understanding the information a light meter is giving in relation to exposure. The meter read-out system itself can be confusing. Some photographers refer to EV (exposure value) readings, others t-stops (transmission) and others in zones. In reality they all mean the same. Of all the variations the most common usage is f-stops. All meters usually default to f-stops and all camera lens apertures are calibrated in f-stops. It is important to understand if the exposure is increased by one stop, either by time or aperture, the amount of light entering the camera has doubled (2x). If increased by two stops the amount of light has doubled again (4x). If increased by three stops the light doubles again (8x) and so on. This simple law applies with the opposite result to decrease in exposure. It is also important to set the meter to the chosen ISO rating (measure of sensitivity to light).

Average exposure If the lighting is even most image sensors are able to record a full range of tones. Where the tones are evenly distributed the most appropriate exposure is often the average indicated by the light meter. When light or dark tones dominate, however, underexposure or overexposure may occur as the average exposure is no longer appropriate. It is essential to understand how the light meter reads light to have full control over exposure.

Itti Karuson

Activity 5

Using a diffuse light source (cloudy sky) take individual reflected light meter readings of three pieces of card, one white, one black and one mid-gray. Adjust the card to avoid specular reflections (the card should not appear shiny). The black card should give a reading different by four stops to the reading off the white card. The mid-gray card should be between the two. If the mid-gray card is two stops apart from each, you have a mid-tone the meter sees as the average tone (18% gray). Make an exposure of each of the three cards using the reflected MIE of each card. Photograph the white and black cards again using the MIE of the gray card. Label the results with the MIE, the actual exposure and the tone of the card.