The Difference Between In-Camera and Handheld Light Metering
In-Camera: We recently received an email from a student who was trying to understand the difference between light and light metering in the hand.
To begin with, it is important to understand that there are two different ways of measuring light to get a properly exposed image. You can measure the reflected light of the subject, how does the camera light meter work or can you measure the light falling on the subject (which we also call the measurement of incident light) with a portable light meter in the position of subject.
An important factor to understand is that the meters are calibrated to measure a neutral gray tone. A light meter thought “supposes” that the subject reflects the same amount of light that reflects the gray card. However, if the subject is bright and bright white or black with a matt non-reflective finish, the counter give a false reading that the photographer will then be correct.
For simplicity, the following examples do not mention opening or closing the aperture, but similar results can be obtained by changing ISO and / or by selecting a different shutter speed.
With the light meter reflected from the camera, if the subject is bright white, the photographer must select an aperture of approximately 2 stops larger than the counter indicates. This is because the camera “thinks” it looks a neutral gray, it is not bright white, then it will indicate an aperture that exposes the scene as gray, not white. Imagine photographing a snowy winter scene in the sunlight or the white sand dunes in the Sahara. If the camera gauge indicates f / 16 as the correct aperture, you may need to open the f / 8 aperture for snow is whiter than gray. However, if you use a portable incident light indicator, you need an accurate reading of the sunlight that illuminates the scene, and you will need no adjustment. The incident light meter should read f / 8 in the same situation where the meter on the camera may indicate f / 16. This is because it reads direct sunlight, not light reflected by the object.
If you take a darker neutral gray object than – say, a black bear in the dark forest, deeply shaded – camera meter can give a reading of that subject surexposera. This is because the black bears reflect less light than a gray card, but the camera still think the scene is a neutral gray tone. So if the camera meter indicates f / 2.8, it may be necessary to select an aperture of f / 5.6 to obtain a correctly exposed image. Again, a portable camera should give you an accurate reading of f / 5.6, as it does not care about the tonal value or reflectivity of the subject, just the value of the subject’s light illumination.
The same principles are true with the portrait. Blond woman in fur and wearing a white wedding dress put in the sunlight with a bright white wall behind is likely to be incorrectly measured (underexposed) by the light meter reflected from the camera while a dark brown color To the skin in a black tuxedo raised against a black curtain probably measured (overexposed) by the camera. The photographer must understand this and increase or decrease the exposure accordingly if based on the camera meter or use an incident light indicator for more accurate results.
The measurement in the camera can be complicated depending on the measurement mode selected by the photographer. Spot metering takes a reading of a small “dot” in the frame. Average metering modes, including center-weighted average and Matrix (Nikon) or evaluative (Canon), will measure light from the full range and reach dark and bright areas. Lesson text suggests measure 2.3 in time of the subject’s cheek, just below the eye, so that the image of the best skin tones is exposed.
One more note – a meter normally reads the “constant” light room. Do you need a meter with metering capability to measure the instantaneous flash burst of light from a flash or flash.