This small ‘learning’ stems from a query sent in by SP reader Amol K. from Amravati. Amol wrote, “A friend of mine, who is a bird photographer, prefers to use an Incident Light Meter.
He says that he gets better exposures with it. Can you explain if that is correct, and if so, how does an Incident Light Meter provide better exposures?”
Let me put it this way; an Incident Light Meter does not necessarily provide “better exposures”, but, in certain cases, makes it easier to determine the correct exposure.
To understand why that is so, let us go back to basics. What is the main reason for camera metered exposures to go wrong? In my opinion, one of the main reason is the brightness (or darkness) of the background. If the background is too bright, the subject in the foreground is ‘underexposed’. If the background is too dark, the subject is ‘overexposed’. This of course assumes that the main subject is occupying a very small space in the frame (as it generally happens), and that the subject is mid-tone.
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So, supposing we could have an exposure meter that just does not read the background, wouldn’t that solve our problem? The answer is ‘yes’. This brings us to the Incident Light Meter! To use an Incident Light Meter, you point the half-white plastic dome (the hemisphere) to the source of the light (and not at the subject), or at the camera. The meter then reads the light falling on the subject. Now observe the meter’s design. By virtue of its design, it just does not allow the background illumination to be metered. Since the background illumination cannot be metered, it cannot provide you with a wrong meter reading! As simple as that.
 Since the subject occupies a small area of the frame, the camera meter is reading more of the background. |
 The Incident Light Meter is pointed towards the light source/camera
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Now let’s talk about bird photography. With a bird in flight, the background (the sky) is always very bright (even when the sky is blue!). If you were to use a in-built camera meter (which is always a Reflected Light Meter), the meter would ‘read’ the sky and underexpose the bird. The Incident Light Meter, as explained above, does not ‘read’ the sky; it can only read the light falling on the bird and hence will correctly expose the bird.
Does this explain why most professional bird photographers use an Incident Light Meter?
Incident Light Meters add to our cost and hence many photographers do not use it. But in case you are interested, just know that a Reflected Light Meter reading (camera meter reading) using a 18 percent Kodak Grey Card would be identical to a reading using the Incident Light Meter.
The camera meter reads more of the background rather than the subject. If the background is not a mid-tone (is too light or too dark), the exposure will likely be wrong.
The Incident Light Meter is pointed towards the light source/camera, and hence is unaffected by the brightness/darkness of the background. This results in ‘better exposures’.
Since the Incident Light Meter is pointed at the source of the light (or the camera), how do we meter for, let’s say, a sunset, or for top-lighting, or for side-lighting?
Sunset/Sunrise
If you point the hemisphere to a sunset/sunrise, and use the indicated reading, the result would be an underexposure (in this case, the source of light is itself the subject). The solution is to override the meter’s recommendation by + 2 to 2.5 stops.
Top-lighting (as in a studio setup)
Point the hemisphere to the light source. This will ensure that the highlights are not overexposed (something very important to all digital photographers).
Side-lighting
Here too, you can point the meter to the light source and use that reading. This will ensure that highlights are not overexposed. Some photographers point the meter half-way through the light source and the camera. This enables them to have a bit more of shadow detail, without overexposing the highlights (due to the exposure latitude).
 Side lighting and meter pointed to the light source. |
 Side lighting and the meter pointing between the source and the camera. |
Is an Incident Light Meter a must?
As we have seen earlier, a camera-meter reading off a 18 percent grey card delivers the same exposure that an Incident Light Meter provides. Hence, in that light, the Incident Light Meter is not a must, but is a convenient way to measure exposures.
Another important feature of the Incident Light Meter is that it can also read the light from a flashgun. Camera meters cannot do that. So if you do a lot of flash work, you may be better off possessing an Incident Light Meter.
Rohinton Mehta
