We are frequently asked on our LIoP photography courses how to use an external light meter. Is it really necessary since we have built-in light meters in our cameras?
Here is the low down:
The Light meter is a very useful tool, especially if you are working in the studio or with film cameras that don’t have a built-in light meter.
There are two basic types of Light metering: Reflective and Incident
Reflective Light metering is measuring the intensity of the bounced light from the surface of a subject or a reflection of a scene. This is how our cameras measure the light.
Incident Light metering is more commonly used with an external light meter. It is an accurate reading and measures the amount of light that is falling onto the subject, from the subject’s stance. It is not the bounced or reflected light. With incident readings the subject’s tonality, colour and contrasts all stay consistent regardless of the background, reflectance, brightness or textures.
If you are working in the studio and using strobe lights you cannot measure the amount of light with your camera as the lights fire at a 1/125th of a second. In the studio, you ignore your camera’s built-in light meter. For example, if you are taking a portrait on a white background, you will need to overexpose the background by two stops to ensure it is white. So, if you decide to use ISO 100, Shutter speed 1/125th and then an aperture of F.8 for the portrait, your background will need to 2 stops difference at F16. How do you measure this? The answer is with an external light meter.
To use an external light meter you input your ISO and Shutter speed – Remember, the fastest sync speed on studio lights that you can use, is around 1/125th to 1/200th of a second. You can use a faster shutter speed with speed lights and some newer cameras. You are trying to match the F-number on the external light meter as per the desired F number. So, going back to our example, we would be measuring the light from the background at F16 and for the Subject at F8.
On even the basic but great light meter models you can measure daylight, flash and flash with sync cable.
To get the best readings from our camera in high contrast scenes we can use spot metering, which is only measuring a small percentage of the reflected scene. This means that on a snowy day, for example- we can measure the amount of light bouncing off our subject only and our snow will get overexposure. So, why not use our camera’s spot metering instead of external light meter? This is because of the spot metering angle. When a lens is changed the spot meter angle changes too. It can be 15 degrees or more. An external light meter can have a 1-degree spot metering, which allows the most accurate readings in complex scenarios.
To sum it up: It is important to get your hands on an external light meter and understand how it works with both daylight and flash. You can then really show off and start mixing both sources of light. Metering modes in your camera are complex but once you understand them your photography will only improve.