When you go to take an images with your camera it uses an internal light meter to determine the exposure settings that are best for the scene.  Have you ever wondered where those numbers come from or how the light meter is determining them? The answers will affect how you shoot and what exposure you ultimately want to use.

There are 2 types of light meters: reflective and incident.  Reflective light meters read the light being reflected off of the subject while incident light meters read the light falling on the dome of the meter, usually held at the subject.

Camera with a built in reflective meter and a hand held incident meter.

All in-camera meters are reflective light meters as well as spot meters (we’ll discuss spot meters in a future post.)  When they measure the reflected light they calculate the exposure assuming the subject is 18% grey, sometimes referred to as neutral grey.  Green grass on a sunny day is an example of an element that would be 18% grey for a reflective meter. Meaning it will be correctly exposed by a reflective meter.  They will calculate everything to be grey, even whites and blacks which means that when your primary subject is white you are going to want to over expose the cameras reading.  Conversely if the subject is black you will want to under expose.

Knowing what the exposure is for 18% of any subject has value.  By using this exposure and knowledge of the zone system you can figure out the exposure for any element in the scene.  We will be covering what the zone system is and how to use it in a future article.

Sekonic incident light meter.

Incident meters like the one pictured above measure the amount of light hitting the dome. When you take your reading you want to hold the meter with the dome in front of the subject with the dome facing towards the camera.  This way you are reading the light the camera will be recording. If you point the dome straight up, for example, it will give you a different reading, causing your exposure to be off.

When using an incident light meter you are trying to find the correct exposure settings for your ISO, shutter speed, and aperture. So, you’ll pick 2 of those three and the meter will give you the value for the third.  Additionally you always pick the ISO and then either of the other two. So for example I set the meter for 200 ISO and F5.6. I take the reading and it will give me the shutter speed I need to complete my exposure.

When using an incident light meter to set you exposure you will want to have your camera set on manual.  Then input your exposure values into your camera from the light meter. If you do not have the camera in manual then the in camera meter will continue to make exposure change using the built in reflective meter.  Take meter readings often when shooting this way because exposure values are always changing.

There are several different companies making incident meters that work with smartphones.  They can either be attached directly to the phone or via bluetooth. It is a nice way to leverage the computing power of your phone to better your photography.  I particularly like the bluetooth versions because I can leave them out while shooting and monitor any changing lighting conditions from my shooting position. The one I am currently using from Illuminati Instruments also has a color meter built in which is a nice bonus.

Illuminati Bluetooth light and color meter.

So, when would you want to use an incident meter over your in-camera reflective meter?  When you have a subject that is either very white or very black you can use an incident meter to get a base reading for your camera settings before you get started.  When Shooting in studio environments I will use the incident light meter to get starting point exposure as well. When shooting macro images I sometimes use the mini receptor to get readings deep down in the flower to make sure I am still getting detail in that part of the image or if I’m shooting black insects.  I will also take it on location scoots so I know what my light levels are like before the day of the shoot. Another use for them is to verify that your in-camera meters are working correctly. These in-camera meters are very good these days but it doesn’t hurt to check from time-to-time that they are still working as you expect.

For the macro world there are not very many options available currently. Sekonic used to make a mini receptor, pictured below.  This little sphere will plug into two of the older Sekonic light meters, the L-358 and L-508. Then you have an incidental light reading for your macro photos. This is a part of the reason I still use the L-358.  These do come up on the used market now and again so keep your eye out as they are great for getting light readings in places like the inside of a flower.

Sekonic L-358 with mini receptor attached.

Figuring out the exposure you use is a creative choice at the end of the day.  Understanding light metering is just one more tool that help guide you in creating the images you have envisioned.  Do you use an incident light meter? When and why do you user your light meter over the one in your camera? Share your experiences in the comments below.