I have heard the phrase “I don’t need no stinking light meter” more times than I can count, your LCD display and histogram are all you need for a perfect exposure right? Would you be surprised if I told you that your camera was lying to you? We first need to know why our LCD and histogram is wrong before we can believe that using a light meter will be of benefit.

That Cheatin’ Histogram

Let’s look at what happens when we take a picture with our DSLRs. The exposure meter in the camera is using reflective light instead of incident light so a man’s tux will show a different meter reading than a bride’s dress even if the exposure for both of them was correct since the white dress will reflect more light than the tux, thus throwing the exposure meter out of whack. Secondly, when we press the shutter the camera creates a JPEG image with all of your picture settings and an S-Curve applied. Yes, this happens even if you are shooting in RAW mode since the display on the LCD is actually the JPEG image which is also the image the camera uses to calculate the histogram. I can’t count the times that my exposure meter was dead center, the preview looked great, and the histogram was good but when I brought up the RAW image in Lightroom, the image wasn’t quite exposed as well as I expected. Anyone who shoots a lot can attest to this issue. Yes, your LCD and histogram are usually accurate enough, but if you want to be accurate all the time then you need a reading from the point of few of the subject by measuring the actual amount of light hitting the subject and not the amount of light reflected by the subject.

What does a light meter tell you?

I asked a handful of photographers who had never used a light meter what they thought a light meter would tell them and surprisingly few actually knew even the basic functions of all light meters did with “Measure the light I guess” being the top answer. Even the most basic of light meters will allow you to set the ISO speed you want to use, then you set the shutter speed or aperture to what you want to use and the meter will give you the correct settings for a proper exposure. More fancy ones can also fire your flash equipment and some will even calculate the amount of flash versus ambient light.

Let’s say we have a nice bright sunny day and we set our ISO to 200, and our aperture to f/16, if we take a reading we will probably get something like 1/200th of a second as our shutter speed for a good exposure. With most digital light meters you can then make adjustments to the shutter or aperture and it will change the values on the screen to tell you what to change the other setting to.

If you aren’t familiar with the relationship between ISO, Shutter Speed, and Aperture, you should read the article we have about it (Link: The Exposure Triangle).

Testing the theory

Ok, I am sure many of you are skeptical that your camera, even a high end pro-series camera isn’t going to be dead-nuts accurate, but let’s take a few tests and see what our camera is telling us.

The following is a series of images of a product that has a black front and a light top, not something a camera’s meter is going to be good at so it will help illustrate the point well. Simply adjusting the camera’s settings to center the camera’s exposure meter at ISO 200, f/5.6 the camera gave us a setting of 1/400th shutter speed. Shooting the gray side of a WhiBal card it changed to 1/400th of a second, and shooting the gray/white/black side we got a setting of 1/640th of a second. By doing this test we can see that the camera’s exposure meter is going to change based on how reflective the subject is. In each image you can also see Photoshop’s histogram to see where the image’s exposure actually came in at.

Image 1 – ISO 200 f/5.6 1/400th – Slightly Underexposed (Based on WhiBal Gray)

Image 2 – ISO 200 f/5.6 1/640th – Underexposed (based on WhiBal White/Black/Gray)

Image 3 – ISO 200 f/5.6 1/200th – Showing good exposure (based on light meter)

Now some people may say that a perfect exposure should be centered in the histogram, however, digital sensors retain more detail in the highlights than they do in the shadows so you actually want your highlights to be fairly far to the right without going into the last little bit of the histogram which is exactly what we got with the final image.

But how does the meter know what the background is?

One of the most commonly asked questions is how does the meter know what the background of the subject is, since that should affect the exposure right? Well, let’s just try some experiments and see what we get. The following images were taken using the exposure settings provided by the light meter which were identical for both images, take a close look at the histogram for each image.

ISO 400 f/4.0 1/125th – Properly Exposed yet histogram says its underexposed

ISO 400 f/4.0 1/125th – Properly Exposed yet histogram says its overexposed

Of course the white paper is providing some bounce light that is providing more details around the edges and since I only metered for the center of center of the phone. So yes, the background can affect the exposure because you may not be metering for any reflected light, although this is typically only a problem with a situation like this. If I wanted to expose for the edges, then I could have metered at the edges. Does this mean the background affected the exposure? Yes, but only because the background in this situation actually is acting as a light source that is reflecting light into the edges around the product.

Will a light meter work properly in every situation?

So far we have looked at using a light meter for incident lighting, where this can break down is with highly reflective surfaces or in situation where the lighting is uneven, in this case a reflective meter is going to be more accurate. Most all light meters can do both types by adjusting or moving the light sensor dome. In reflective mode, the meter is going to act just like the meter on the camera.

Like anything else, a light meter is just another tool that is designed for a specific purpose. Used properly, it can dramatically improve your exposures and you camera setup time. The following image was taken in bright daylight with a large diffuser to the camera right and a large beauty dish flash to camera left. Combined with the ambient light, that is three different light values. With the light meter getting a reading taken at the subjects face, and the meter facing the camera, the settings provided were certainly dead on.

Model: Taylor Thorne

Ok, I want one, what do I do?

Light meters, especially used ones can be had for pretty reasonable prices. I see meters like the Sekonic L-358, which is a pretty nice meter with lots of functionality, often sell for $150-$200 on Craigslist. Cheaper models with less features, but still fully capable can be had for as little as $50-$60.

Model: Taylor Thorne

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