Flash Meters

Posted onDavidbyJanuary 22nd, 2009 with Comments Off

One of the most used tools I use in my photography, other than my camera, is my light meter. There has been a lot of debate about the need for an incident meter in the digital era with the presence of the histogram in most DSLR cameras on the market. I don’t believe that everyone needs one, nor do I believe that one is needed for all occasions. However, I do believe that if you really want to improve your images, especially if you use strobes, you will benefit greatly from the an incident flash meter.

Meter vs Histogram. A histogram is the graph that shows up on your LCD display on your camera (if you have it set up to do so) or in most image editing applications. It’s purpose is to tell you where along the exposure line your image information is at. For instance, if you have more peaks at the left end of the graph you have a lot of dark area in the image. If you have more peaks at the right end you have a lot of bright area in the image. The problem with using this as your only tool for judging exposure is that it doesn’t tell you where the peaks should be, only where they are.

The subject of this photo is black. It should be rendered as black, not gray. The histogram for this shot shows almost no information at the right end of the graph and a lot at the left end. If I were to adjust the exposure to get the graph to move to the right I would end up rendering the subject as gray, not black. Basically, in this case, the histogram is not giving me much in the way of useful information. It might tell me if I have rendered the subject too black and have lost any detail, but not much else.

The incident flash meter measured the light hitting the subject, not the light bouncing back off of it. Because of this it was not fooled by the black subject and gave me a reading that was dead on.

Using the exact same setting I took a photo of a white subject under the same light and conditions. It is rendered as white, but with detail, which is what I want. The meter was not fooled by the white subject. In fact I didn’t even have to take a new reading, although I did just for kicks. Since I was measuring only the light it didn’t matter what subject was placed there.

In the case of the white subject the histogram looked very different. There is a black background so there was still a fair amount of information on the left end of the graph, but this time there was also a fair amount of information at the right end. Because the exact same lighting and other conditions were used and the same settings on the camera were used both images came out with perfect exposure, yet the histogram looked very different.

These are not real world photos.  They were used to illustrate a point.  There is a real world parallel, however.  In wedding photography you typically have a bride in white and a groom in black.  If you use the in camera meter and the histogram to set exposure for shots of the bride and groom separately you will get very different exposure settings and the skin tones will not match.  Doing weddings was the first place I really started appreciating my meter.

I do find the histogram very useful and I have my camera set to automatically show the histogram with the instant preview on the LCD screen. However, I use it in conjunction with the flash meter, not instead of it.

I hope this has been helpful to people and shows just what a meter does and what a histogram does. Used together they are powerful tools indeed.  In fact, using them together may just warrant it’s own post.