Many of you will be solely relying on the in camera metering system, whatever flavor that might be, to provide you with exposure information. Some Photographers will be shooting Program Mode where the camera makes all the decisions and as sophisticated as DSLR’s and Mirror-less cameras are today you will obtain pretty good results most of the time. Then there will be other Photographers concerned with specific aspects of the image and how their exposure decision impacts that:
Back in the days of shooting film a Photographer had a little more freedom with color negative film than with Chromes (Slide). But film has a different graph for light sensitivity with a definite heel and toe (as it has been called) at the brightest and darkest sensitivity levels. That gave film a little edge on handling images with greater dynamic ranges or differences between the brightest spot in the image to the darkest. In the digital realm our cameras can only handle about 9 Exposure Values of Dynamic Range before real problems start to manifest for the photographer who is striving to achieve their vision for the picture. Improper exposure can ruin a perfectly composed image with lots of emotion to convey to the viewer. On the other hand, a technically perfect picture with no message leaves the viewer flat, but that is a topic for another time!
In order for us to obtain a “Optimal Exposure”, which probably is not the “Correct Exposure” that our camera’s metering systems calculated through complex algorithms and in camera software comparing the metered scene to scenes stored in camera by the manufacture and through the magic of computers decides which one is the closest and selects it to give us the best chance at a good exposure.
Our camera’s utilize reflected light that is read by the sensor to determine the correct exposure combination of Aperture and Shutter Speed. So if we is take a picture of a snow man in the snow and do not make exposure corrections (+1 – 1.5) the snow will be gray. Or if we are photographing a dark subject on a dark background the image will be gray, unless of course we made an exposure compensation of minus 1 to 1.5. Why? Reflected light meters want to make everything 18% Gray or in the middle of the Zone System’s 10 levels of black to pure white (you can read up on Ansel Adams for a better understanding of the Zone System). Our cameras have a “Histogram” that shows a graph with all of the exposure data for a range of 255 – Black With No Detail on the left all the way to Zero or White with no detail on the right. The higher the peaks in the graph the more digital data is in that area for Red, Green and Blue combined.
There is a school of thought called “Expose To The Right” or ETTR. To accomplish this we need to increase the Shutter Speed, Aperture or ISO (Increasing the ISO is theoretically increasing our camera sensors sensitivity to light. It is a lot more complex than that and something I will not delve into today.) What ETTR advocates is moving the Histogram as close to the right as possible without exceeding the right boundary. Once we exceed the right side of the Histogram we experience “Clipping” or there are highlights in the image that will reproduce as pure white with no detail. We have “Clipped the Highlights”. We can also under expose so drastically there is no detail left in the shadows and they will be pure black.
If we used a light meter that measured the light falling on the subject we would obtain an exposure that would render the mid tones of the image as the 18% gray and the highlights and shadows would fall where they are supposed to. Or that snowman would come out white. We can simulate this by using an 18% gray card and spot metering it with our camera or filling the frame with it to obtain a exposure reading.
We could all gain a world of knowledge and good information if we would use an 18% Gray Card and begin to understand how our camera’s metering system functions. After all, we want to best or “Optimal Exposure”!
Today, our digital cameras