Exposure, Part III - Light meters and placing the world into zones

All camera metering systems are reflective metering systems as opposed to incident metering systems, which because of their limited use, I will not talk about here in detail. I always meter with the spot meter in my camera which figures out the exposure from the amount of light in the central 5% of the frame. If your camera doesn't have such a function, you can fake it by zooming in on various parts of the image before metering. This turns your fancy matrix metering or center weighted metering into what is effectively a spot meter.

Before you do anything. Change the metering mode to spot, at least center-weighted if possible. What follows will not work with "intelligent" metering modes such as matrix metering. More on why matrix metering makes poor photos later.

First, how does the light meter determine the exposure? The meter determines the exposure necessary to render the area that is metered into a middle gray [zone V] in the final image. For digital cameras outputting JPEGs directly, one could argue that the meter knows exactly what amount of light will give a middle gray in the image because the same camera firmware that determines the exposure processes the data from the sensor into the image. If you calibrate your monitor right, it would show up as middle grey too.

Whenever you meter off some part of the scene and expose the photo according to the exposure values given by the metering system for that part of the scene, we say that you place that part of the scene in zone V. Since each photo has only one exposure , the other parts of the scene that are brighter or darker automatically fall into the other zones [yes, I do HDR too and I add digital lighting but we are talking about JPEGs straight out of the camera for now]. Without post-processing, there is nothing you can do about it, if part A and part B of the scene differ by 3 zones, they will always differ by three zones no matter what zone you place part A in.

All is nice and good if you put a card with a value of middle gray in every shot, meter from that and then remove the card to shoot the picture. This essentially turns the reflective metering system to a incident metering system because you are controlling the reflectivity of the surface on which you meter. But this is not what we usually do and most of the time, it is simply not possible. Imagine draping a large square of middle gray cloth on a distant mountain and metering off that then packing it up just before the exposure.

All still is nice and good if you photograph scenes that have elements that are middle gray or average to a middle gray [the classic example being a Dalmatian], sadly, that's often not possible either. Consider my nine shots of a white wall in part IIb. The meter reading places it in zone V so it turns out to be a perfect shade of middle gray, smack in the middle of pure white and pure black [joys of the linearity of digital images]. The meter is doing a wonderful job, but it has one critical flaw, it has no way of knowing that I want my white wall to be white. If I want the wall to be white, it had better be in zone VIII [or more], so you will have to increase the exposure by 3 stops based on the meter reading. In M mode, you would dial in your required exposure and in shutter/aperture priority modes you would set the +/-EV to +3EV to get the same results. In that way, you have just placed the wall in zone VIII.

Photos rarely have only 1 zone. Those that do are probably not very interesting either. When it comes to the real world, scenes will most often have more than 9 zones of dynamic range. Even with the much greater dynamic range of our eyes, we still cannot encompass the full range of light we encounter. The most common example is that a flash light in a dark room makes you loose you night vision, everything around the flashlight goes black, you are out of dynamic range. Our brains are hard wired to do everything to attenuate the brightest spot in the scene to our comfort level, be it grabbing sunglasses [use an ND filter when f/22 is still too bright], squinting, pupil contraction [decrease the aperture] or simply a matter of shifting perception [think of your eyes changing ISO]. Better safe than sorry. Working with the much smaller dynamic range of the camera, the photographer will need to decide what goes black and what goes white, to sacrifice some detail so as to preserve the moment. That, is the art of the exposure.