So my last HDR tutorial was on dynamic range and the application of Ansel Adams ‘and Fred Archer’s zone system to understand exposure. I discussed how a perfect exposure (0) was in zone 5, the center of the zone system.
But what does this have to do with taking photographs you might ask? Simple. Each step in the zone system equates to one stop up or down the scale. So for instance you have the following scenario.
You have the scene before you perfectly exposed. And you have an object that is in zone 7. And you decide that you want to properly expose for the object. You know that the object is in zone 7, or overexposed by 2 stops. So for the object to be in zone 5, you would need to underexpose the base exposure by two stops.
Now, you are probably not going to do something like this very often. This process is called exposure compensation, and it is related to the exposure compensation dial on your camera (another discussion for another time). This is commonly done in winter scenes, where you need to over expose the snow by around two stops to get snow that is more white than gray.
But if you focus your exposure meter on the scene in general, and configure your exposure settings (ISO, F-Stop and Shutter speed) to reach proper exposure, it will be 0 on your camera’s meter, which will match zone 5 in the zone system.
You can look at your scene and judge what parts of the scene are in what zones. And if you are really curious, you can point your exposure meter to any part of the scene and determine what the differences are in exposure.
Take for instance a sunset photographic opportunity. You decide that the subject is a cacti in the foreground (I do live in the Sonoran Desert!) and you expose for your subject.
But looking at the setting sun and bright clouds, you quickly discover that your scene has both bright areas as well as areas in deep shadow. You may find out that your scene has areas in zone 0 or 1, and areas in zone 9 or 10.
But your camera’s meter only will show you up to either -3 to +3 if you have a Nikon, and -2 to +2 if you have a Canon.
If you have properly exposed for your subject – what we call base exposure, you can’t find out what the exposure for those parts of your scene is in zones 9 or 10 based on the exposure of your subject without doing some math in your head as you begin to calculate how many stops difference there is between your subject and the bright clouds in the sky near sunset.
This is where you have to learn the reciprocals of the famed exposure triangle, i.e., know that f/1.4 to f/2 is one stop, or that going from 1/60 sec to 1/120 sec is one stop, etc. This is helpful to know when you change your exposure settings that you will be changing your exposure one or more stops of light, and either going stops above base exposure or below base exposure.
Or you can just judge based on the zone system and know that the part of your sky that is bright or pure white is most likely in zone 10 and those that are deep black are likely in zone 0.
I know, this is really complicated! And you may be asking why do I need to know this stuff? The short answer is so that you know how many photographs you will need to take and what exposure settings to capture enough of the dynamic range to process to capture the dynamic range of the scene before you.
Zone System and Stops
So let me put this all together in such a way that it might make sense. Let’s say that I am taking a picture of this saguaro cacti facing the setting sun. And let’s say that my base exposure is f/16 at 1/8 sec at ISO 100. (NOTE: Remember, base exposure is always 18% gray tone, and so regardless of what the subject is and how much light is falling on it, the camera will always set your base exposure to 18% gray). And I am shooting with a Canon camera, so my light meter only shows me -2 to +2 range.
|My Initial Base Exposure||-5||-4||-3||-2||-1||0||+1||+2||+3||+4||+5|
So based on the above chart, my meter only shows me 5 of the zones – zones 3 through 7, and I can point my spot meter towards the dark deep shadows and my meter indicator blinks off past -2 showing me that the deep shadows are beyond what the meter can measure at my base exposure. So I know that if I decrease my shutter speed, my base exposure will begin to move closer to what is near zone 0 in my current base exposure.
So I go from 1/8 second shutter to half that, 1/4 second.
|My Initial Base Exposure||-4||-3||-2||-1||0||+1||+2||+3||+4||+5||+6|
Remember that my saguaro cacti was properly exposed with my shutter set at 1/8 second. By halving the speed, I have increase the amount of light by double, and now based on this exposure setting my cacti is over exposed by a full stop. Meaning that it is brighter, much brighter. But those parts of my scene that were in zones 0, 1 or 2 have gotten brighter also.
So if I double the amount of light coming into my camera, I over expose my base exposed subject, but I also was able to increase the exposure of those dark areas of my scene.
And if I was to go the other direction, I would begin to decrease my exposure of those parts of my scene in zones 9 and 10, making them darker. This is the basis of HDR photography, by adjusting your exposure settings you can create images that can cover the dynamic range of the scene.