For the digital photographer, there are 3 ways to change the tonal values of an image, namely shutter speed, aperture size and ISO. With that comes the concept of stops. A change in one stop is any method that will increase or decrease the amount of light getting to the sensor by a factor of two. If you change the shutter speed from 1/100 to 1/50, then the shutter is open twice as long and you have increased your exposure by one stop. Similarly if you change your aperture value from f/5.6 to f/4, you have doubled the area of the aperture [note that the aperture values form a geometric progression of sqrt 2 because they are an indication of the diameter of the aperture which is related to the area of the aperture by an exponent of 2] and hence twice as much light will pass through your lens and so you have increased your exposure by a stop. Technically, doubling the ISO value does not result in more light coming through the camera nor does it make an area twice as bright, but the ISO curves of digital camera are linear enough over the dynamic range that for our intents and purposes, I will stretch the definition a bit and claim that if you increase the ISO value by a factor of 2 the image will be twice as bright.
Now back to zones. The difference between two consecutive zones is one stop. This is how I got from pure black to pure white with my camera. The subject was a white wall with some texture. Start from zone V which is whatever the automatic metering system says is the "correct exposure", which was 1/40, f/5.6 at ISO 100 for this shot. Get the zone Iv image by exposing at one stop lower, i.e. 1/80, f/5.6 at ISO 100. The other images were generated similarly.
The single most important thing that you need to know from this article is that it does not matter what the zone V exposure is. I can generate the same set of images if I used the overcast sky as my "white wall". In that case zone V might be 1/160, f/22 at ISO 100. The I would simply get the zone IV image by changing the shutter speed to 1/320s. In an ideal world, the grey in the zone V shot of the wall would be the same grey as the zone V shot of the sky. If you already see where I am getting at, then perhaps the rest of the articles on the zone system will seem trivial. If you are still puzzled, maybe I am a bad writer or maybe you have to read on.
Theoretically, you can vary the ISO and keep the shutter speed and the aperture constant, or vary the aperture and keep the other 2 parameters constant to get this set of shots but varying the shutter speed gives you much more range because while there are a total of 6 stops of ISO and aperture [ISO100 - 3200 and f/4.0 to f/22], the shutter speed goes from 30s to 1/8000s covering almost 19 stops. Note that I could not get zones 0 and X. I and IX were pure white and pure black devoid of detail to my eyes when I look at them on my screen. Don't use the color selection tool in Photoshop to test whether your whites are actually white because what we are concerned is how the images look on your screen. It doesn't matter if the white image actually has detail, if you can't see it on your screen, it's pure white; unless of course, you look at your images by rolling a colour selection tool over it instead of looking at it.
So I find out my Canon 30D together with my crummy monitor can generate images of a dynamic range of 9 zones. Not too bad when compared to a b&w print after all [of course we are not talking about the contrast between zone I and zone IX which is much higher on a gloss print than on an LCD monitor].
Now that we know what zones are and how many zones my [and your, if you do the test] system gives me [you], we can move on to talking about how we actually use the zones.
No comments:
Post a Comment