Latitude
In order to master exposure, there are a few concepts that require a full understanding. One is the concept of a stop of light, which we’ve already covered. Another concept is that the medium you use, whether film or digital, doesn’t “see” the world the same way your eyes do. This is the reason your pictures come out differently from how you remember the scene.
If we go back to a stop of light, you should remember that it’s a doubling or halving of any given amount of light. To take that further, let’s relate it to a range of light levels. We know that one stop more of light is a doubling of the light. Two stops is another doubling. In other words, two more stops is four times the light. It follows, then, that three stops is eight times the light (2x2x2)
The human eye can typically discern a range of light from dark shadow to bright highlights of about 12-14 stops. That’s quite a range. Slide film, on the other hand, can “see” a range of only four to five stops of light, depending on the particular film. Print film and digital can “see” about 6-7 stops (though digital sensors are getting better and better every day).
We call this range of light, what the film or sensor will reliably record, “latitude”.
Many scenes in nature have a range of light well beyond the latitude of the sensor or film. So the obvious question is: If this is so, how can you successfully photography these scenes?
To begin to answer this question (which I will in future lessons) you first must understand the limitation of your film or digital camera. (Here I’m going to use film in generic way to represent whatever medium you’re using, be it slide film, print film, or digital)
In the days before digital, we taught that the best way to learn exposure was to use slide film. The reason is that processing slide film is done in an objective way, meaning that there is a set chemical solution, a set temperature, and a set time for development. This means, barring any mistakes in the processing, that what you shoot is what you get.
Using print film, the negative is first processed. This too is done objectively. Then the print is made, and this can be very subjective. Machine printers use a sort of “auto exposure” system to make the prints. This means that, even though you may have purposefully varied your exposure (what we call bracketing), all the prints could come out looking the same due to the machine auto exposing everything. You won’t learn anything from this.
With slide film, this can’t happen and you’ll easily see the results of your exposure choices (good and bad). That’s why, if you’re using a film camera, I highly recommend you use slide film to learn about exposure.
As I said earlier, slide film has a latitude range of about 4-5 stops. For the sake of this discussion, I’m going to use four stops. This means that on slide film, the range from black with some detail to white with some detail is only about four stops. That’s not a whole lot, and if you’re photographing a scene with a wider range than that, you’ll end up with many shadow areas going black and many highlight areas going white.
On slide film, when part of a scene is over exposed, it goes completely white; the film itself becomes completely transparent. There is no information, no detail to work with. This is usually a BAD THING (though not always).
The same concept holds true for digital. If the highlights are over exposed, what we call “burned out”, the sensor records them as pure white. This means there is no detail. There are no magic Photoshop tricks to fix this other than painstakingly reconstructing the area using various mystical, arcane, and hours-devouring cloning techniques. It’s much easier to get it right in the first place.
Fortunately, for digital shooters, you can see any major exposure problems, especially burnt out highlights, while you’re still in the field. Learn to use and interpret your camera’s Histogram. You’ll need to pull out your manual again for this.
Also, digital shooters have the advantage of greater latitude. You still need to avoid blowing out the highlights, but there are about two or so extra stops to use in the shadow areas. In other words, you have a usable latitude range of +2 to -4 stops, maybe even more depending on your Photoshop skills and how much you paid for your camera. In addition, when you get a little experience under your belt, you can use multiple digital captures at different exposures, combining them later in Photoshop. This virtually eliminates the problem of latitude.
The BIG CONCEPT to understand here is that what you see and what you can successfully photograph are not necessarily the same. Learn the limitation of your film or sensor and learn to see like they “see”. This is the real first step in mastering exposure. Once you understand this concept of latitude, you’ll be able to use some of the tools available to mitigate it. (Things we’ll be covering in the future) More importantly, you’ll begin to learn when a scene is simply not worth photographing.