The Myth About The Perfect Exposure
Exposure is the length of time the film or sensor is exposed to the light. When talking about a photograph, we're specifically talking about how well it was exposed. In a non-technical discussion, you might hear: “You really exposed the film for just the right amount of time for that photo.” Okay, we never hear that, 'cause that would be silly. Instead you'd really hear someone say: “Perfect Exposure. Good job.” Sadly, this leads many beginning photographers to believe that there is only one perfect exposure and they are being commended for finding it. Well, I hate to be the bearer of bad news, but it's all a myth – a misconception of the truth. But I'm here to set the record straight.
There are dozens of ways to achieve a perfect exposure. So many, in fact, that I prefer not to refer to it as a perfect exposure. I call them proper exposures as the term does not implicate one specific exposure. The characteristics of a proper exposure is quite simple: All necessary detail is present within the shot, especially in darker and brighter areas. Be aware that not all details are necessary for the shot.
I'd like to take a brief aside here to discuss a little technical aspect of exposure. Assuming the light source is the same, the exposure for a given shot is controlled using the three corners of the exposure triangle: ISO, Aperture and Shutter Speed. It's rare that you would change the ISO setting, so lets assume it's fixed. The other two corners are therefore directly proportional. If the aperture size decreases, the shutter must remain open longer to expose the subject properly, and vice-versa. Each proportional combination – an exposure recipe if you will – of aperture and shutter speed settings will result in a properly exposed subject. And that is the root of our discussion.
Therefore, a photographer could, in theory, choose to use any number of those exposure recipes, and all would be technically correct. But the creation of a photograph never ends with a technicality. The aesthetic nature of the photograph depends on far more. So the photographer must look at the other aspects of a photo that are directly related to each exposure recipe: The quality of light, the depth of field, motion blur and so on. The photographer shall have a specific photo in mind and be able to eliminate the recipes that are incapable of producing that final image. Recipes can be eliminated on the basis that there's not enough depth of field, or the light would fall off too quickly and so on.
Take a look at the three photos I have shown here. For straight out-of-the-camera comparision, I have not made any post-processing corrections to these photos except to convert them to black & white (and I didn't use any of my tricks either). I'm working from a tripod shooting a still life and so the length of my exposure will not be a concern. I am also working from an external flash using TTL , so you'll note that the shutter speed doesn't change either. In this case, the flash is actually filling in light automatically to compensate for the aperture. For all intents and purposes, the three shots all have a proper exposure. The depth of field changes are quite apparent, the church is a blurry blob at f/1.4 but quite well defined at f/22. On the other hand, you'll note that the light cuts off more sharply at f/22. The flash is ramping up the power to compensate for the smaller aperture, and so the ambient light is getting overpowered. The unfortunate side-effect is that we lose a lot of detail on the right face of each building and we lose some of the shadows cast.
But again, it's all aesthetics and ultimately up to the photographer. None of these are technically wrong, it just depends on what you're looking to say. As for me, I would probably chose the middle shot (f/11) for the softer edges and detail around the darker buildings. Others may choose a wider aperture and focus on the church instead of the building in the foreground. There are dozens of proper exposure recipes to choose from. Each will tell a very different story. So stop trying to achieve the mythological perfect exposure and focus instead on determining what story you wish to tell.