Field Guide 101: Exposure - Depth of Field

Misty Morning

Yellowstone National Park, Wyoming 2006

Depth of field is something many beginning photographers struggle with. So first let us define depth of field so there is no confusion. Depth of field is the distance in front of and beyond the plane of focus. There is only one plane in the photograph that is in focus and there is apparent focus toward the camera and beyond the focus plane. Typically the subject will be the on the focus plane. Apparent focus means that more of the image appears to be in focus. For a more detailed description, including the math behind depth of field, visit Wikipedia, Depth of field.

The depth of field is determined by the f-stop. By increasing the f-stop you reduce the aperture diameter and increase the depth of field, which reduces the amount of light. Each stop up doubles the amount of light and reduces the depth of field. It is a good idea to memorize the whole f-stops: f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. I found in the film days I used one-third stops but in the digital world for speed purposes I have now switched to one-half stops. The reason being that with film I needed to be more accurate with my exposure but with digital there is a little bit of fudge factor given the way you want to expose to the right. You still want to get a correct exposure; it’s just that with digital there is more room for error and correction later.

More about exposing to the right later.

Photographers use depth of field for effects like blurred background, known as bokeh, or trying to make the entire scene look as if it is in focus. For an example of an in focus scene take a look at the photo at the top of this article. I used f/16 to get the desired effect. I also was shooting at 45mm. The wider angle the lens the more apparent focus. For this reason, and slow shutter speeds with good tripod use, most landscape photographs appears to be entirely in focus. Telephoto lenses have the opposite problem; very shallow depth of field even at the widest of apertures. Macro lenses suffer from shallow depth of field in spades.

If you were writing a book at this point I would take a couple photos of a flowers illustrating what happens when you open up the f-stop or close it down. Instead I will leave this as an exercise for the reader to do. Use whatever lens you have, put the camera in aperture priority and take the same photo at each whole f-stop. Then go back to your computer and compare the results. If you have more than one lens, or a zoom lens do the same thing at various zoom levels until you understand how the f-stop affects your photograph.

Field Guide 101: Exposure - Basic Exposure Theory

Natural Bridges Sunset

Natural Bridges State Park, Santa Cruz California 2006

One of the keys to photography is properly exposing the film or digital sensor. Since most people shoot digital now days because of the better performance over film, including myself, I'll focus on digital photography. Once you go digital it is very difficult to go back. I stopped using film in early 2007 when I purchased a Canon 5D digital body. The reason I used film for so long was my film body was much lighter than my digital body. The 5D changed all this for me and now I take my 5D on long hikes and backpacking where weight really matters.

Getting into photography today means you have many sophisticated automatic metering systems in your camera that can be taken for granted. Far too often photographers use the camera's automatic settings to determine proper exposure without any idea what the camera is doing. Even the most sophisticated system is fallible so the key is to learn how to use the automatic systems and learn how they work so you can expose photographs under difficult lighting conditions. So the first thing is for us to get started with a good understanding of exposure. We will be looking at manual, aperture priority, and time priority modes of the camera because they are all important in modern nature photography. But the number one thing to be aware of is you need to be in control of taking the photograph, not the camera. Even when using one of the priority modes the photographer needs to know how the camera will be exposing the scene. The priority modes are actually more important because you are letting the camera choose how to expose the scene.

I use Canon cameras so anything I write about will be geared toward that equipment. Nikon and other cameras have similar settings but you should refer to your owner’s manual for detailed usage.

For a good textbook definition of exposure head on over to Wikipedia's definition of exposure. Most textbooks also show a nice bell shaped curve where properly exposed film is exposed in the center. Let me just say this isn't exactly correct, and things have changed a bit with digital. Correct exposure has nothing to do with the exposure meter in your camera. The exposure meter is simply a tool to use for you to evaluate how the image will be exposed. Correct exposure is exposing the film or digital sensor exactly the way you want to expose it. In the digital world you can get more information in your RAW file if you expose to the right and adjust later in Photoshop. Don't blow out the highlights, unless you intend on blowing them out. You can see if the highlights are blown out by the flashing alert on most camera preview screens.



The goal here is to know exactly what you are capturing leaving nothing to chance. Once you gain a level of control over your camera and exposure where you know what will happen you have succeeded in controlling exposure. Sure you might bracket your exposure, you might take a number of different photographs from different angles, after all we are dealing with nature and often times if you wait a few minutes something happens and things get better. But once you can control the camera you have reached a new level of proficiency to build upon. Reaching this proficiency allows you to take advantage of new oportunities and to begin learning other new things.