Since there are so many combinations which give a "correct" exposure, how do you decide which one is best for you? This is part of the creative decision, a decision that distinctly changes your photographs. When it comes to determining the right exposure for backgrounds, foregrounds, and the amount of depth in your image that remains in or out of focus, things get a little more complicated. Determining depth of field is often guesswork, but we’re here to help you get the guesswork out of calculating depth of field, adding this creative skill to your photographic composition abilities.
Lens open wide at f 1.4
Lens open medium at f 8
Lens open smallest at f 22
While the shutter and aperture work together to capture enough light to expose your subject in your photograph, the shutter speed controls the speed of movement across your film and the aperture controls the depth of field in your photograph. Depth of field is the range of apparent sharpness from front to back in your photograph. Objects within the depth of field appear sharp, and those outside of it are out of focus. The larger the f-stop, the smaller the aperture, and the larger the depth of field. To repeat, the larger the f-stop number, the larger the depth of field. The smaller the f-stop, the smaller the depth of field. What this means is that if you want as much as possible to be in focus, stop down as far as your lens will allow, usually f22 or more.
In most cases, a large depth of field is wanted for scenic photographs. We want to see the detail in the flowers in the foreground and the mountains in back. Choosing a small aperture like f22 is a good choice, maximizing the amount of the image in focus from front to back. At other times, an out of focus background eliminates distracting elements so that our attention is on the subject, as in a close up of a single wildflower. I might shoot this at f4 or maybe f5.6. In some cases, it just doesn’t matter which aperture to use, so I pick one in the middle like f8 or f11. Look in the Popular Photography magazine lens tests and you will see that almost all lenses are at their best around f8 and f11, exhibiting greater sharpness and less distortion.
Artistic Control with Depth of Field
Like all things in life, there are tradeoffs between aperture and shutter speed. We have to have a certain quantity of light to make a proper exposure. If we close the lens down to get more depth of field, we may not have enough light to stop the motion of our subject. Open the lens for a fast shutter speed to stop the motion, and the depth of field may suffer. Or we make knowing decisions by understanding how our equipment works, and therefore we know a larger aperture will give us a shallow depth of field and a slow shutter speed will blur our subject. These creative possibilities allow us to play with exposure to create the effect we want. So we have to strike a balance and get as close as we can to the vision we have in our mind.
Depth of field also has some technical limits due to constraints of equipment and close focusing and working distances.
To review, depth of field is the amount the image is in focus from back to front. It is controlled by the aperture of the camera. The smaller the aperture (f22, f32 or greater) the greater the depth of field. Landscape photography measures depth of field in feet or meters. Close up photography measures depth of field in millimeters. The closer you are to your subject, the shallower the depth of field. The further you are from your subject, the greater your depth of field.
Aperture of f2.8
Aperture of f32
To show you the difference in aperture for extreme close up photography, we photographed a US quarter against a US dollar bill at 1:1 magnification or life size. At an aperture of f2.8, focused on the quarter, only the front of the quarter is in focus and the dollar bill behind it is out of focus. At this aperture, our depth of field is almost zero. When we close down our aperture to the smallest opening, in this case to f32, the dollar bill is now in focus. At extreme closeness, our depth of field is barely the depth of the coin.
Depth of field now becomes part of the compositional elements as you choose what to put into focus and what not to. The out of focus elements can become just as important to the image as the part that are in focus. For example, let’s examine this series of three photographs of the same flower. Moving through the flower from front to back, adjusting only our focusing point as the depth of field is at maximum, look at how the picture and composition completely changes. Which one you like best is up to you, but the point is that you can choose where to focus to change the composition of the image.
When the depth of field is in millimeters, so shallow that even a focusing shift can create a completely different image, you have what is called "selective focus". This is where the focus point becomes the subject rather than the subject itself. This is not a series of pictures of a red flower, but much more – artwork.
Backgrounds and Depth of Field
As your depth of field narrows, your background becomes even more important and critical to the success of the image. Pay close attention to the details in your background and around your subject. When in focus or partially in focus, these have shape and attract attention. By blurring them with a narrow depth of field, they fade to nothing or take on a ghostly shape. Sometimes this is effective, other times the ghostly shape attracts even more attention than the subject, especially if it is brighter than the main subject.
Photographing dark subjects against a light background or the reverse helps the subject stand out against its background. But photographing dark subjects against dark backgrounds (or the reverse) may cause the subject to be lost. Pay careful attention when photographing small animals and insects, noticing when black antenna disappear into black backgrounds. Choosing your background can make the difference between a successful close up image and a throw away.
Working with depth of field preview, you can determine what is in focus and what is out of focus before taking the photograph. Another technique is to unfocus your lens and look through the viewfinder to see if there are any bright or dark spots that will be distracting. Then re-compose and re-focus on your subject.
The depth of field preview button helps you see if your chosen aperture will hold the subject sharp, while controlling how out of focus the background is. By closing the aperture to the chosen f-stop, the image seen in the viewfinder gets darker, with you can see what is in focus to help you in your aperture choice. The final photo will look normally bright, because you are compensating for a smaller aperture with a longer shutter speed.
When you increase your magnification, you enter a new world of closeup photography where what is in focus is the main subject, allowing everything else to fade off into blurs. This process of deciding what to have sharp in the image and what not is called selective focus. Developed extensively by nature photographer, Mary Ellen Schultz, her amazing studies of the inside of flowers, where focus and depth of field is measured in millimeters if measurable at all, have become classic works of art as she took abstract art and impressionism to new heights. Here are some samples of our work which hint at her magic.
As you work with this technique, we highly reocmmend that you use the capability of the depth of field preview in your camera body to help you choose your focus point and determine exactly what is in focus as well as what isn’t. At extreme magnification, lighting becomes a problem. Use a flashlight or studio lamp to assist you in focusing and composing, then turn it off to capture the natural light.
Expanding upon this notion of selective focus, you can also create your own selective focus "blurs" to enhance your images. Especially when working with wildflowers, you are often challenged with difficult lighting situations as well as distracting foregrounds and backgrounds. By photographing with out-of-focus colorful flower pedals in the foreground, you can create a watercolor wash of color that will hide distracting elements. If the neighboring flowers don’t cooperate, you can pick up some fallen or damaged flowers (we do not recommend picking wildflowers for this process) and hold them up in front of your lens to recreate the same quality. By understanding how your lens sees, and working with the depth of field preview or by experimentation, move the flowers in your hand forwards and backward between the lens and the subject until you achieve the desired result.
Brent holds up a damaged and fallen flower in front of his subject. Understanding his depth of field range, he moves it around until it blurs the distracting foreground, isolating the subject flower in the background.
Calculating Depth of Field
For more on understanding depth of field in relationship to close up photography, check out our series of articles on macrophotography and close up photography called "I Long To Be Close To You."