Part 6 - Photographic Lenses & Depth of Field

The subject of Depth of Field was explained earlier in relation to f/stop settings selected when deciding correct exposure. As you begin taking pictures and evaluating the results, you will soon discover that the optical effect created by Depth of Field is a critical component in your photography. While f/stop settings alter the amount of light entering the camera and define depth of field (the amount of the scene that is in focus from front to back), there is one more factor that adds or subtracts from the picture's total amount of depth of field.

Lens Focal Length

You selected one or more lenses when you purchased your camera. This decision was likely based on what you and a helpful store expert determined most appropriate for your photographic interests. The conversation probably went something like this: “I’m an avid outdoor and wildlife enthusiast. I want to be able to take close-up pictures of wildlife and also be able to capture the beauty of the landscapes I visit.” Given these parameters, the store's recommendations probably included either 1) a wide-angle and telephoto combination or 2) a zoom lens that could perform both duties.

It’s doubtful that the subject of depth of field was mentioned. However, because of the optical physics involved, focal length plays a major role in determining how much of your image is in focus. Focal length is expressed in millimeter numbers. You heard them in the store:

• A normal lens for a typical DSLR falls in the range of 50mm to 58mm. A normal lens "sees" what your eyes see - excluding your peripheral vision.
  
  
  What the 52mm “normal” lens sees.
  
  
• A wide-angle lens has a focal length of less than 50mm (the smaller the focal length number the wider the angle of view). In other words, the smaller the focal length number, the wider the lens and the more it “sees” of the original scene. Giving the impression that the camera is farther away from the subject. (FYI: a wide-angle lens with a focal length of approximately 21mm "sees" everything you would typically see in a scene -- including your peripheral vision.)
  
  
  What the 18mm “wide-angle” lens sees.
  
  
• A telephoto lens has a focal length greater than 58mm. The larger the focal length number, the "longer" the lens and the less it “sees” of the original scene. Giving the visual impression that the camera has moved closer to the subject.
  
  
  What the 200mm “telephoto” lens sees.
  
  
• A zoom lens has the ability to vary the position of glass elements that make up the lens to cover a range of focal lengths. In essence, a zoom lens acts as many lenses from wide, to normal, to telephoto.

Lens Focal Length's Impact on Depth of Field

These features and benefits ultimately lead to your lens purchase decision. Now consider the impact a change in focal length has on depth of field. The pictures below will illustrate this optical reality.


This picture was taken with a 52mm “normal” DSLR lens. Notice the rose in the foreground is very sharp. The background garden, however, is slightly out of focus. The aperture setting was f/8 and the shutter speed was 1/1000th.


Compare this 200mm telephoto picture taken of the same scene. The camera was physically moved away from the front rose until it appears to be about the same size as captured with the “normal” lens in the first picture. Notice the “shallow” depth of field. The background is very out of focus. The aperture setting was f/8 and the shutter speed was 1/1000th.


Finally, the picture above was taken with an 18mm “wide-angle” lens. In this case, the camera was moved closer until the foreground rose was approximately the same size as the rose in the “normal” lens photo. In this version of the photo, every object in the frame is in focus (extreme depth of field). The aperture setting was f/8 and the shutter speed was 1/1000th.

You can see clearly the optical phenomena that impact depth of field as the focal length of the lens changes. Although each picture was taken at exactly the same f/stop and shutter speed, the amount of depth of field in each case is obviously different. The rule is: As focal length increases, the depth of field at each f/stop decreases. And, conversely, as focal length decreases, the depth of field at each f/stop increases. So, wide-angle lenses inherently have more depth of field at each given f/stop while telephoto lenses have less depth of field at each given f/stop.

It’s no wonder that portrait photographers are likely to use telephoto lenses to ensure the subject is sharp and the background is out of focus. It’s also understandable that architectural and landscape photographers most commonly use “wide-angle” lenses to capture the breadth of their subject and maximize depth of field to ensure the entire image is sharp (in focus) from front to back.

The same depth of field and optical rules apply to zoom lenses. The longer the zoom length (focal length increases as you zoom "in"), the less depth of field at each given f/stop. Zoom "out" to your shorter focal lengths -- wider angle -- and the depth of field increases. Magic.

Now you have two major factors impacting how much of your scene is in focus. Photographers who master depth of field will produce the most compelling and memorable pictures. The two major factors are:

• The f/stop you select when determining the correct exposure. The smaller the aperture (larger f/stop numbers) the greater the depth of field.
  
  
• The lens you are using. The longer the focal length (increasingly more telephoto) of the lens, the less depth of field it has at each f/stop setting. The shorter the focal length (wide-angle), the greater the depth of field at each f/stop setting.
  

There are many ways of checking the specific amount of depth of field any f/stop will produce with a given focal length lens. Some lenses are engraved with markings that allow you to calculate the depth of field distance. Very accurate tables are available in photo books and on the Internet that will give you the precise distance(s) for each focal length and f/stop combination.

I suggest you pull out your camera manual and become intimately familiar with your camera's depth of field preview button. It takes a while to become comfortable using this camera feature. Here's what is happening inside the camera.

When you look through your viewfinder, you are actually seeing the scene through the lens' widest aperture setting. This is to make it easier for you to compose and focus your image.

As you manually depress the "depth of field preview button", the lens physically "stops down" to the f/stop you selected (the metal leaves that make up the aperture mechanism in your lens are being closed to the selected f/stop position). Unless you selected the widest aperture on your camera, the viewfinder image WILL get darker. Learn to look through the darkness and you will visually see what WILL and WILL NOT be in focus when you take the picture. So, take the time to use your depth of field preview button. You'll thank me later.

In my next post, I'll discuss the shooting modes available on your camera, and how they influence these discussions on depth of field and motion stopping.

If you have questions, comments or suggestions don't hesitate to drop me a line.