THE SOLUTION
To understand the depth-of-field preview button, try this exercise:
1. Attach a 70mm or longer lens to your camera.
2. Set your aperture to the smallest possible f-stop (f/2.8, f/3.5, or f/4, for example).
3. Focus on an object close to you, leaving enough room around it so that an unfocused background remains visible in the composition.
4. Press the depth-of-field preview button while looking through the viewfinder. Note that nothing happens.
5. Now decrease the aperture to f/8, press the button again, and take another look. Pay special attention to the out-of- focus background. You probably noticed the viewfinder getting darker because you shrank the lens opening, allowing in less light, but did you also notice the background becoming more defined? If not, set the aperture to f/16 and press the button again, paying special attention to the background. The viewfinder will get even darker, but that once-blurry background should be razor sharp. You’re seeing the impact of aperture. Each time the aperture gets smaller, objects in front of and behind your focal point become more defined. In other words, the area of sharpness (the depth of field) extends.
If there is one bit of advice that bears constant repeating, it is this: Pay close attention to your aperture choice! Many times, the wrong aperture has ruined an otherwise wonderful exposure. Let’s say you were after a striking image of a lone tulip against a background of out-of-focus tones, shapes, and colors, as in the second image here. This is what each and every one of us would see if we stood in this spot and shot with our 70–200mm zoom lens at the 200mm focal length with the aid of a small extension tube (a metal tube fixed between the camera body and lens to improve macro photography capabilities). However, just because this is what we would see as we looked through the viewfinder, there’s no guarantee that this is what we will record on our image sensors.
If in your excitement of framing up a shot like this, you fail to notice that you are using an aperture of f/16, you will definitely record the image of this same tulip, but with a much busier background, as shown in the first image.
Fortunately, most of us have depth-of-field preview buttons on our cameras. In this scenario, I depressed my depth-of-field preview and could see right away that f/16 was offering too much depth of field. While still pressing the depth-of-field button, I increased my aperture while looking through the viewfinder, watching as the once-busy background slowly faded to blurry tones. At f/5.6, I felt the background was reduced to the softer, blurrier tones I was seeking.
My first attempt offered too much depth of
field.
70–200mm lens, f/16 for 1/10
sec.
As I shifted to f/5.6, the background faded to a
blur.
70–200mm lens, f/5.6 for 1/100
sec.