I’ve just returned from a wedding interview and the bride-to-be asked me a question about one of the images she saw in my slideshow (see below). She asked how I was able to create such a blurry background, yet have the subjects in clear focus. I explained the difference between f-stops, as well as focal distance, and she was very interested; so much so that I thought I’d post the photo here and give some lens data. When photographers talk about a fast lens they are referring to its minimum f-stop (aperture); how wide open the lens will go. The term fast simply means it’s easier to shoot at a faster shutter speed with an f2.8 lens than it is with an f4 lens. The wider a lens opens, the more light gets in, which means a faster shutter speed is necessary to properly expose the image. For example, say an image shot at f4 needs a shutter speed of 1/125th-of-a-second for proper exposure. That same image could be shot at 1/250th-of-a-second if an f2.8 lens was used. Not only does this make it easier to capture sharper images while hand-holding a camera, the f2.8 lens will have a softer bokeh (ie, blurry background) than an f4 lens. This is where depth of field comes in. Depth of field is simply referring to how much of the image is in focus, based on linear distance from lens to subject. A shallow depth of field (ie, f2.8) will only contain a small area in focus while an image shot at f16, for example, would pretty much render everything in sharp focus, provided the subjects were standing completely still. However, if I were to have shot the image below at f16, the bride and groom would not have looked so sharp, since everything behind them would have been in focus as well. Additionally, at f16, I would’ve had to have used a very slow shutter speed (1/30th-of-a-second, for example), and the entire image would have suffered from motion blur.
I used a Canon 70-200mm f2.8, Image Stabilized lens on my Canon 5D MarkIII for this shot, and I was zoomed all the way in to 200mm. Image stabilization helps reduce motion blur when hand-holding a camera. I’ve already explained how shooting at f2.8 creates a blurry background and a nice bokeh. Using a telephoto lens compresses the foreground and background, further enhancing the bokeh when the lens is zoomed to its maximum distance. By compression, I mean optical compression, not digital compression; it’s the effect of the glass itself that creates the compression, not a digital algorithm (which a lot of point-and-shoot digital cameras use, making enlargements look very fuzzy). For this image, which was taken in Flowery Branch, Georgia, I had the couple stand on a bluff overlooking the lake behind them. I stood about 30 feet away and zoomed in so they almost filled the frame. The woods and lake in the background are over 100 yards away from the couple but they look closer, thanks to the compression-effect of the telephoto lens. Shooting at an f2.8 aperture meant I only had a small area that would be in focus, but the telephoto compression actually gave me a bit more leeway. If I’d used my 16-35mm lens for this image, shooting at f2.8 would have been a disaster: I would’ve had to have stood within a few feet of the couple, and only a part of their faces would be in focus since I wouldn’t have the benefit of compression.
So that’s Depth of Field 101, with a bit of telephoto compression thrown in 🙂