Sound advice: SLR cameras go into depth - of field

  • Article by: DON LINDICH , Special to the Star Tribune
  • Updated: August 19, 2011 - 1:48 PM

Answers to your multimedia questions.

Q I recently purchased a Pentax K-r digital SLR camera that you recommended (two-lens outfit with 18-55mm and 50-200mm lenses). I moved up from a compact and the pictures and operational speed are worlds better. However, I have a lot to learn. I purchased it to take better pictures of my baby daughter and want an outdoor portrait of my husband holding her with a nice, soft background behind them, as you see in magazines. How do I do this?

A There are several factors that affect your ability to blur the background, also referred to as controlling depth of field. You can go for soft backgrounds (shallow depth of field) or as much in focus as possible (deep depth of field.)

The first thing to consider is the camera's sensor. The smaller the sensor, the more depth of field at a given distance or camera setting. In fact, the tiny sensors in some compact cameras create so much depth of field that it's almost impossible to blur the background in some circumstances. Your K-r has a pretty big sensor, so this will not be a problem.

The second is the focal length of the lens. Telephoto lenses (higher numbers) produce less depth of field. Wide-angle lens settings create a large amount of depth of field.

Third is distance from the subject and the background. The closer you are to the focus point, and the farther away you are from the background behind them, the softer the background will be.

Finally, there is the aperture, or f-stop. Lower numbers (bigger apertures) create less depth of field. Higher numbers, meaning small apertures, create more depth of field. Putting it all together you want a long focal length, you want to be close to your subject, and you want a big aperture.

Place your subjects and set your 50-200mm lens somewhere between the 100mm and 200mm setting. The higher the number, the more background blur you will get but the farther away you will be from your subjects. Back up until you get the composition you want. Use a tripod if you can.

Set the camera on "AV" (aperture value), and set it to the lowest possible number. For your lens, it would be 5.6 at 200mm. You can also use one of the camera's scene modes; set it to "Portrait."

Make sure you are getting a shutter speed of at least 1/250 of a second. Slower speeds may cause blur if you hand-hold the lens when it is zoomed out that far. If you need to bump up the ISO (sensitivity of the sensor to light) to get the higher shutter speed, do so. Bumping up the ISO can have a detrimental effect on image quality, especially with small-sensor compact cameras. But with the K-r or most other digital SLRs or interchangeable-lens cameras, you can use ISO 400 with little practical difference.

Focus on the eyes and take lots of frames, preferably with the camera on high-speed motor drive mode, and capture that magic moment.

Submit questions and read past columns at www.soundadviceblog.com.

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