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Photography Question 
Melinda Meredith
 

Focus & DOF issue when shooting twins


I am fairly new to photography so please excuse the rookie question. I am trying to photograph my twin babies. I am having a problem blurring my background while keeping both babies in focus and not blurred due to depth of field. Any help would be greatly appreciated!


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7/25/2008 8:41:26 PM

 
Alan N. Marcus   Hi Melinda,
Depth-of-field is influences by focal length. Short lenses (wide-angle) display expanded depth-of-field. A zoom set to near minimum magnification is likely working as a wide-angle. Conversely, a long lens contracts depth-of-field. Stated another way, likely your camera sports a zoom lens, try shots of your twins with the zoom set well past the middle position. Pour on the magnification to suppress depth-of-field.

Depth-of-field is influenced by aperture setting. The aperture is a variable hole size, it simulates the human iris. Either you (manual) or your cameras “Auto”can change the size of this entry opening based on the intensity of the ambient light or flash. You can exercise control. Small opening limit the amount of light entry and greatly expand depth-of-field. Small apertures are f/22 – f/16 – f/11 – f/8 (f/8 is the largest of this list). Conversely large apertures (f/2 – f/2.8 – f./4 – f/5.6 emit copious amounts of light and diminish depth-of-field (of this list f/2 is the widest).

Depth-of-field is influenced by subject distance. Objects close to the camera are recorded with expanded depth-of-field. Conversely, objects further away from the camera record with minimum depth-of-field. You need to train yourself to overcome the natural tendency to compose your shot “exactly” in the viewfinder. What I am saying is, compose your shot allowing some extra room around the subject. This act forces you to step-back thus flatting depth-of-field. The extra room you created affords the opportunity of crop and improves composition during using editing software. Along with increased subject distance, increased background distance is could be the solution. If you can place the twins an extended distance from the background, likely the background will record out-of-focus.

Most modern digitals have a portrait mode. This setting forces the camera to use a larger than normal aperture that minimizes depth-of-field. This act alone my allow you to reach your goal.

Depth-of-field is a zone or expanse that extends both before and behind the point focused upon. It is not split down the middle. The zone extends 2/3 to the rear and 1/3 forward of the point focused upon. You should practice selective focusing. Compose your shot of the twins, an assistance temporarily holds a target like a magazine just in front of the twins. You center the camera on the magazine, press the shutter release stopping just ahead of taking the picture. The camera will focus on the magazine. Continue to hold while the magazine in removed from the scene. The camera retains this focus. Re-compose and continue the shutter release stroke. You have focused ahead of the twins; depth-of-field carries to the principle subject but not to the background.

Summation: A. Use a long lens (telephoto setting). Increases camera-to-subject distance You increase subject-to-background distance. B. Use large apertures. Ambient can sometimes be dimmed, this forces the camera set to “Auto” to use a large aperture. C. Setting the camera to a fast shutter speed reduces the time light can play on the digital sensor. The camera likely will automatically select a large aperture to compensate. Increase camera-to-subject distance. When flash is used, try covering the flash output lens with facial tissue. Try one or more thicknesses. This cuts down the light forcing the camera to use a larger aperture.

It won’t hurt a bit if you read your camera manual on depth-of-field.

Alan Marcus (marginal technical gobbledygook)
ammarcus@earthlink.net



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7/26/2008 11:36:27 AM

 
W.   
Hi Melinda,

http://en.wikipedia.org/wiki/Depth_of_Field

However: because of their very short focal length, and their limited largest aperture, a background that is totally out-of-focus is impossible with point & shoot cameras. You will need an SLR for that.

Have fun!


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7/27/2008 9:57:03 AM

 
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