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Thursday, January 18, 2007

May 2006 HelpLine

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ƒ-Stops And Bad Crops

    * Digital Camera ƒ-Stops
    * Straight-Line Troubles
    * Cropping Casualty
    * PPI Canon Vs. Nikon
    * Out-Of-Shape Histogram?
   

Digital Camera ƒ-Stops

Q)  Is there a scientific reason that all or most digital cameras go down only to ƒ/8? I've been a photographic nut for more than 60 years and always loved being able to stop way down to increase my depth of field.

Pete W.
Via e-mail


A)  When you say "all or most digital cameras," I assume you're talking about fixed-lens compact digital cameras, not digital SLRs. And yes, there's a scientific reason for the limitation. The image sensors for the compact digital cameras are much smaller than those of digital SLRs and 35mm film cameras. With the smaller image sensors come smaller lenses (both physically smaller and shorter in focal length) with physically small lens openings.

With these smaller lens openings comes increased diffraction (the tendency of light to change intensity and direction when going around a barrier like an iris-the opening in the lens that creates ƒ-stops). Diffraction causes softening of the image, and this diffraction limits the ƒ-stops that you can have on a compact digital camera. (Diffraction is also the reason why it's never a good idea to completely stop down a lens-the sweet spot of a lens is always less than the maximum ƒ number.)

I should mention that depth of field at a particular ƒ-stop is inversely proportional to a sensor's diagonal dimension. A compact digital camera at ƒ/8 with an 11mm diagonal sensor is equivalent to ƒ/32 on a 35mm camera (using equivalent focal lengths). This, then, is a challenge for small digital cameras-limiting depth of field!

 


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