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By Michael Guncheon
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Page 1 of 4
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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