On Tue, 20 Aug 2002 21:39:10 GMT, "Jeremy Knight"
>"Neuman - Ruether"
>news:3d6b66d6.8942529@newsstand.cit.cornell.edu...
>> On Sun, 18 Aug 2002 20:25:03 GMT,
>> info@internet-real-estate.com wrote:
>> >In article <3d609a3b.2919303@newsstand.cit.cornell.edu>, d_ruether@hotmail.com
>wrote:
>> >> The physical size of the lens has little to do with
>> >> "light gathering". The widest relative aperture
>> >> ("f-stop") available indicates the "speed" of the
>> >> lens, not its size...
>> >You've never used telescopes much, have you. The bigger the objective
>> >lens, the more light gathering ability.
>> Different issues...
>> With essentially parallel rays through the lens (long
>> teles), there is some association of lens front size and
>> "light gathering" ability - though the actual f-stop (the
>> most relevant measure) is still related to both diameter
>> *and* FL; with short FL lenses, there is little reasonable
>> correlation between lens front diameter and its "speed"...
>> >A pair of binoculars with 70mm objectives will be MUCH brighter than the
>> >less expensive ones with 35mm objective lenses.
>> Well, yes, unless the 70mm objectives are twice the
>> FL (magnification) of the 35mms, in which case, they
>> will have the same brightness, all else being equal...
>> (10X70 and a 5X35 binoculars will have the same
>> brightness...).
>> >Two cameras ... both set at f4 for example, but one with a much larger
>> >objective lens ... WILL be much brighter.
>> ????????????
>> I don't think so................! ;-)
>> The relative aperture (all else being equal, like
>> transmission efficiency, illumination evenness,
>> accuracy of opening, etc.) determines the brightness
>> of the light passing through, regardless of front
>> size or FL (but depending on their relationship...).
>>
>> David Ruether
>Well...
>
>The absolute aperture of the lens does determine how many photons come into
>the camera. The brightness per unit area is the same for two cameras set at
>f4, but the picture area is 4 times larger for a 70mm f4 lens compared to a
>35mm f4 lens. This means more photons per pixel for the big lens, and
>greater light gathering power when every photon counts.
>
>This is why owls have physically large eyes, and why insects that depend on
>vision have to go home at dusk. Not enough photons can get into their tiny
>eyes.
>
>Jeremy Knight :: Berkeley CA
> customarily@attbi.com
> jeremy@echidnamedia.com
Well...;-)
Now you are introducing imaging *systems*, and not just
the lens, which changes the question... Or, for a simple
lens system and unspecified sensor area, your example
is correct, but for cameras, where the sensor area is
specified, it is not. A 70mm f4 lens and a 35mm f4 lens
are covering the same sensor area, and the relative
aperture of f4 is the determiner in the system, with both
lenses passing the same brightness of light to the sensor
regardless of their absolute aperture differences...
It is interesting to note the variations, but the discussion
was, as I recall, about whether or not a bigger diameter
lens on a video camera meant that it necessarily had
greater "light gathering ability", and the answer to that
question is that it does not...