From: Michael Lindner 
To: John Sherman , atm{at}shore.net
Reply-To: Michael Lindner 


On Thursday 17 July 2003 08:52 pm, John Sherman wrote:
> Now imagine two f/2.8 lenses, one has a 50mm focal length and the other has
> a 100mm focal length. Now, you want to make an image of a person on a piece
> of film, say. If you use the 100mm lens the person will be twice as tall as
> if you used the 50mm lens. Of course the aperture is the same, and
> therefore the total amount of light gathered is the same. But the 100mm
> lens spreads that light out over four times as much film (the image is
> twice as high, and twice as wide).

Almost...the first example (50mm f/1.8 versus 50mm f/2.8) is correct, but
the second example has a flaw. The 50mm f/2.8 and 100mm f/2.8 lenses are
the same "speed" - neither is faster. They do not have the same
aperture. In order to be f/2.8, the aperture is the focal length over 2.8,
so the 50mm aperture is 50/2.8 or about 18mm. The aperture of the 100mm is
100/2.8 or about 36mm. So the light gathering power (which is proportional
to the square of the aperture) is 4x as much for the 100mm lens. Of course,
the image scale is proportional to focal length, so as you said, the image
from the 100mm lens is twice as high and wide, with 4x the area, so the
light per unit area is the same, and therefore the exposure time the same
for both f/2.8 lenses (indeed, for ANY f/2.8 lens).

Clear skies.
--
Michael Lindner
http://www.starastronomy.org
http://home.att.net/~mikel http://www.atmsite.org
http://home.attbi.com/~dcass/atmfaq/atm-faq.htm

--- BBBS/NT v4.01 Flag-4