Re: [Leica] The F-Stop's Here

["Dan Post" <dpost@triad.rr.com>]

Buzz-
The f 'stop' is actually a ratio of the focal length to the size (diameter)
of the aperture. So, simply put a lens of 50mm focal length and a maximum
aperture or 'fstop' of 2 would have an aperture with a diamter of 25mm.

This same aperture of 25mm, if found in an 100mm lens, would yield an f
ratio of- 4.

If you think back to the so called 'inverse square law' you can see that
doubling the focal length, or optical distance of the light coming out the
back of the lens, with the same size aperture of the 50mm lens will result
in a two 'stop' reduction of exposure- of 1/4 the light!

I always taught anyone using a camera to think of the camera as a small dark
room, with a window on one wall. The exposure was dependant on two things-
how big was the window in realtion to the distance to the opposite wall, and
how long did you leave the window open!

These same ratios work with lights (both flash and studio floods). For
example, if you want a 3:1 liughting ration for a portrait, then you can
place your key light at, say 5.6 feet from the subject. If  you consider
this to give you two units of light, and all you had was another light of
the same wattage (or guide number, etc.) then by using it as a fill light at
a distance of 8 feet, then it would illuminate the subject with half the
amount of light as the light at 5.6 feet!
This would give you a 3:1 lighting ratio. If you wanted the fill light to be
even less, then move it twice as far as the main light, out to 11 feet, and
it only puts 1/4 (two stops less)  of the light as the closer one.

These ratios are pretty simple to remember- all the lenses I have already
have them engraved on the barrel!

Dan