Originally Posted by eddym
Correct me if I'm wrong, but I believe that the lighting setup you describe actually creates a 3:1 ratio.
In order to end up with a 2:1 ratio, one needs to have a fill light and a main light that, individually, provide the same amount of light.
That way, the shadowed side of the subject receives one unit of light (light from just the fill), whereas the other side receives two units of light (light from the fill, plus the same amount of light from the main).
“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”
Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2
I think Eddy refers to a case, which I made reference to, where the two light sources are at the two sides of the subject, while you (Matt) refer to a case where the main light is lateral, and the fill light is frontal, near the lens axis, so that the fill light embraces both sides and "adds" its light to the one projected by the main (a more typical setup for portraits I suppose).
Oh, and I begin getting the usefulness of the flat cover.
If one uses the dome for determining the ratio, in the case implied by Matt with the fill light "frontal" and the main light "lateral", one must turn off one light to measure the other light.
If instead of using the dome one uses the flat cover, the different power of the two lamps will be read precisely, without need to turn off the other light. With hot lamps that might even avoid stress to the lamps (turning on and off in rapid succession).
EDIT In fact this was the conclusion that was already implied in post #13 by 2F/2F even if I did not catch it immediately.
The dome, covering 180 degrees, takes that flare/spill into consideration. This is why I prefer it to the disc for measuring lighting ratios.
Originally Posted by MattKing
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
- Rob Tyner (1944 - 1991)
The dome can't "see 180 degrees" 2F/2F because it can't see behind the meter body, there were however in the late 1950s a proposed version of Western Master meters with two hemispherical receptor domes one facing forward, and one back to take 180 degree light into consideration in one reading that was never manufactured, but this Duplex method can be achieved by taking two readings one in each direction and averaging them .
Originally Posted by 2F/2F
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The domed cover measures "omnidirectionally" and includes extranious light. The flat cover is used by aiming it directly at the light source(not the camera)to ascertain the illumination ratio. No, you dont need the flat cover, but if you are a stickler for perfection and need to know the actual lighting ratio, use it.
Normal dome 180° vertical x 180° Horizontal
What it doesn't normally see is 360° x 360°
Mark Barendt, Beaverton, OR
"We do not see things the way they are. We see things the way we are." Anaïs Nin
Originally Posted by markbarendt
Originally Posted by benjiboy
The dome does, actually, have a 180 degree field of view, what it doesn't have is a 360 degree field, and what would be the point of having that?
The flat disc is also used for checking exposure for flat work(art). by checking each corner & center it tells you if the light is even edge to edge.