Various ways to use an incident light meter

[MattKing]

True, and since most portraits fall in that category, it's a very simple process:
Turn on main light, point the dome at it and take a reading. Turn off main light.
Turn on fill light, point the dome at it and take a reading.
For a 2:1 ratio, fill should meter one stop less light than main.
Turn main light back on. point the dome at the camera, and take a reading.
Set the camera and take a picture.

More complex lighting setups will call for more complex metering procedures, hence the need for the flat panel.

Eddy:

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).

I believe

[Diapositivo]

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).

Fabrizio

[Diapositivo]

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).

Fabrizio

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.

[2F/2F]

One point, for clarity.

If the light source illuminating the shadows on one side of the subject is in the nature of a diffused "fill" light, it will also be contributing illumination to the other, highlight side. Thus a change to that fill light will not only change the ratio, it will also necessitate a change to the camera exposure settings.

So it is important to differentiate between lighting setups that have two sources that illuminate separate sides, from lighting setups where one source is more of an all-over "fill".

The dome, covering 180 degrees, takes that flare/spill into consideration. This is why I prefer it to the disc for measuring lighting ratios.

[benjiboy]

The dome, covering 180 degrees, takes that flare/spill into consideration. This is why I prefer it to the disc for measuring lighting ratios.

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 .

[Rick A]

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.

[markbarendt]

Normal dome 180° vertical x 180° Horizontal

What it doesn't normally see is 360° x 360°

[benjiboy]

Normal dome 180° vertical x 180° Horizontal

What it doesn't normally see is 360° x 360°

Exactly

[Galah]

The dome can't "see 180 degrees" 2F/2F because it can't see behind the meter body...

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?

[John Koehrer]

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.