Quick Question about aperture readings.....

[ferio-moreno]

if i have 2 light sources pointing towards the same subject, what would the appropriate aperture reading be say if I had the lights like so :


Fill Light ( F/5.6) + Key Light (F/8.0) = F11???

and

Fill Light (F/8.0) + Key Light (F/11.0) = F16???

is this correct? Can someone explain this a little better? thx.

[Ty G]

Double the lights doesn't necessarily mean double the light output. I don't know your strobes, so I don't know if those aperature numbers come from the strobe setting or a light meter.
Before I start, if you are using the same shutter speed to get those aperatures, then the key light would actually be the 5.6 one and vis versa. The smaller the number, the more light comes in and the brighter it is.
If you use a light meter to get those aperatures, then the fact that your key light of F5.6 (remember I reversed them for you) is for proper exposure for that area where the light is hitting,(i.e. the model's cheek); and your F8 fill is telling you that if you wanted that area properly exposed, you would shoot F8. If you shot that area calling for F8 at f8, then the area that reads F5.6 would be overexposed.
I hope like hell I'm making sense.
SO, lets say you had a F5.6 light falling on a models cheek or chin and an F8 light as a fill light by the ear or under the eye in the shadow. You shoot that at F5.6 and the cheek area would be properly exposed and the shadows will be just a little underexposed.

[RTP]

then the key light would actually be the 5.6 one and vis versa.

Maybe I'm reading you wrong, but I disagree. If a light is measured as f/8, that means an aperture of f/8 would give a correct exposure. If a light is measured as f/5.6 then an aperture of f/5.6 would give a correct exposure. The f/8 light is brighter and therefore needs a narrower aperture to get a correct exposure.

The best way of getting correct exposure with both lights is to take another reading with both of the strobes firing.

[Lotto]

From my pass measurements, f8 main + f5.6fill = about f9, f11+f8= f13, depends on the placement of the fill light.

[rabidcow]

Lotto is more correct here. One light at 5.6 and one at 8 will yield a total output of 8.5. Somewhere between f/9 and f/10. I meter to a 1/3 ratio for most head shots and I meter my fill to 5.6.5 and main fill combined to 11. This puts my main alone at 8.5.

[Curtis N]

I'm puzzled by your question. If you can meter the lights individually, then you can certainly take a collective reading, with the meter pointed toward the camera, after the levels and ratios are set.

There is no precise answer to your question, since the individual readings are taken with the meter aimed at the light. The correct shooting aperture will depend on the various angles involved.

[PacAce]

if i have 2 light sources pointing towards the same subject, what would the appropriate aperture reading be say if I had the lights like so :


Fill Light ( F/5.6) + Key Light (F/8.0) = F11???

and

Fill Light (F/8.0) + Key Light (F/11.0) = F16???

is this correct? Can someone explain this a little better? thx.

If the key light is f/8, then adding another light of the same intensity would double your light or increase it by one stop. That means you would need f/11 for two lights at f/8. However, your other light is only f/5.6 (one stop less) or 1/2 of the light intensity of the key light. So, the combined light intensity is one and a half times the key and would require f/9.5 if you have the camera set to 1/2 stop increments. If set to 1/3 stop increments, f/9 would just barely overexpose the shot and f/10 would just barely underexpose the shot. But for practical purposes, either of these would do also.

For the f/11 key + f/8 fill use f/13. :)

[forkball]

In my experience... it really depends on how much of the key contributes to the fill side, and how much the fill contributes to the key side. For my more traditional portraits... it usually works out like this:

key - f/8
fill - f-5.6
Combined with meter at center aimed at camera and lumisphere fully extended is just about at f/9. If I modify the key and main so that neither spill into the other's area (with grids or eggcrates) I set the camera for the key setting.