Metering for more than one light

[Josh66]

OK, so I've heard the 'meter one light at a time' advice, but I'm sort of confused about what happens next.

I have a meter (L-358) and 2 flashes.

When I meter the second light, do I just totally ignore the settings from the first light, and set the camera to the settings for the main one?

What happens if the light overlaps? Won't those areas be overexposed?

 

[pgriz]

Light is additive. If you for example get f/8 for light 1 and f/8 for light 2 and they are set up to overlap, then the overlap will need f/11. The parts lit by only one light will require f/8.

Try it... meter both lights separately, then meter them together. You'll find that if they are of equal brightness on the place of overlap, you're effectively doubling the light, requiring halfing the aperture to balance. Clear as mud?

 

[Josh66]

Let's say they are an equal distance from the subject, same camera angle, but one light is set one stop lower, and they overlap.

Is there even a point to metering them separately in that case?



If I'm understanding correctly, if they mostly overlap - meter the overlapped area. If not, meter one at a time. Is that right?

 

[tirediron]

it depends on why you're metering; are you metering to determine the exposure or to achieve a ratio? If want to know what exposure I need for the subject's face, I'll hold the meter in front of his/her face, pop the lights and set the exposure the meter gives me. If on the other hand, I want a specific ratio, say for instance, fill light 1.5 stops below key, then I'll set my key, take a reading, and then turn the key off, and meter and adjust the fill 'til I get 1.5 stops below.

 

[Josh66]

are you metering to determine the exposure or to achieve a ratio?

Both. Determining exposure, I'd say I'm comfortable with. Ratios are still kind of new to me.

 

[tirediron]

Fair enough, then start by getting your key light in position and at the appropriate level. Let's say that's f8 and you know you want your fill one stop below. Turn off the key, set up your fill, and adjust it 'til you get f5.6. Remember however, when you're metering your key, you will normally meter at the subject's face. Depending on what you're doing with your fill, you might be metering at the subject's legs or elsewhere.

 

[pgriz]

If the lights are not equal, but say 2 stops different, then meter for the main light and ignore the contribution by the second as it will be minimal. Look at it this way: if both lights contribute equally, then half comes from one and half from the other. Either one by itself is therefore 1 stop less than both together. If one light is 1 stop less than the other, then the overlapped area is getting 75% of its light from the main, and 25% from the fill. If the fill is 2 stops less than the main, then the overlapped areas are getting 87.5% from the main, and only 12.5% from the fill. At this point, you can pretty safely ignore the fill and meter only for the main.

When I do product or still life photography, and have different amounts of light, I start with the main, and adjust the power of the flash until I get the f/stop I want to use (for DOF reasons). Say it is f/8 Then I decide how much fill I want (say 1.5 stops less compared to the main), and adjust the power of the second flash until I get the f/stop I want to have (f/4.5 in this case). Then I check with the meter in the overlapped area to make sure I didn't goof - it should still give me a reading of f/8. At this aperture, the overlapped area will be properly exposed, and the fill areas will be 1.5 stops underexposed, which was my goal to start with.