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Question on flash metering

FORUMS General Gear Talk Flash and Studio Lighting
Thread started 01 Feb 2009 (Sunday) 20:44   
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sito
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Today I was playing around with several flashes and my Sekonic lightmeter L-358 and a question popped out. First I set my camera at ISO 200, f/8 and 1/60. Then I set my main light at 45 degree towards the subject and set its power until the flashmeter read f/8. Then I set the fill light. I adjusted its power until the Sekonic read f/5.6 (1 f/stop lower than the main light). After setting both lights, I took a reading of the two lights combined and the Sekonic read f/8 plus 7/10 (approximately that is f/10). To get a real f/8 with a shot of the two flashes combined, I needed to dial down the power of the two lights until I got an f/8 on the Sekonic. I think this is normal, for I have a DVD by Sekonic where there is a photographer stating the same effect.

However, I do not really understand why shooting two lights (one of which is underpowered) gives me more power than the main light. I would appreciate if anyone could explain that to me.

Post #1, Feb 01, 2009 20:44:35


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TMR ­ Design
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Think about it. Even though you're adding less light with the fill at 1 full stop lower than the main, aren't you still adding light? You can't add light and not see an increase in light, thus a smaller aperture reading on the meter. The fact that you are seeing 7/10 stop additional represents the small contribution to the exposure, but a contribution nonetheless.

Post #2, Feb 01, 2009 21:02:52


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johnz
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I was playing with a light meter also this weekend, and faced some issues questions too.

In this example, aren't you still supposed to keep your camera at f/8 if you are looking for a correct exposure on the subject + some rim light? ( If that was what the OP was trying to achieve ).

Post #3, Feb 02, 2009 04:58:18


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bohdank
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If the second light overlaps the coverage area of the main light, you are adding to the light.

Post #4, Feb 02, 2009 06:59:46


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Wilt
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Light is like water...add more water, the bucket fills higher -- even when the flow rate from the smaller hose is a lot less than the main hose!

What most amateurs do not fundamentally understand is that if you were in a darkened room and opened the shutter (Bulb) and popped the flash 1 time, vs. 2 times, vs 4 times, vs. 8 times, vs 16 times, you are exposing the sensor/film to accumulating light that results in an exposure of 0EV, +1EV, +2EV, +3EV, and +4EV respectively. That is why the overall exposure is +0.7EV from your 1/60 f/8 single light reading. Bohdank was 100% right with his comment.

Post #5, Feb 02, 2009 10:16:35


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sito
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I am getting the point. Thanks a lot. Question now is: What is the recommended workflow of lighting setup? Should I set first the main, then fill, then dial down both lights to match the flashmeter?

Post #6, Feb 02, 2009 10:26:05


20D - 10-22mm - 17-85mm - 50 f/1.4 - 70-200 f/2.8L IS - 100mm Macro - 1.4x II - 420EX - Canon 550EX

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Wilt
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Read just the main. Then read just the fill. Use the readings to establish the contrast ratio of the lighting.

Read both lights together, set the aperture of the lens to that value. Shoot.

Post #7, Feb 02, 2009 12:02:18


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johnz
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Nice! Thanks, somehow i was under the impression that the f/stop is adjusted by the main light. This makes actually much more sense.. :)

Post #8, Feb 02, 2009 15:17:23


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steveathome
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sito wrote in post #7237740external link
I am getting the point. Thanks a lot. Question now is: What is the recommended workflow of lighting setup? Should I set first the main, then fill, then dial down both lights to match the flashmeter?

johnz wrote in post #7239345external link
Nice! Thanks, somehow i was under the impression that the f/stop is adjusted by the main light. This makes actually much more sense.. :)



If you want to shoot at a specific aperture, then IMHO Sito's post is the way to go.

Once you have your ratio set up, reduce both strobes equally to the desired output/aperture, this will maintain the same ratio.
(ie if your final meter reading is 7/10 over, reduce both strobes by 7/10)
Having strobes with accurate incremental control is an advantage.


If you tend to use similar set-ups, you can set your key light at the required amount that you normally need to drop (in this case7/10) from the beginning.
This method will also assist, if your strobes do not have accurate incremental control.

Hope this makes sense.

Post #9, Feb 02, 2009 15:53:39




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PacAce
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Or, you can just work the aperture values backwards. If you want the camera set to, say, f/8, and you want a 2:1 main-to-fill ratio, meter the main light for f/5.6 and 5/10 (or half a stop under f/8 ). Then meter the fill for f/4 and 5/10 (one and a half stop under f/8 ). The combined reading, assuming the main and fill overlap, will then be exactly f/8. :)

Post #10, Feb 02, 2009 16:18:21


...Leo

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steveathome
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Thats what I meant in my last paragraph, but just maybe didn't put it into English very well :o

Post #11, Feb 02, 2009 16:21:33




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Papa ­ Carlo
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Wilt wrote in post #7238358external link
Read just the main. Then read just the fill. Use the readings to establish the contrast ratio of the lighting.

Read both lights together, set the aperture of the lens to that value. Shoot.

You forgot to mention that only main or only fill shoud be read with the dome down and directed towards the light and the resulting light with the dome up and pointed towards the camera. In my case it often happens that final resulting reading is lower than the main light. For example Main F8 hair light F4.5 the result F5.6.

Post #12, Feb 03, 2009 06:59:50




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TMR ­ Design
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Not everyone meters as you're describing. With digital there are many (myself included) that meter for the highlights and the taking aperture is derived by metering towards the main source of illumination and not the lens.

These is always great debate and discussion over this and I don't want to see this turn into that debate again but if you're metering for highlights as I described then what you're describing does not occur.

Post #13, Feb 03, 2009 07:05:54 as a reply to Papa Carlo's post 6 minutes earlier.


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Papa ­ Carlo
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TMR Design wrote in post #7244190external link
Not everyone meters as you're describing. With digital there are many (myself included) that meter for the highlights and the taking aperture is derived by metering towards the main source of illumination and not the lens.

These is always great debate and discussion over this and I don't want to see this turn into that debate again but if you're metering for highlights as I described then what you're describing does not occur.

So i your case do you lower the hemisphere or not ?

Post #14, Feb 03, 2009 09:12:38




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TMR ­ Design
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Hi Papa Carlo,

If I'm metering a single light source and no other strobes are firing I do not lower the dome. If I'm metering a light source and other lights are firing then I do lower the dome and will go as far as shielding the dome so as not to have contribution from those other sources. When I'm metering for my final taking aperture I have the dome raised.

Post #15, Feb 03, 2009 09:17:39 as a reply to Papa Carlo's post 5 minutes earlier.


Please call me Robert or Rob, not TMR
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Question on flash metering
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