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FORUMS General Gear Talk Flash and Studio Lighting
Thread started 25 Nov 2017 (Saturday) 16:53
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Metering to balance flash and ambient light

 
DaviSto
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Nov 25, 2017 16:53 |  #1

I am trying to understand how to use a flash/light meter to assist in balancing flash and ambient light ... and I've wandered into a swamp of deep confusion. I know that there is something very simple that I am missing ... but what is it?

Mine's a Sekonic meter and I looked to the Sekonic site for help. Here I find (for the outdoors fill-flash case):

"The first step, then, is to take a reading on the subject. On an overcast day, the foreground and the background should be receiving approximately the same amount of light. A typical reading on a dull, cloudy day at ISO 100 would be about 1/60 at f/5.6. Most photographers prefer their fill flash to be about two-thirds to one stop below the ambient light, but this is entirely a creative decision. Assuming that you prefer one stop, take flash readings until the meter reads "f/4", one stop below the ambient level of f/5.6."

It is at the last sentence that terminal brain-fade sets in. How is it possible to add light using a flash-gun(s) and then get a lower light meter reading than the initial ambient reading? The ambient light hasn't gone away.

At this point, all further advice and instruction becomes quite useless to me. I just can't get my head around the basic idea.

I know you can chimp your way around these issues and just use trial and error to get a good mix of ambient and flash ... and I do. But I would like to be more systematic about things. I'd like to understand what I am doing.

I'd be most grateful for advice.


Comment and (constructive) criticism always welcome.

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MalVeauX
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Post has been last edited 22 days ago by MalVeauX. 2 edits done in total.
Nov 25, 2017 17:08 |  #2

Remember the metered exposure value is essentially an average of falling light at that particular location. It will contain shadows and highlights in your actual exposure where that light isn't falling (under a chin, their entire face if the sun is behind them, etc, the entire ambient histogram is not F5.6 at every point (for example). If you're then applying fill flash at a lower value, such as F4, you're filling the shadows up to F4 (maybe the deep shadow under a chin was nearly black for example, even though you metered F5.6 at the subject's forehead/nose), but not past F4, and not matching ambient exposure, so you're not overtaking any light brighter than shadows up to F4. You're lifting shadows (filling shadows) up to a value with light.

Very best,


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DaviSto
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Post has been last edited 22 days ago by DaviSto. 4 edits done in total.
Nov 25, 2017 17:23 as a reply to MalVeauX's post |  #3

Thank you for a very quick response ... but I am still in the swamp.

The way the Sekonic advice is phrased, I'm assuming that I, notionally, would take an initial incident light meter reading at the subject's location and get (in Sekonic's example) a reading of T 1/60, F/5.6, ISO 100. Fine.

Then, with the meter at exactly the same position, I would fire my flash gun(s) and take trial readings (again, measuring incident light) at different power levels until I get to T 1/60, F/4.0, ISO 100. Now I am, supposedly, set.

But ... how is that possible within the laws of physics? I'm adding light ... but I am supposed to get a lower incident light meter reading.

Head is spinning.

Edit: Is it possible that the Sekonic advice is just some huge 'misnprit' and that the first case is supposed to be F/4.0 (ambient alone) and the second is supposed to be F/5.6 (with flash added)???


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OceanRipple*
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Post has been last edited 22 days ago by OceanRipple*. 2 edits done in total.
Nov 25, 2017 17:39 |  #4

DaviSto wrote in post #18504028 (external link)
Thank you for a very quick response ... but I am still in the swamp.

The way the Sekonic advice is phrased, I'm assuming that I, notionally, would take an initial incident light meter reading at the subject's location and get (in Sekonic's example) a reading of T 1/60, F/5.6, ISO 100. Fine.

Then, with the meter at exactly the same position, I would fire my flash gun(s) and take trial readings (again, measuring incident light) at different power levels until I get to T 1/60, F/4.0, ISO 100. Now I am, supposedly, set.

But ... how is that possible within the laws of physics? I'm adding light ... but I am supposed to get a lower incident light meter reading.

Head is spinning.

Edit: Is it possible that the Sekonic advice is just some huge 'misnprit' and that the first case is supposed to be F/4.0 (ambient alone) and the second is supposed to be F/5.6 (with flash added)???

'Head is Spinning' is correct given how badly written their piece is.
https://www.sekonic.co​m ...-daylight-fill-flash.aspx (external link)

Your Edit is not quite right tho'.

Note their last line includes " .. take flash readings until the meter reads "f/4" .. ". They've got themselves into a muddle.

Does your meter offer 'Flash percentages'?

Maybe see Joe Brady here:
https://www.youtube.co​m/watch?v=Y01cwu4RouI (external link)




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MalVeauX
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Nov 25, 2017 17:42 |  #5

DaviSto wrote in post #18504028 (external link)
Edit: Is it possible that the Sekonic advice is just some huge 'misnprit' and that the first case is supposed to be F/4.0 (ambient alone) and the second is supposed to be F/5.6 (with flash added)???

If ambient (incident light, falling light, doesn't matter if your subject is dark or light, incident light will meter the same) was metered at F4, and flash was shot at them and metered F5.6, then the flash was 1 stop brighter than the incident light and potentially overexposed (depends on the subject brightness/darkness, color, etc).

I think what's maybe the block you're hitting is what incident light is, compared to reflected light.

Very best,


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DaviSto
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Post has been last edited 22 days ago by DaviSto. 2 edits done in total.
Nov 25, 2017 17:55 |  #6

OceanRipple* wrote in post #18504032 (external link)
'Head is Spinning' is correct given how badly written their piece is.
https://www.sekonic.co​m ...-daylight-fill-flash.aspx (external link)

Your Edit is not quite right tho'.

Note their last line includes " .. take flash readings until the meter reads "f/4" .. ". They've got themselves into a muddle.

Does your meter offer 'Flash percentages'?

Yes ... it's the L-478D.

Up 'til now I have been trying things out mostly sitting at a desk rather than in the real world outdoors. Because the ambient level is low indoors, my trial runs are not coming up with very interesting % results. The faintest 'dab' of flash at 1/128th power is giving a flash percentage of 90% or so. Thinking about it, the problem is probably that, in playing around to try to understand this stuff, I have the flash gun too close to the meter (I've been too lazy to get out of my chair!).


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MalVeauX
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Nov 25, 2017 18:04 |  #7

DaviSto wrote in post #18504048 (external link)
Yes ... it's the L-478D.

Up 'til now I have been trying things out mostly sitting at a desk rather than in the real world outdoors. Because the ambient level is low indoors, my trial runs are not coming up with very interesting % results. The faintest 'dab' of flash at 1/128th power is giving a flash percentage of 90% or so. Thinking about it, the problem is probably that, in playing around to try to understand this stuff, I have the flash gun too close to the meter (I've been too lazy to get out of my chair!).

Incident meter indoors is basically nill. So flash will always produce essentially 100% of the exposure when looking at ratios indoors.

Very best,


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DaviSto
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Post has been edited 22 days ago by DaviSto.
Nov 25, 2017 18:11 |  #8

MalVeauX wrote in post #18504037 (external link)
If ambient (incident light, falling light, doesn't matter if your subject is dark or light, incident light will meter the same) was metered at F4, and flash was shot at them and metered F5.6, then the flash was 1 stop brighter than the incident light and potentially overexposed (depends on the subject brightness/darkness, color, etc).

I think what's maybe the block you're hitting is what incident light is, compared to reflected light.

Very best,

I am looking at this entirely as an incident light question ... no spot readings involved, no use whatsoever of the camera's reflected light metering system ... just the stand-alone Sekonic meter with the white dome up, one or two flash guns and a radio trigger (the camera is sat safely in my bag).

I use incident light metering extensively in my available light shooting and I am comfortable with it and understand how to use it (I'll use the meter's spot metering facility very, very occasionally too ... but, mostly, it isn't needed for what I shoot).

The step-up that I am trying to make is to use incident light metering as part of the process of adding flash lighting into the mix. The Sekonic web-site explanation is amazingly clear at first reading ... but it doesn't work. And the more I mull it over, the less sense it makes.

Edit: Is your thinking that -- without making any serious attempt to spell it out -- Sekonic is advising you take a reflected light reading off the principal subject without flash ... and then an incident light reading with flash? That seems to be a complicated way of going about things. It might have its uses, I suppose. But the Sekonic piece is meant to be advice at the most basic level (which is exactly what I need) and comes across that way ... except it seems impossible to apply.


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Lotto
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Nov 25, 2017 18:36 |  #9

Then how do you measure the flash power at f4 when the ambient already at f5.6?


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DaviSto
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Nov 25, 2017 18:39 |  #10

Lotto wrote in post #18504073 (external link)
Then how do you measure the flash power at f4 when the ambient already at f5.6?

Indeed.


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MalVeauX
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Post has been last edited 22 days ago by MalVeauX. 3 edits done in total.
Nov 25, 2017 20:42 |  #11

That's not what I was trying to suggest, I was just trying to make sure you were looking at the light as incident completely and not as reflected, because incident light again doesn't care if your subject (say a person's face) is any color or light or dark and it's just measuring falling light and it's averaged.

How are you rationalizing the idea of adding light to light? Are you thinking adding a less luminous light to a more luminous light add together and are a sum of the two? Or are you thinking something else? It would help to unravel what's blocking the idea.

I try to think of it like a histogram and your example of F5.6 is the average of the whole thing. There are peaks & troughs representing shadow and highlights in there maybe down to F2.8 and up to F11 or something. And when you are using a light to fill shadows, they're the troughs around F2.8 and F4 in that histogram that you're targeting (on the subject). Since the light doesn't result as the sum of all light, you're not increasing luminosity on areas of the subject that are already as bright as F5.6 (or brighter even), you're just filling those troughs in the histogram that were below F4, up to F4. Maybe my wording on this is not perfect, but maybe it helps with the idea.

Very best,


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Wilt
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Post has been edited 21 days ago by Wilt.
Nov 25, 2017 21:58 |  #12

Lotto wrote in post #18504073 (external link)
Then how do you measure the flash power at f4 when the ambient already at f5.6?

Yeah, the writer at Sekonic certainly messed that one up!

quote=DaviSto]Edit: Is it possible that the Sekonic advice is just some huge 'misnprit' and that the first case is supposed to be F/4.0 (ambient alone) and the second is supposed to be F/5.6 (with flash added)???

Less likely a 'misprint' and more likely an undetected 'brain fart'


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dmward
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Nov 25, 2017 22:46 |  #13

i have no idea how modern meters attempt to measure both ambient and flash exposures.
What I do know, from using selenium cell incident meters for measuring ambient and a primitive flash meter for measuring the strobe contribution is that its critical to get your ambient exposure set to give you the image that you want overall. Then set the flash to add light where you want it to complement the ambient. If its fill that's one thing. If its key that's another. Fill is a bit less than ambient, key is a bit more.

In my view flash should not be the obvious key light with ambient except for special effect.


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bobbyz
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Post has been last edited 21 days ago by bobbyz. 2 edits done in total.
Nov 26, 2017 00:14 |  #14

I just use the % flash/ambient thing reported on my L-358 to control where I want.

Balanced (at least to me)

IMAGE: http://www.bobbyzphotography.com/img/s9/v90/p2240840741-5.jpg

More flash

IMAGE: http://www.bobbyzphotography.com/img/s7/v152/p2240840739-5.jpg

Still more flash
IMAGE: http://www.bobbyzphotography.com/img/s11/v36/p2240840742-5.jpg

There is a thread here where it shows how % translate to Flash vs Ambient ratios, I will see if I can dig it. I never bother now and just rely on simple % and look at the LCD.

Here is where there is just a kiss of flash.
IMAGE: http://www.bobbyzphotography.com/img/s10/v98/p2414466073-5.jpg

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DaviSto
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Post has been last edited 21 days ago by DaviSto. 4 edits done in total.
Nov 26, 2017 06:12 |  #15

bobbyz wrote in post #18504192 (external link)
I just use the % flash/ambient thing reported on my L-358 to control where I want.

Balanced (at least to me)

I'm pretty sure that is where I will end up (all being well). But I wanted to have a more basic understanding.

Anyway, I think I've worked my way round to understanding what the Sekonic advice was actually meant to be saying (although it certainly didn't actually say it). It should have read something like below.

"Suppose the ambient light falling on the subject gives an incident light meter reading of 1/60, F/5.6 at ISO 100 (fairly typical for an overcast day) and, for simplicity's sake, that the background is similarly illuminated. Although it's actually a matter of personal preference and intended effect, for fill flash a lot of photographers like to have flash contribute about one stop less light than ambient to the overall subject illumination. So, if we could notionally switch off ambient and read flash in isolation from ambient, we would be looking for a light meter reading from just the flash of 1/60, F/4.0 at ISO 100.

When the flash illumination is set to this level, ambient light will be contributing two thirds of the total exposure value and flash will be contributing the remaining one third (or roughly 30%).

We can't actually switch off ambient and get a flash reading in isolation, of course; we have to meter flash and ambient together. If flash is to contribute one third of total subject illumination, then the total exposure value for the shot (metered at the subject) should be half a stop higher than for ambient alone. In our example, that means adjusting the flash power and/or positioning to give a flash meter reading of 1/60, F/6.8 at ISO 100. The camera should itself be set at 1/60, F/5.6 and ISO 100 to take the shot (i.e. to match the ambient only reading).

This amounts to a lighting ratio of about 30% and will usually result in an acceptable fill-flash exposure. But it should be regarded as just a good starting point to achieve any particular intended effect."

Does this make sense?

BobbyZ, it would be great to hear what sort of lighting ratios you would suggest for different levels of fill illumination ... and to get that pointer to the earlier lighting ratios thread.

Edit: Was this the thread? http://photography-on-the.net ...showthread.php?p=18​363259


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Metering to balance flash and ambient light
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