Key/Fill ration question

CambsPhoto

CambsPhoto

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Fairly straight forward question but I'm having a tad of trouble with exposure on this one.

In a studio environment with a key and fill light (both 500 watt monoblocks) I have key light set at f5.6 after taking a flash meter reading for given shutter speed of 1/160th second.

I put in fill light and, in this instance, I want the ratio to be the same i.e. f5.6 also. So both heads are set to half power and are at the same 45 degree axis either side of camera and both giving a reading (when taken individually while the other is turned off) of f5.6.

So, individually they both give a flash meter reading of f5.6, but when they are both turned on at the same time and I take a reading from the centre of the subjects face I get f8, which stands to reason as some of the light from each will be spilling onto the other side of the face also.

So, what f-stop should I be setting the camera to? If, I were to have the fill light set to take a flash reading of f2.8 i.e. two stops lower then the key at f5.6, I'd be setting the camera to f5.6, right? So that would suggest that both heads set to f5.6 would also suggest that I set the camera to f5.6, correct?

I'm asking as if I set camera to f8, the reading I get when both flashes fire into the flash meter, the images seem about a stop or so under exposed. At F5.6 they seem about half a stop over.

What's the school of thought on this, with two lights set at the same distance at the same power and both, individually, give f5.6, but both together give f8, what f-stop do I set camera to?
 
To clarify, two lights either side of camera both same distance from model, both give a flash reading of f5.6 individually, but together they give f8. What f-stop do I set camera to?
 
CambsPhoto said:
What's the school of thought on this, with two lights set at the same distance at the same power and both, individually, give f5.6, but both together give f8, what f-stop do I set camera to?
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Depends on what you are exposing for... it's still just a meter and it's just telling you what setting will place midtones in the middle of the exposure.
I would typically say f/8 as that should prevent blowing out whites/highlights.
 
Oh, and incident meters have a bit a variability to them as does the camera's ISO value; which means they may not give exactly what you are expecting unless you calibrate the meter to your camera.
 
sk66 said:
Depends on what you are exposing for... it's still just a meter and it's just telling you what setting will place midtones in the middle of the exposure.
I would typically say f/8 as that should prevent blowing out whites/highlights.
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Makes sense. When I took test shots at both I prefered something in the middle, about f6. In Capture One with 'highlight alert' turned on, even at f5.6 nothing flashes up in highlight allert.
 
A question about actual lighting, not gadgets - makes a welcome change:)

I'll try to give an answer, but as I don't know anything about the subject it could easily be wrong.

Assuming that the meter is correctly calibrated and is being used in incident mode, the correct answer is f/8.
mike weeks said:
First thing is that fill ones not come from the other side, it normally comes from where the camera is so that if FILLS the shadows caused by the off axis Key light
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Correct. The function of a fill flash is to fill in shadow areas (as required). Except in very specialised situations, this means that the fill light must be situated on axis with the camera lens (can be either above or below, usually behind for convenience).
If you're lighting a human face then placing a light each side creates a strange lighting effect, because the person is being lit by two suns, and there's only one sun on this planet. And of course this lighting arrangement also gives everyone a fat face, because the edges of the face are inevitably brighter and this draws attention to the width. There are times when this lighting setup can work extremely well, for example with some fashion images, but generally it isn't great.

if that's the effect that you specifically want, consider having the lights at equal effective brightness and in the rimlight position - a bit behind as well as at the sides, and pointing forwards a bit, quite dramatic. This will nearly always need a fill light too, which can be literally anything - if you only have 2 flash heads you could use a flashgun on camera for fill.

For most portraits though, most of the time, a good starting point is a single flash directly in front of where the subject is looking, fairly high. This shapes the face, creates the right shadows in the right places and is entirely natural, because that's a natural angle not only for the sun but also for most common artificial lighting, e.g. room lighting. A fill light is nearly always needed, just set it to whatever level of brightness works (don't worry about setting it to a specific ratio, just do what looks right to you).
mike weeks said:
There are 2 schools of thought

1. Meter towards camera
2. Meter towards light

Neither is the perfect solution but in my experience metering to camera preserves the highlights
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Mike is being polite to the people who don't understand simple physics - in all normal situations, the meter must point towards the camera for the correct exposure. The only time that it helps to meter towards the light is when the light is coming from a very extreme angle, e.g. behind, and in this situation the meter reading will indicate the level of brightness of that specific light, not the necessary exposure.

FWIW, I view meters as nothing more than a tool that can be useful sometimes. I very rarely use them.
 
sk66 said:
Then I would expose at f/5.6 and pull it down in post if desired. That will give the sensor the maximum amount of light and maximize data/accuracy/quality.
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sk, if it were a choice of underexposing a little and overexposing a little would you go for the latter, to give the sensor more detail? Nice tip on getting maximum amount of light to the sensor.
 
CambsPhoto said:
sk, if it were a choice of underexposing a little and overexposing a little would you go for the latter, to give the sensor more detail? Nice tip on getting maximum amount of light to the sensor.
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I would choose to overexpose the histogram, not the sensor (i.e. not causing clipping). This becomes much more important the darker the subject/scene is; because if you record it as all dark tones you actually generate very little data/accuracy, and you can be quite limited with what you can do with the image (post editing).

Edit: this is actually the basis of ETTR, exposing to the right, pushing the histogram right of where it normally should be.
 
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Thanks, sk66/ Mike, forgot to mention, I'm shooting in RAW so can grab detail back from where it can't be seen, but I'm not aiming to overexpose that much. From my tests the histogram looks fine, even slightly under if anything, but the highlights don't flag anything in the hightlight-warning.
 
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