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Photography Question 
Jon Dane

f stops with flash

okay stupid question

I understand shutter speed and aperature on the camera but when someone is talking about their studio lights and says "main light is set at around F6 and my fill at F5"?

thanks for assistance

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1/14/2009 3:31:18 PM

Pete H
BetterPhoto Member Since: 8/9/2005
  Studio flash meters indicate a F number once ISO is cross referenced.

(i.e) Dial in your ISO on your flash meter, fire flash and the meter will show you f/stop to set.

One warning: All meters are not created equal, but with a few trials, it is pretty easy to find if your's is under or over exposing..then it's a simple matter to dial in the bias on the flash meter.


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1/14/2009 6:36:34 PM

Jon Dane   okay
so I do my main light alone and then my fill light so they are at different f stops
and then when I do the pic and they are sync that's what will happen

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1/14/2009 8:25:25 PM

Pete H
BetterPhoto Member Since: 8/9/2005
  Pretty much Jon.

In a two light setup, you may want one light to read f/11 and the other f/5.6.
You check each light individually.
The difference in f/stops creates the lighting ratio. The above would be a 2 stop difference or 2:1...classical butterfly portraiture and rembrandt lighting uses 3:1. Theatrical portraiture use 4:1 and even 5:1
No real hard and fast rules here unless you desire to imitate the masters.

Attaining just the right (look) is more than just getting the right ratio (i.e) Hard light, soft, diffused, size of light source all come into play.

Hope that helps a little,


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1/14/2009 9:36:02 PM

Alan N. Marcus   Hi Jon,
The classic portrait setup creates an illusion of a picture taken using just one light. This light is generally positioned high and off to the side to simulate high sun. The classic view of just one light takes its cue from the old European fine art masters. We who work in the photographic world are handicapped when we work with just one light. Likely the shadows casts by a one light setup will be too deep and the resulting image will present too much contrast. Our countermeasure is to somehow soften the shadows cast by the one light setup. To accomplish this, the photographer can bring to bear a second light fixture.

Now the one light placed high is called the “main” or “key” light. The second lamp is called a ‘fill”. To preserve the one light illusion, the fill is caused to cast a light that is subordinate to the main. Generally the difference in brilliance, at the subject plane, is measured in f/stops. Each f/stop is a 2x change in light energy. Stated another way, the main plays on the subject with a greater intensity.

The intensity difference between the main and fill allows the photographer to set the contrast of the finished image. If the main and fill are allowed to be equal, the resulting image will be flat and lifeless. This is true because the human face is a three dimensional object, however our media is two dimensional. Thus we need shadows cast by the nose and hollows to give our images an illusion of depth.

Generally the difference between the main and fill is set whereby the fill is subordinate by one f/stop. This relationship is called a 3:1 ratio. The 3:1 setup is called the “bread and butter” setup because generally clients and critics like it.
Next is the 5:1 set-up. This is fashioned by causing the fill to be two f/stops subordinate. The 5:1 is a more constrasty lighting, often considered quite masculine. Next is the 9:1 ratio. This is very contrasty lighting. Produced by causing the fill to be three stops subordinate. The 9:1 ratio is very contrasty while preserving some shadow detail. It is considered to be theatrical lighting.

The placement of the main is an art form. We place the main based on how best shadows enhance the subject’s face.

The placement of the fill is easy. The fill is placed at lens height somewhere on an imaginary line drawn camera to subject. The fill may stray off to the side to prevent the fill from casting shadows of equipment or the photographer on the subject. This placement and subordinate light energy maintains the one light illusion by filling shadows from the camera’s viewpoint.

Alan Marcus (marginal technical gobbledygook)

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1/14/2009 11:43:38 PM

Alan N. Marcus   Lighting Ratio

Why one f/stop difference is called 3:1:
Why two f/stops difference is called 5:1:
Why three f/stops difference is called 9:1:

The “main” placed high to simulate high sun casts shadows. The shadows reveal that the face is 3 dimensional. The shadows are present because the face has contours. It is shadows that create the illusion of depth.

We can measure the intensity of the main light using a light meter. The meter likely returns data in the form of f/stops however this intensity can be traced back to foot candles or watt seconds or standard candles or other units of light measurement. For giggles, let’s describe the setup’s main light intensity as 1000 units.

Now the frontal areas like forehead and cheeks receive this 1000 units of light. The shadows cast by the protrusions like the nose and rills are void of main light i.e. they don’t receive much of this 1000 units. Unless scatter or reflected light trickles in to the shadow areas, they will be dimly lit. The problem here is the human eye has a far greater dynamic range than film or digital chip thus we (the photographer) can see detail in these shadow areas whereas the camera can’t.

The countermeasure is the application of a second lamp to soften the shadows. This lamp called a fill is placed near the camera at lens height. This fill lamp is adjusted to delivers 1 f/stops less light to the face. 1 f/stop reduction = 50% reduction = 500 units. Now we have 1000 units from the main + 500 units from the fill. Thus the frontal areas receive 1000 units from the main + 500 units from the fill = 1500 units of light. Now the shadow areas, not illumined by the main, receive just 500 units from the fill. We calculate the light on the face as 1500 in frontal area and 500 in shadows areas. We write this as a ratio 1500:500. Now ratios like fractions can be reduced for simplicity. We divide both sides by a common denominator. In this case 500, doing so we get a simpler ratio i.e. 3:1.

If the fill is reduced one more f/stop in intensity to 250 units, the face receives 1000 units from the main + 250 from the fill = 1250 units in the frontal area and 250 units in the shadows. We write this as 1250:250 both sized divisible by 250 thus the reduced simplified ratio is = 5:1.

If the fill is reduced one more f/stop in intensity to 125 units, the face receives 1000 units from the main + 125 from the fill = 1125 units in the frontal area and 125 units in the shadows. We write this as 1125:125 both sized divisible by 125 thus the reduced simplified ratio is = 9:1.

More truly marginal technical gobbledygook from
Alan Marcus

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1/15/2009 7:50:09 AM

Jon Dane   Thanks this has been great help.
Next question would be...
so now my camera aperature really has nothing to do with these fstops for flash
I played around today and my lights (i think) are at a 3:1 they are at f4 and f5.6 but my camera to get a good read iso 160 f9-f11 125
So do I just play around with my camera fstop or is there some "scientific" way to do it?

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1/17/2009 6:00:01 PM

Pete H
BetterPhoto Member Since: 8/9/2005

Your camera F/stop is set according to the main or key light.

If your main is registering f/8 and your fill f/5.6; your camera is set to f/8. It's all based off the main light.


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1/17/2009 8:10:58 PM

James N. Carpenter
BetterPhoto Member Since: 11/28/2006

Simple way to set your ratios. Set your camera to the desired F/stop. F/8, F/10, etc. Set your main light with the meter to that same setting. Divide your desired ratio (say 3:1) into that F/stop. (8 divided by 3 = F/2.6) Set your fill to F/2.6 and take a shot. Check your histogram and picture to see if you have the desired effect.

Hope this helps.


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2/13/2009 9:43:46 PM

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