Portrait Lighting

[iND]

When using two studio lights and metering for a portrait.

If I have a Main Light to camera left and Fill Light on camera axis and using f11 as shooting aperture.

If I meter the Main to f8 and the fill to f5.6 this should come close to f11 (but not exact)

In my calculation f8 + f8 = f11, but then there would be no contrast.

I would be off by f8 - f5.6 if I used f8 + f5.6.

Realize the light scale between f.56 to f8 is not linear.

So just for the sake of brain twister what is the value of f8 - f5.6 ?

and what is the value of x in f8 +fx = f11

[Sean Setters]

If I'm reading this correctly, the exposure value of f/8 - f/5.6 should be f/5.6. That's because when you double the light intensity of an exposure at f/5.6, you achieve f/8, therefore f/8 - 50% of itself (f/5.6) would equal f/5.6.

Therefore, I believe your original setup of f/8 + f/5.6 = f/9.5 (or 1/2 stop above f/8, 1/2 stop below f/11)

Therefore, f/8 + f/8 = f/11. However, that won't give you a ratio for main/fill differentiation. What you'd really need is something like f/9 + f/7.1, I think (but you'll need to double check me on that)

[iND]

Agree x = f8
The problem comes when trying to use the interval between stops.
Say you divide the interval from f5.6 to f8 into thirds.
The amount of light in those thirds is not equal.
When wanting to add light to F5.6 to reach f8 the last third takes more light than the first third.
This is all very well a discussion for mathematics and flow dynamics and in reality we would make the adjustment in the studio.
But if we want to do this problem in a physics class it is very interesting.
Light should follow the same physics as a fluid though a pipe minus the friction at the sides of the pipe.

This being said I am using a constant light source in my studio for portraits and I would like to adjust them without simply turning more light on or off and do not really want to move them.
What I would like to find is a dimmer type box that I can attach.
Realize that these lights are 1200- 2000 watts each.
Thank you

[cls]

Hi, I’d like to begin with a disclaimer: Use this info at your own risk (although not too much of a risk - in worst case you’ll end up with some bad exposures).

I’ve never experienced studio lights or metering in real life, but I’m interested in photography, math and general problem solving so I gave it a shot.

My understanding is that you begin with two known parameters: The desired ratio between two strobes and the desired aperture when both strobes fire. Based on some assumptions and simplifications I ended up with two fairly simple formulas:

f1 = sqrt[R/(1+R)] * f
f2 = sqrt[1/(1+R)] * f

R is the desired ratio and f is the desired resulting f number. f1 and f2 are the f-numbers corresponding to each of the single strobes.

An example to illustrate: Assume that you’d like a ratio of 3 between main and fill, and that for some reason you’d like to use f/6.3 for the shot. According to these formulas you’d get f1 = 5.5 and f2 = 3.2. (In practice you’d have to round off 5.5 to the nearest f-stop 5.6.)

Adjust the power of the main light only to get the exposure right at f/5.6 and adjust the fill light only to give the right exposure at f/3.2. Then stop down to the desired f/6.3 and fire both strobes. If I’ve got it right you should get something similar to what you wanted. (But on the other hand, I might as well be completely wrong...)

[peety3]

My degree is in Electrical Engineering, yet my head hurts. Set up the lights, apply some first-guess settings, then push the little round button and review on the screen! Add salt/pepper to taste, and then "Please Enjoy!" already!

[clemmb]

Just buy this for $23 +shipping
Measure lights and shoot

[cls]

I can do math, I just choose not to when I can avoid it.

In the method I was taught a ratio of 1:3 between two lights puts the fill light 1.5 stops lower than the key light which is only a small contribution. To wind up at f6.3, or at least very close on the first try, I would just set up the key light at the full stop below, in this case 5.6, and the fill light at 1.5 stops lower, in this case f3.5 and just see what happens. I would likely wind up repositioning at least one light anyways which would subtly change the ratio as well as adjusting the ratio on the fly anyways until I was satisfied with the balance for that particular shot.

For me it is good enough to get reasonably close to the ratio and aperture I think I might want with this simple method. Sure if I had a Cine meter with Lux and Cd/m^2 displays I could get each light dialed in just exactly to obtain the perfect ratio. I am not convinced it is worth the expense, time, or effort as I do not know of anyone who could tell the difference between lights at 1:3 and 1:2.9 anyways. And what if it wasn't the ratio I really wanted for the shot? Would I take the time to reset the ratios perfectly to some other number, which may or may not be what I want in the end, or would I just tweak settings on the fly relying on the scale of my light adjuster to keep it close to the total combined exposure I want?

Seems like a reasonable method to me. I believe that in practice it must be better to use some method like yours rather than to calculate some "exact" settings. Regarding those formulas I was just curious to see if I could derive something useful. (Unlike you I happily do some math when an interesting problem occurs ;-)

[Thom Deevers]

Ok - set your exposure for the value of your main light. The fill light generally is 1 stop less to fill in the shadow detail. So if your main is f/8 set your camera to f/8 and shoot away. That will set your 1:3 ratio.

Adding the fill light will not bump your exposure to f/11.

Don't worry about the math. It will hurt your head.

General rule - set the exposure of your main light as the f/stop on your camera.