Metering a few light sources for light ratios

[marius_s]

Hello,

Since google gives me too conflicting results, i've decided to ask this question here. Let's consider a simple example:

- we have a model and 2 light sources
- model is facing the camera directly
- lights are on both sides in front of the model, facing the model at 45 degrees
- both lights provide the same output (flat lighting) and need to be converted for a 1:2 portrait lighting

This is where i get a bit confused with metering. Let me tell you what i would do:

- set a meter to 1/160 at iso 100
- turn on the left light
- put the meter next to the model's right cheek, turn it towards the light and take a reading
- let's say it metered f4.0
- now switch to the right light and take readings while adjusting power output to get f5.6
- turn on both lights
- put the meter next to the center of model's face and turn it directly towards the camera
- take a reading for final exposure

Yet something feels wrong to me here. Is that a correct way of doing it? I guess the main questions are:

a) do i turn the meter towards the light source when measuring individual lights?
b) do i place the meter at the relevant part of the subject's face when measuring individual lights or should it always be in the same position?
c) do i even need to calculate the "final exposure" or should i use the one from the more powerful ligh source?

Thanks for your advice

 

[Dao]

First of all, I do not have much experience for studio setup. But my question to you is, why don't you just measure the final exposure? Like measure both lights at the same time for overall exposure. Put the meter near the area you like to meter and fire the strobes at the same time (or turn all continuous lights on).

 

[gsgary]

your camera will be set to the light with the smallest aperture, forget 2 lights from the front (very flat lighting, set one light at 45 to sitter facing slightly downwards and the other from behind higher at 1- 1.5 stops lower than main light and add a reflector to opposite side at the front or infront of model to fill under her chin

 

[Helen B]

a) do i turn the meter towards the light source when measuring individual lights?

Yes, and use the dome down, or cupped in your hand if there are other light sources present if you want to measure that light. However, for the key you will probably measure all lights together, so the dome will be up and the meter facing the camera for that measurement.

b) do i place the meter at the relevant part of the subject's face when measuring individual lights or should it always be in the same position?

At the relevant subject location, often the face.

c) do i even need to calculate the "final exposure" or should i use the one from the more powerful ligh source?

The are some circumstances when you could use the brightest light, but you would usually have taken the 'bright' reading in the same way as the overall reading. As always with light measurement, it is best to understand what you are doing and why, so that you aren't just following rules and you can use the most appropriate technique.

You may find ratios aren't your thing, and just light by eye.

Ratios are traditionally measured as brightest : darkest, ie key+fill : fill. Some people have difficulty with that, and the simpler technique of simply measuring key : fill has come into use. You need to know that both exist, and which one is being referred to. It is, however, all a bit academic though useful, and there are many opportunities to argue about how to measure lighting ratio in both normal and special circumstances.

 

[marius_s]

gsgary, sorry, but this is not what i asked. This was just the most simple example i could come up with.

 

[marius_s]

Dao, i can't measure JUST overall exposure, since i need my lights to have an intensity ratio (meaning one has to be N times brigher than the other). So the only way to calculate this is to measure each light separately.

 

[marius_s]

Thank you, Helen. I didn't understand why i would move the dome down though, but i guess i don't know that part about light meters yet, so i'll just go ahead and google it

 

[MLeeK]

So that the light from other sources isn't affecting your measurement.
Do you actually have a meter or are you using the in camera meter?
What exactly does the assignment read that you have to do?

What are the settings on the back of your lights? They should easily tell you what the output is and you should be able to set the ratio according to that.
Here's a tutorial that may also help you a bit: Portraits with Two Lights: Adding a Fill Light

 

[gsgary]

gsgary, sorry, but this is not what i asked. This was just the most simple example i could come up with.

Basically your fill light will be 1 stop lower than your main light, do you know what a stop is ?

 

[Derrel]

You have ONE BIG problem: You wrote: - we have a model and 2 light sources
- model is facing the camera directly
- lights are on both sides in front of the model, facing the model at 45 degrees
- both lights provide the same output (flat lighting) and need to be converted for a 1:2 portrait lighting

Okay...the problem is that the two lights are going to basically cancel one another out if they are left at the same distance. This is a terrible way to light...two lights, equal power, and equal distance, aimed in at the subject at 45 degrees....ummmm....that's kind of like a copy stand lighting setup where the goal is perfectly flat, even lighting with an exact 1:1 ratio....TOTALLY FLAT, totally EVEN lighting.

What you ought to do is angle the MAIN light in at the subject from 20 to 50 degrees,depending, and then aim the FILL light straight ahead, from a position right NEXT to the camera.

 

[Helen B]

Thank you, Helen. I didn't understand why i would move the dome down though, but i guess i don't know that part about light meters yet, so i'll just go ahead and google it

You move the dome down (if it is retractable, of course) when measuring to find the strength of one light, if other sources of light are present. The alternative method is to shield the dome in your hand. The idea is that the receptor should only receive light from the source you are measuring. It can also be done with a flat receptor - which is how the dome behaves when it is retracted.

Generally, when you are measuring for exposure you have the dome up, so that it can do what it is designed to do: integrate light coming from all angles except from directly behind, with light coming from the front having more weight than light coming from the sides and rear sides (the dome should not be blocked from light coming from the rear sides).

You do sometimes use the dome down (or a flat receptor) when you are taking an exposure measurement - and you do it when the subject is flat. Flat subject - flat receptor on your light meter. 3D subject - 3D receptor. The intention is to get the receptor to mimic the subject. Technically the dome has a cardioid (heart-shaped, so they say) response and a flat receptor has a cosine response.

A 2:1 lighting ratio can be interpreted in two ways:

Internet: The key is twice the fill, so there is about a stop and a half difference between any areas lit by the key + fill and the areas lit by only the key, if there are areas lit by both lights. This would be called 3:1 using the classic method. If there are no areas lit in common then it would be 2:1 using the classic method. This variation is the problem with this method, if anyone really cares enough.

Classic: The key + fill is twice the fill, so there is a stop difference between the key + fill and the fill, or a stop difference between the bright side and the dark side, irrespective of how the lights are arranged. It doesn't matter if the key and the fill do not illuminate common areas - the lighting ratio simply describes the brightness difference in the lighting.

 

[table1349]

Classroom Articles - Search: lighting ratios

 

[Tee]

Marius:

Here's 2 good tutorials from Mark Wallace about light meters and ratios. These are great vids to get you on your way.

http://youtu.be/zQBa2y9NYIE

http://youtu.be/IdcdWkc7xEI

 

[kundalini]

What you ought to do is angle the MAIN light in at the subject from 20 to 50 degrees,depending, and then aim the FILL light straight ahead, from a position right NEXT to the camera.

This is my general setup. You'll find that having the Fill on the same side of the camera as the Main to give you an easier setup to create the lighting ratio. I'm partial to darker, moodier images so a 3:1 ratio is fine with me and particularly with males, a 4:1 ratio can look good.

 

[marius_s]

So that the light from other sources isn't affecting your measurement.

Do you actually have a meter or are you using the in camera meter?
What exactly does the assignment read that you have to do?

What are the settings on the back of your lights? They should easily tell you what the output is and you should be able to set the ratio according to that.
Here's a tutorial that may also help you a bit: Portraits with Two Lights: Adding a Fill Light

Then there's no need to remove the dome then as i only have one light source at a time when measuring them individually.
There is no assignment whatsoever, i just wanted to know where to point the meter when measuring individual lights ;] And yes, i do have one
I know there's settings on the lights themselves, but imagine that the're ain't - my method would work in both cases. The only problem is that it's a little more difficult than looking at the output values on the lights themselves.

Thanks for the explanation about the dome - makes sense now