Lighting head distances from subject

Started Jun 26, 2008 | Discussions
riddell Veteran Member • Posts: 3,762
Lighting head distances from subject

Ok. I know what I am doing here through experience, but I just want a more formal, technical explantion. I know that there is an offical term / law used for these rules. If someone gives me that, then I don't mind going away and reading about it myself.

What I actually want to understand is why?

If I have a flash head and a subject, you can can in effect increase the amount the subject is lit without adjusting any settings by moving the flash head nearer the subject, but why if the head is placed too close, does the light seem to get harsher? and why can I more easily burn out the whiter parts of the subject without effectively lighting the darker parts?

Also I don't quite understand the relationship when say I have a reflector, which say has a spread of 100º that surely means that everything with that 100º arc should be lit (lets assume I have a quality reflector setup here at this point with nice even light) so why again does that seem to alter dependant upon distance?

As I said I know through experience how to avoid and / or use this to my advantage, but I just don't know why? and would like to!

john g Regular Member • Posts: 430
Re: Lighting head distances from subject

Hi Riddell;

I think I can explain some, but I should say that most people on this site have different opinions from me on these points.

1. When you move the flash head forwards you ARE increasing contrast. What you see with your eyes is correct. People here say that it softens the light, but as you can plainly see, it does not. For any given diffusion method, moving the light forward increases contrast, or, you can think of it in terms of "Specularity". Your ratio of highlights to shadows increases. You can call it "harsher" or "hotter", if you like. The proper approach then is to choose you light source based on contrast or size and do a final adjustment with your eyes when you observe how the light looks on the set. If it's a little too hot, back the light out, if it's a little flat (or lacks "presence"), bring it in. Sets should look a little softer than seems to be right in real life.

2. Regarding Coverage, certain light sources have "hot spots". Beyond that, there is a balance between distance and coverage based on the inverse square law. Because most lighting equipment for still photography is fairly simplistic you can usually calculate Coverage based on the size of the light source. So, if you're trying to light a full length portrait and you believe that you can cover the whole body with a 100° reflector @ 3 meters, forget about it. If you need a lot of coverage use a big light, or better yet, create a better lighting setup using several smaller, less covering lights. More like a "pools of light" approach.

Good luck!

riddell wrote:

Ok. I know what I am doing here through experience, but I just want a
more formal, technical explantion. I know that there is an offical
term / law used for these rules. If someone gives me that, then I
don't mind going away and reading about it myself.

What I actually want to understand is why?

If I have a flash head and a subject, you can can in effect increase
the amount the subject is lit without adjusting any settings by
moving the flash head nearer the subject, but why if the head is
placed too close, does the light seem to get harsher? and why can I
more easily burn out the whiter parts of the subject without
effectively lighting the darker parts?

Also I don't quite understand the relationship when say I have a
reflector, which say has a spread of 100º that surely means that
everything with that 100º arc should be lit (lets assume I have a
quality reflector setup here at this point with nice even light) so
why again does that seem to alter dependant upon distance?

As I said I know through experience how to avoid and / or use this to
my advantage, but I just don't know why? and would like to!

Peter Berressem Forum Pro • Posts: 10,647
Re: Lighting head distances from subject

riddell wrote:

If I have a flash head and a subject, you can can in effect increase
the amount the subject is lit without adjusting any settings by
moving the flash head nearer the subject, but why if the head is
placed too close, does the light seem to get harsher? and why can I
more easily burn out the whiter parts of the subject without
effectively lighting the darker parts?

If I understand the question correctly, you are talking about the 'inverse sqare law'
http://en.wikipedia.org/wiki/Inverse-square_law

The lighting contrast increases due to the fast fall-off from the 1) shorter distance (of the source) to the 2) nearest part of the subject in relation to the 3) remotest part. The longer the distance 2) to 3) in relation to 1) - 2) the less the fall-off, i.e. the contrast.

Also I don't quite understand the relationship when say I have a
reflector, which say has a spread of 100º that surely means that
everything with that 100º arc should be lit (lets assume I have a
quality reflector setup here at this point with nice even light) so
why again does that seem to alter dependant upon distance?

The more distant the source the narrower is the effective angle the subject receives light. Say, when the subject at close distance is lit by 50° of the total arc it will receive just 10° (from the full 100°) at a larger distance.

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cheers, Peter

Germany

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