Eliminating fall off on models legs

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I have the problem where with my two strobes I'm getting quite bad fall off on my models on the mid thigh and lower. The problem is I can't move my strobes any further back due to space (im shooting in my living room) and as I'm trying to reduce fall off on my black background.

Im guessing a reflector or two might help but not sure where I would position them? Are there other solutions?

I've attached a very bad drawing of my setup!

 
It's about falloff of light due to the effect of the inverse square law.
You don't say, but my guess is that you're placing your light high to create the modelling effect you want on the face, and because the light has much further to travel by the time it reaches the lower parts, much less light is reaching the subject.

There are 3 possible answers
1. Place the light further away - this may not be possible if you don't have enough height and anyway will affect the quality of the light
2. Use a second light, to light the parts that other light can't reach (apologies to Heineken:)) - the best option
3. Use a reflector. Obviously, as a reflector can only reflect light that reaches it, it will need to be placed where it picks up light that has 'missed' the subject - but this isn't difficult if you're using a softbox or umbrella, it becomes impossible if you're using a more precise lighting tool such as a honeycomb or spotlight. Bear in mind that a reflector can never provide 'equal' lighting, again because of the Inverse Square Law - it has to travel much further, and also quite a bit of light is lost in reflection.

Edit: The 4th answer of course is to place the light lower, but again this will affect the quality of the light
 
Thanks for the reply Gary.

You're right I have been placing the light high to get a nice modelling effect on the face. Although as they are still full body shots I've been taking I've been mindful not to set the softboxes too high.

Option 1 isn't possible due to space constraints, the softboxes are already as far back I can get them while reducing spill onto the background.

I'm assuming option 2 would mean for me getting a 3rd light, as I use two lights either side of the model to get that "pop" effect on her against the black background. I'm abit reluctant to get a 3rd light just yet as i'm still mastering 2!

So looks like the best solution in this case is sacrificing the quality of light by lowering the softboxes and two reflectors aimed at the legs?
 
Inverse square law goes right out the window once you start using light modifiers.

Inverse square law only applies to bare light sources. The moment you add a reflector, a softbox, etc, the equations don't add up.

Personally, I'd just pick up a cheap speedlight as an optical slave like a YN460-II, throw it in a 24" softbox on a very short light stand, that'd give you maximum control (total cost under £100).

Or, go with the reflector. :)
 
Kaouthia said:
Inverse square law goes right out the window once you start using light modifiers.

Inverse square law only applies to bare light sources. The moment you add a reflector, a softbox, etc, the equations don't add up.
Personally, I'd just pick up a cheap speedlight as an optical slave like a YN460-II, throw it in a 24" softbox on a very short light stand, that'd give you maximum control (total cost under £100).

Or, go with the reflector. :)
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Actually, that isn't right. The ISQ is a law of physics and as such is immutable and applies to all light sources.
But I know what you mean, because, as Newton clearly explains, the ISL applies to any point source of energy (not just light) and, even if a theoretical point source of light actually exists, we don't use it for photography.
And when we get well away from a point source of light, for example a softbox, although in practice the ISL still applies there are millions of separate point sources of light involved. Take a large softbox say 1ft from the subject. The light from the closest part only has to travel 1 foot, the light from 1 side may have to travel 4 feet and the light from the opposite side may have to travel 3 feet - and the light from all parts between those extremes has to travel a different distance. Because of this, although the ISL still applies, it doesn't follow the linear rules as set out by Newton.

This means, in practice, that when people say something like "If you move the softbox twice as far away you'll only get a 1/4 of the light", they are wrong:)
 
Garry Edwards said:
Actually, that isn't right. The ISQ is a law of physics and as such is immutable and applies to all light sources.
Click to expand...

The inverse square law says that when you double the distance from your light source, you quarter the amount of light being received. When you use grids, reflectors, softboxes, umbrellas, this does not hold true. The inverse square law is based on a point light traveling in a straight line. When you add modifiers, it is no longer a single point light source traveling in a straight line.

So, yes, it's right. :)

http://www.portraitlighting.net/inversesquare_law.htm

PortraitLighting.net said:
Sources that use optical or physical means to collimate light will not follow the inverse-square relationship, at least when based on their physical location. Such devices include, optical spots (elliptical, Fresnel) and sources whose beam has been modified by a grid. When working very close to large, diffuse sources, such as softboxes and scrims, the fall-off rate will be somewhat less than predicted by the inverse-square law.
Click to expand...

Garry Edwards said:
This means, in practice, that when people say something like "If you move the softbox twice as far away you'll only get a 1/4 of the light", they are wrong:)
Click to expand...
So you start off your post by saying I'm wrong, and then end it by agreeing with me? ;)
 
Kaouthia said:
The inverse square law says that when you double the distance from your light source, you quarter the amount of light being received. When you use grids, reflectors, softboxes, umbrellas, this does not hold true.

So, yes, it's right. :)

http://www.portraitlighting.net/inversesquare_law.htm




So you start off your post by saying I'm wrong, and then end it by agreeing with me? ;)
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Let's not get into a silly argument about semantics...

But you're wrong, the ISL does apply to point sources of light, as set out by Newton, and it applies to every single point source of light produced by a studio light too - just not to the entire light or, more properly, to the modifer it happens to be fitted with.

I didn't agree with you, I just pointed out that if what you mean is that it doesn't always apply in linear terms, e.g. when very large light sources are used, then that's true.

It's also true to say that it doesn't apply in linear terms to true parabolic reflectors, such as searchlights, and the parabolic reflectors used to transmit sound over long distances, nor does it apply in absolute terms to lasers.

You'll always find tutorials on the interweb that try to re-write the laws of physics - personally I prefer to rely on physicists.
 
Yes, as I said, and as you confirmed, a point source. As a light contained within a softbox is NO LONGER a point source, the simple equation of doubling the distance and getting a quarter the amount can no longer be applied (something you also confirmed).

Therefore, that equation, the inverse square law, this one..

Inverse Square Law said:
The intensity (or illuminance or irradiance) of light or other linear waves radiating from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only one-quarter the energy (in the same time period).
Click to expand...
cannot be used to give accurate results.

Once you throw it in a softbox, it is no longer an accurate method of calculating the light the subject will receive at different distances from that light source, which is what I initially stated (in slightly more casual terms), so I was not incorrect.

So, what are we arguing about?
 
Kaouthia said:
Inverse square law goes right out the window once you start using light modifiers.

Inverse square law only applies to bare light sources. The moment you add a reflector, a softbox, etc, the equations don't add up.
Click to expand...

I think it is very misleading to say that. While it is true, strictly speaking, that the inverse square law doesn't apply when using modifiers such as a softbox, generally speaking it is more true than not.

If you forget the stuff about Newton and point light sources for a moment, generally speaking if you move a studio light back to double the distance, the pool of light that it projects will be roughly four times the area - or at least something close to it.

In practical terms, that is the inverse square law in action, and it is an extremely useful rule of thumb. Sure, there are exceptions but in practise they are rare and you ignore it at your peril.
 
Kaouthia said:
Or ignore it at the cost of a reliable light meter. :)
Click to expand...
If by that you mean that a meter will give you a reliable answer and calculating using the ISL formulae won't, you're right.

But where the ISL comes in very handy is that by being conscious of its principles we know that something will or won't work without wasting time trying it - for example We know that, as in this thread, the light needs to be moved further from the subject if the falloff of light is too great. And we also know that if that can't be done or shouldn't be done (because changing the distance also changes the quality of the light) then adding a reflector will mitigate the effect but the loss of light caused by the ISL will make it impossible to get equal levels of light (or anywhere near it).
 
Edit: Oops! Crossed post with Garry - didn't mean to gang up on you John :D

Kaouthia said:
Or ignore it at the cost of a reliable light meter. :)
Click to expand...

Haha yes ;) Of course! I'm not suggesting for one moment that you shouldn't use a light meter, but I think that an understanding of the inverse square law is, perhaps, the single most important thing to know about how studio lights work.

It's the main reason why studio lighting is not like daylight, where it makes no difference where your subject is in relation to the light source, ie the sun - a few feet in a few million miles makes no odds.

But in an artificial lighting situation, it explains for example, why using on-camera flash often gives very dark backgrounds, or why that brilliant white studio backdrop you've just set up behind the subject comes out grey. Or going back to the OP, if you position a light just above the subject's head and expose for that, their legs just a few feet further away will be a lot darker.

Perhaps we should call it something like 'close distance light fall-off' but it's the principle of the inverse square law at work :)
 
Well this thread has definitely been an education!

Tfboy I am using rectangular shaped softboxes.

I did another shoot yesterday, having the softboxes lower (at the cost of the quality of light on the models face) did the trick. I'll invest in one or two reflectors in the near future to have abit more control of the light aswell.
 
what about ditching the seperation light in favour of a big reflector and then using that as the leg light
 
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