In lighting, distance from light to subject is key. We don’t have a lot of control over distance when we shoot outdoors in sunlight because, wherever we happen to be in the world, we’re about 93 million miles from the sun and we can’t do much to change that.
But, in the studio, it’s all very much under our control. So I thought it might be an idea to demonstrate just how much difference the positioning of the light can actually make to any shot.
I originally planned to make a video about this, but dropped the idea because it’s difficult to give good, hard and detailed info in a video, so opted for this written tutorial instead. The easiest way of explaining the detail and the maths is to do so by showing equasions, but I’ve just used a lot of words instead 🙂
The shots above were taken with the softbox and light fitted to one of our Parallelogram Boom Arms, this is a useful tool because it maintains the angle when the height of the light is increased. I couldn’t make the ceiling any higher than it is so the Parallelogram boom arm didn’t allow me to demonstrate the changes as we move the light much further away. Because of this, I then took some shots with the flash head and softbox fitted to an ordinary stand, and moved to double the distance for each shot. For this experiment, I set up the lighting as per the shot on the left. My mannequin isn’t the most attractive subject, her skin is more reflective than real skin and her eyes are painted (so don’t show real catchlights) but at least she keeps still and doesn’t complain…
To some extent, this experiment isn’t producing typical results and the reason for this is that the softbox is square to the subject, i.e. it isn’t pointing downwards and so isn’t producing the typical shadows that a softbox normally produces when used for portraits. It wasn’t possible to point it downwards because I would have had to maintain the same angle at different distances and would have run out of ceiling height very quickly – which is why I used the Parallelogram boom arm in the first shots, and of course tilted my ‘model’ to suit.
This setup shot was with the 60 x 60cm softbox just 37.5cm from her nose – my measurement here was from the front of the softbox, not from the flash tube, because the front of the softbox was my effective light source.
And here’s the shot that I took at 37.5cm.
The black bit on the right is the edge of the softbox.
At such as short distance, even this small softbox has a ‘wraparound’ effect caused by the fact that the light is hitting the subject from multiple different places. The larger the softbox (or other light source) and the closer it is to the subject, the greater the wraparound effect, and once it’s has been moved to about the diagonal measurement of its front surface (which in this case is about 85cm), it lost its wraparound effect and in effect stops behaving like a softbox.
You’ll notice that there is much brighter exposure on the bridge of her nose than on her ear, that’s because the light has travelled 37.5cm to her ear and has then travelled another 7 cm to reach her ear, which means that a lot of the light has been lost.
And you’ll see too that the background is pretty dark. The background is in fact unlit white vinyl, it’s 4m from her nose to the background and so even less light is getting that far back.
And this one is shot at 75cm
The distance from the front of the softbox has doubled and you can see that because of this, although there is exactly the same distance between the various reference points in the photo (tip of nose to ear, and tip of nose to background) the ratio of the distance has now changed, and because of this there is now much less difference between the exposure of the nose, the ear and the background.
Because of the increased distance, although the softbox is still producing some wraparound effect, it’s fading fast.
And this one is shot at 150cm
And now I’ve doubled the distance again, and you can see that the light is now falling off even more slowly because there is now even less difference in the amount of light reaching the nose, the ear and the background, so the ear and the background are getting lighter.
Because of the increased distance, the wraparound effect has now really gone.
And this one is shot at 300cm
At a distance of 3m, the falloff of light between the nose and the ear is becoming far less significant, and the background is now affected far less by the fall off of light too.
At this distance, it doesn’t really matter that I used a softbox, because the light from it doesn’t have a ‘softbox look’ to it.
And finally, at 600cm.
At a distance of 6m, the softbox is so far away that there is really no real difference between the exposure of the nose and the ear, so the subject has really gone from 3 dimensional to 2 dimensional.
And the background has become even lighter too.
If I hadn’t run out of studio space I could have just kept doubling the distance until I reached the point where the background hadn’t darkened at all and where there would have been absolutely no modelling on my subject.
I think that most people know about Newton’s Inverse Square Law, in the sense that most people know that if they move the light twice as far away from the subject then about 3/4 of the power is lost, which means that the f/number needs to be 2 stops wider – but the Inverse Square Law isn’t the nuisance that it sometimes seems to be, it’s actually a very useful creative tool that can be used to create emphasis on a near (to the light source) part of the subject and leave a further part of it in relative darkness, and this gets us as close as we can be to creating real depth in 3 dimensional subjects – the other main tool that we can use to create that third dimension is of course by limiting the depth of field, throwing the rear of the subject (or the background) out of focus to draw attention to the part that really matters – and of course we can combine both selective focus and selective lighting, because neither effect is affected by the other.
It’s worth mentioning too that the Inverse Square Law doesn’t just affect the fall off of light over distance, it also affects the relative size of the softbox or other light shaping tool – if you move it twice as far away from the subject then, in effect, it becomes just a quarter of the size it was before you moved it, and so it loses its wraparound effect and, with the increase to distance, behaves less and less like a softbox. In fact, if you move it far enough away it will virtually just become a point source of light, just like the sun. The sun, at 800,000 miles wide, is 100 times wider than the earth but because it’s also 93,000,000 miles away it looks tiny and produces very hard lighting when it isn’t obscured by cloud or fog.
Someone wanted to know how much difference it made to have a much larger or smaller softbox, so I did an identical test using one of our 150cm octa softboxes, which is pretty massive. The space where a photo should be is due to the fact that I couldn’t get a shot closer than 75cm with the large softbox, it was in the way of the camera.
Top 60 x 60 cm softbox @ 37.5cm – 600cm. Btm, 150cm Octa softbox @ 75cm – 600cm
that there isn’t really all that much difference between the amount of fall off of light with these two very different softboxes. There is in fact some difference, but this is really caused partly by the greater wraparound effect of the larger softbox and partly because the light from the larger softbox is starting out from a much larger place and so there are millions more point sources of light, all travelling from different places and at different distances, which means that the Inverse Square Law isn’t working in a strictly linear sense with a large softbox at close distances.
The real difference is in the lighting effect between different sizes of light shaper at different distances, you can see that the light from the 150cm Octa softbox is softer, it wraps around the subject more and the specular reflections are larger too.
The Inverse Square Law states that that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. This means, when applied to any radiated energy source such as heat, magnetism, sound or (in this case) light, that when you double the distance, you lose 3/4 of the light. But, there are qualifications to this: Newton was referring to a point source of light, and we don’t use point sources of light in photography, and this law is only strictly applied to a vacuum (free space is physics talk) and we don’t have that either, so light is sometimes affected by pollution. When the light source is not a point source, the inverse square rule is often still a useful approximation; when the size of the light source is less than one-fifth of the distance to the subject, the calculation error is less than 1% so in effect becomes a point source.
What does this mean in the real world of these tests? Well, here are the actual light loss results, compared with the theoretical light losses, based on the Inverse Square Law. In each case, the power of the flash head was adjusted to give a starting point of f/16 @ 37.5cm
Here are the figures for the 60 x 60cm softbox
75 cm f/8 d5 (expected figure: f/11)
150 cm f/5.6 (expected figure: f/5.6)
As you can see from these figures, there is a fairly substantial “error” of 0.5 stop when doubling the distance of the light from 37.5cm to 75cm, due to the fact that we’re not using a single point source of light, there are multiple point sources, in various different places and at various different distances from the subject. By the time the light is 150cm from the subject, the “error” has gone and the exposure is what the Inverse Square Law predicts it to be.
And of course, the larger the light source, the more exaggerated the ‘error’ becomes at any given distance. Here are the figures for the 150cm Octa softbox
75 cm f/16 d1 (expected figure: f/11)
150 cm f/8 d7 (expected figure: f/5.6) (there is still a large discrepancy between the actual reading and the ‘expected’ reading, so I carried out a test at 300cm too)
300 cm f/5.6 (expected figure of f/2.8)
So, there you have it – proof positive that, in practical terms, the larger the light source, the less the fall off of light appears to follow Newton’s Inverse Square Law, because the light is coming from innumerable point sources of light that are themselves spread over a large area, and which are travelling from a lot of different directions and which are at different distances from the subject.