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Measuring Light Hardness

Stephen Selby

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People talk about soft and hard light but really it is just a gradation between them. How does one measure the hardness of light falling on the subject as opposed to the hardness of the source?


Clearly hardness of the source would be measured by calculating the angle between the distance of the source and the size of the source.

So the sun would be the Tan-1(Sun Height / Sun Distance) = 0.53 degrees i.e. extremely sharp.

A blonde with reflectors 0.1m diameter positioned about 5 meters away from subject would be Tan^-1 (0.1/5) = 1.14 degrees i.e very sharp.

A softbox 2m high and 2m from subject would be Tan^-1 (2/2) = 45 degrees i.e. soft


However when there are obstacles or diffussion in the way the actual hardness of light falling on the subject is different - so how does one measure this?

In some instances the diffuser or diffusion becomes the new source - e.g. a tweenie bounced off a muslin - the muslin is now the source if the muslin is 2 meters away from the subject and 2m high the light falling on the subject now has a hardness of 45 degrees (i.e. soft).


But when the sun becomes diffused by haze, mist, fog, cloud covering, etc. the effective sharpness of the shadows surely changes - until it is overcast and then the angle of hardness is 180 degrees - totally diffused and no shadows.


I was walking in the countryside on Sunday and the lighting was beautiful. Something I would like to replicate.


It was overcast, not heavy overcast and a low sun was diffused through some more whispy clouds.


So I took out my Lee swatchbook and observed the colours that surrounded me. I measured the angle at 10 degress, and the shadows to be about 4-5 shows darker than the highlights.


But I had no idea how to measure the hardness of the shadows. The clouds were diffusing the sunlight and a rough guess would be 80% if 100% is a perfect sharp line. I presuming the sharpness of the sun is less in winter than in July due to travelling at a lower angle, and hence more diffusion through the atmosphere. Is there a tool for such a thing? Is there a known chart for the hardness of sunlight under various conditions - i.e. December vs July. Does the sharpness actually change between winter and summer due to a lower trajectory and more atmosphere to travel through or only intensities?

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You usually run into practical limitations when trying to create something as big as an overcast sky or as hard as sunlight on a clear day. Generally it's near impossible to overshoot those targets, you just get as close as possible. As for semi-overcast / light haze there are so many degrees of softness even minute to minute that you can make up whatever you want.

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Yes trying to replicate nature is extremely difficult and would be impossible outside or for a large interior set, but I would like to try to get as close as possible to beautiful light I see in nature, then try to reproduce this coming through a small window. When I'm on my long walks in the countryside I study the light, when I see something I really like I note down the colours, the angles, the contrast between the highlights and shadows, and try to make a rough gauge of softness, though if I could be a bit more accurate it might help.


If the sun is hazy, then it can be tricky working out how much it is diffused, whether to add a little hampshire frost, or perhaps something a bit more diffused, what size grid to go for, in order to mimic as closely as possible to what I already like. I do this so I can hopefully reproduce it in the studio, a beautiful winter sunlight penetrating a room etc. If I ever use a greenscreen - and seldom do - it would be nice to try and reproduce the exact light that a plate was photographed on - getting colour temperatures right, and angles is not too difficult - but shadow density and softness is also critical - and this is where I would also like to measure shadow softness more acurately.


Perhaps simply holding up a piece of card at some distance from a ruler will do the job. Clearly a very hard shadow (i.e. sun) will mean that the shadow width is exactly the same as the card, but a softer source will create a penumbra resulting in a wider fainter shadow that I can measure - clearly the wider the shadow the softer the light. I shall give it a go, always worth playing, as that is only the real way to learn.

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Ah here is a toungue and cheek solution to continuity of sunlight direction we've all been looking for:


Build your set on this town square and then you'll have consistent lighting direction (though intensity will change somewhat). Just need to build a periscope version of it to go above the clouds, so that weather can't interfere.



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Although you're coming from another perspective you're working with a very similar set of considerations/solutions as used in 'global illumination' algorithms for CG.


It's not easy.


Lots of sums and sequences that operate to infinity but it sounds like it might interest you or at least give you another search string to try in google.


Speaking of google, they bought up Bot and Dolly recently - the folk who did the moco for Gravity:




Those are LED panels doing the lighting - interesting huh ^_^

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You can only approximate a lot of these daylight effects, mainly because you can't recreate the fall-off in intensity of sources that are miles away from the subject.


As for hazy sun, it can be to whatever degree of softening you want, there is no single level of softness. You could put a 4'x4' 216 frame on a 10K or 20K and get shadows that do not have a sharp edge (unless the light was very far away).


Hampshire is so light as to almost not be worth the expense of putting it on a light, all it would do is barely throw the edges out of focus. A light hazy sky is a much broader source.


Silks are interesting diffusions because they allow a certain amount of specular light to leak through, create both a soft and semi-hard shadow sometimes, especially something like 1/4 Silk. But the light gets spread in a star-shape which is perhaps a bit odd if you see it reflected in a shiny surface.

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