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the nature of light?


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#1 c_conditt

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Posted 02 June 2006 - 07:45 AM

I am thinking about a quite complicated question and no one yet could answer my question:

Lets take this theoretical construct:

We have a room without a window and only one point light source on the sealing, a lamp with lets say 100 watts.

Lets say my stock is 200 ASA and my stop is 2.8 and I take PIC A

Now I put in a new bulb that is much stronger and my new given f-stop ist 11. I take PIC B

Do both pictures look the same?

They should as we have relatively the same amount of light hidding the negative.

---

Background of my question:

My teacher told me that pic a and b will never look the same even not on black and white and also my experiene tells me it is like that.

We tried to light a studiobuild room with a light that was approximetely 2pm o clock summer day. We did not reserved enough light so we had to use 3 x 2k and the atmosphere we got was maybe 5pm o'clock in the evening. Of course our eyes can determine very good the ambience situation of the light but why is it not possible to fake this on the stock by f.e. opening the iris a little bit more. Is this just a color temperature problem or are there any effects that the materials reflective curve do change with the amount of light hitting them making it impossible to fake it with different film exposure.

I also read an Interview with a professional DP who answered the question: Why do we still use so powerful lights even in studio though film stock became so good even in high ISO? His answer: Because of quality.

What quality?

Thank you very much for joining the discussion!

Clemens

Edited by c_conditt, 02 June 2006 - 07:47 AM.

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#2 David Mullen ASC

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Posted 02 June 2006 - 12:12 PM

There are a number of factors involved so it's complicated.

In a perfect closed-world environment, having a room with with a single lamp and two different wattage bulbs, adjusting the iris to match, should not cause any noticable change in look or contrast. However, there are two reasons why there does tend to be a change: one, at wider apertures, the negative picks up more information from low-level secondary light sources (i.e. you're not in a perfectly controlled environment with only a single possible source) and two, the shallow the depth of field, the more out-of-focus the background can become in tighter shots, and an out-of-focus background LOOKS more low-contrast than an in-focus background. This is because the extreme highlights and shadows blur over each other when thrown out of focus.

In the case of shooting outdoors, it's much more complicated because now you're in an environment with a lot of natural sources that are hard to fake. It's hard to fake exactly the projected hard light of direct sunlight and it's hard to fake the look of broad cool skylight filling in the shadows, so your simulations of those elements tend never to be completely realistic (it's easier to fake hard sunlight as a backlight rather than a front-light, which is one trick to remember.) For one thing, the human eyeball is like a mirrorball and can see the difference between a real daylit environment bouncing back into the face and an artificial light, no matter how large you make it -- even a 20'x20' frame filling in the face won't fill the eyeball with its reflection the way that a sunlit landscape would, which creates a natural glare over the eyes.

The main reason why big movies use big lights is that soft lighting techniques are rather wasteful, output-wise, so you need to start bigger, and also bigger lights allow you to light from farther back, which keeps the fall-off in intensity as the actors move more natural-looking (if you're not simulating a close source like a small table lamp.)
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#3 c_conditt

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Posted 04 June 2006 - 04:26 PM

At first thank you very much for you detailed answer & thank you also very much for your hints!
I really appreciate this!

So what I can conclude, on a theoretical basis the contrast ratio created by reflected surfaces dont change with the intensity of light, meaning a 20%gray card will always reflect 20% equal to the amount of light hitting it.
(?)
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#4 David Mullen ASC

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Posted 04 June 2006 - 05:18 PM

At first thank you very much for you detailed answer & thank you also very much for your hints!
I really appreciate this!

So what I can conclude, on a theoretical basis the contrast ratio created by reflected surfaces dont change with the intensity of light, meaning a 20%gray card will always reflect 20% equal to the amount of light hitting it.
(?)


Well, that's not a contrast ratio, which implies a ratio between highlights and shadows. You're just talking about the reflectance of the highlights.

Yes, it doesn't really matter if you light a gray card with the sun or an 20K or a Tweenie if you expose for its reflectance -- the final density on the film should be the same for that object.

But the real world is not a closed system, so there are other factors that come into play. For example, you may light a scene to f/2.0 on fast film and end up with a lower contrast because other ambient sources in the room will expose better and practical lamps in the frame may be bright enough to flare the lens and lower contrast, whereas if you keyed with a stronger light, you'd overpower any natural ambience from other sources and thus increase contrast.

And even comparing something lit with a single 20K versus a Tweenie, you'd have to factor in the larger fresnel on the 20K, so if it is close to the object, it will be a less sharp source than a Tweenie at the same distance, and therefore may create a softer shadow and a larger glare on the surface of the object, all of which may change the impression of contrast, even if you matched the two shots in terms of exposure.
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