light level

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#1 Christian Tanner

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Posted 10 June 2005 - 12:20 PM

Hipotheticaly:
I shoot a scene with, let's say, 1000 footcandles illumination. I close my apperture down so that only 500 footcandles reach the film.

I shoot the same scene with 500 footcandles illumination. This time I leave the apperture wide open to let 500 footcandles reach my film.

Question: Does the 500 footcandles from the first example (stopped down by the apperture) have a different quallity than the 500 footcandles from the second example?

(this is not about depth of field).
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#2 Patrick Neary

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Posted 10 June 2005 - 12:33 PM

all things being equal, and for all practical purposes - no.
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#3 John Pytlak RIP

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Posted 10 June 2005 - 01:26 PM

Hipotheticaly:
I shoot a scene with, let's say, 1000 footcandles illumination. I close my apperture down so that only 500 footcandles reach the film.

I shoot the same scene with 500 footcandles illumination. This time I leave the apperture wide open to let 500 footcandles reach my film.

Question: Does the 500 footcandles from the first example (stopped down by the apperture) have a different quallity than the 500 footcandles from the second example?

(this is not about depth of field).

<{POST_SNAPBACK}>

Unless you change the contrast of the lighting, or its spectral qualilty (color temperature), just changing the light level and keeping the exposure constant by changing f/stop should have NO effect on the image quality (except for depth of field and other lens-related things).

Incident Light Table:

http://www.kodak.com...t/h2/ilit.shtml
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Posted 10 June 2005 - 03:07 PM

John is onto something here. If you light a face to 1000fc on one side and the other is completely black (0fc), then you will have a 1000/0 contrast ratio in fc's. If you reduce to 500fc on the bright side, but the dark side is still 0fc, then you've in effect reduced the contrast. This will look different, all other tings being the same. Nothing can change this since you can't make that side darker than 0fc.

However, in the real world, the spill from the 1000fc source would "fill in" the dark side enough to not make it exactly 0fc, therefore for practical purposes, there isn't much difference.

Anyway, this is just academics I'm rambling.

Edited by AdamFrisch, 10 June 2005 - 03:09 PM.

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#5 Tim J Durham

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Posted 10 June 2005 - 08:32 PM

John is onto something here. If you light a face to 1000fc on one side and the other is completely black (0fc), then you will have a 1000/0 contrast ratio in fc's. If you reduce to 500fc on the bright side, but the dark side is still 0fc, then you've in effect reduced the contrast. This will look different, all other tings being the same. Nothing can change this since you can't make that side darker than 0fc.

However, in the real world, the spill from the 1000fc source would "fill in" the dark side enough to not make it exactly 0fc, therefore for practical purposes, there isn't much difference.

Anyway, this is just academics I'm rambling.

<{POST_SNAPBACK}>

I don't think you can call 1000:0 a ratio. You'll have to give the dark side a value of 1.
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#6 David Mullen ASC

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Posted 11 June 2005 - 12:03 AM

Hipotheticaly:
I shoot a scene with, let's say, 1000 footcandles illumination. I close my apperture down so that only 500 footcandles reach the film.

I shoot the same scene with 500 footcandles illumination. This time I leave the apperture wide open to let 500 footcandles reach my film.

<{POST_SNAPBACK}>

I'm not sure I understand; in your first example, the scene is lit to 1000 fc but you close down the aperture so that 500 fc reaches the film? What does THAT mean? What does it mean to open the aperture up to let "all the footcandles reach the film"? Lenses don't work that way -- even "all the way" open is an f-stop of some sort. On some lenses, all the way open is f/1.4 and on others, it may be f/4. Big difference.

So I'm not even sure WHAT you are talking about!?!

We need more precise information, for example:

If you have a shot lit to 1600 fc, let's say, this means that if your film stock is 100 ASA (following the rule that 100 fc on 100 ASA film at 24 fps / 180 degree shutter = f/2.8) then the correct exposure would be f/11.

So if you cut the amount of light in half to 800 fc, the correct exposure would be f/8.

The difference between lighting to 1600 fc and 800 fc, assuming you kept the key to fill ratio the same, and exposed correctly for the respective light levels, would be the same contrast, the same density on film, but more depth of field at f/11 versus f/8.

Now in the real world, of course, as you increase the light levels dramatically, you tend to overpower any natural ambience in the room so contrast goes up (because shadow detail goes down) unless you carefully add enough fill light to restore it back to the contrast ratio of the lower level scene.

At really low light levels, natural ambience becomes very hard to get rid of or control and contrast tends to be low because if you light to a key of only 10 fc, let's say, and the natural ambient light in the room is 5 fc, then that's only a 2:1 ratio.
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#7 Tim J Durham

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Posted 11 June 2005 - 12:11 AM

We need more precise information, for example:

If you have a shot lit to 1600 fc, let's say, this means that if your film stock is 100 ASA (following the rule that 100 fc on 100 ASA film at 24 fps / 180 degree shutter = f/2.8) then the correct exposure would be f/11.

So if you cut the amount of light in half to 800 fc, the correct exposure would be f/8.

The difference between lighting to 1600 fc and 800 fc, assuming you kept the key to fill ratio the same, and exposed correctly for the respective light levels, would be the same contrast, the same density on film, but more depth of field at f/11 versus f/8.

Now in the real world, of course, as you increase the light levels dramatically, you tend to overpower any natural ambience in the room so contrast goes up (because shadow detail goes down) unless you carefully add enough fill light to restore it back to the contrast ratio of the lower level scene.

At really low light levels, natural ambience becomes very hard to get rid of or control and contrast tends to be low because if you light to a key of only 10 fc, let's say, and the natural ambient light in the room is 5 fc, then that's only a 2:1 ratio.

<{POST_SNAPBACK}>

Do you gents (or ladies) in the motion picture world have to deal with reciprocity failure? If so, at what point? I seem to recall in still photography, it occured (or became noticeable) when you exposed a frame for a number of seconds.
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#8 David Mullen ASC

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Posted 11 June 2005 - 12:30 AM

Only in very long exposures like for time-lapse at night.
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#9 Christian Tanner

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Posted 12 June 2005 - 03:37 AM

@ david: I came up with this question during a discussion I had with some collegues. I was pretty sure the answer had to be "no" (as in: no, there is no difference) - so I just wanted to clarify.

The discussion started because of the question in which light level to shoot idealy (I heard a cuple of times the rumor that the ideal stop to shoot with in studio ought to be 4). The discussion made a cuple of turns as usual and ended with the strange idea that strong "stopped down" light might have a different quality we don't know about (as oposed to "weak, slightly stopped light"...

By the way: does anyone have experience with shooting in very low light levels? What kind of problems did you experience?
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#10 Stephen Williams

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Posted 12 June 2005 - 04:17 AM

Do you gents (or ladies) in the motion picture world have to deal with reciprocity failure? If so, at what point? I seem to recall in still photography, it occured (or became noticeable) when you exposed a frame for a number of seconds.

<{POST_SNAPBACK}>

Hi,

I have done long exposure motion control shoots, 8 seconds per frame without any obvious problem, mabe because the camera was moving!

Stephen Williams DP

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#11 David Mullen ASC

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Posted 12 June 2005 - 10:48 AM

I explained the reasons -- at very high light levels, you have to control the fill light exactly to keep the contrast from getting too high because your strong key is obliterating any natural ambience. At very low-levels, it's the opposite problem.

But ASSUMING you match key to fill ratios, then the only difference between shooting at a deep or wide-open stop is depth of field.

There is also an ILLUSION of lower contrast when shooting in bright light (like outdoors in sunlight) but at a wide-open apertures (by using ND filters) because more of the frame will be out of focus, and when an image goes out of focus, highlights and shadows blur over each other creating more midtones. But for the subject IN FOCUS, the contrast is the same.

Some lenses flare more at wide apertures, lowering contrast.
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#12 Tim J Durham

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Posted 12 June 2005 - 11:19 AM

Hi,

I have done long exposure motion control shoots, 8 seconds per frame without any obvious problem, mabe because the camera was moving!

Stephen Williams DP

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<{POST_SNAPBACK}>

Well, even if you experienced reciprocity failure, you can still expose correctly. It just means that if you meter for a 2 second exposure at f8, for instance, you cannot count on f5.6 being correct at 4 seconds or f4 being correct at 8 seconds. You need to re-meter for each change in iris or shutter. Hence- no reciprocity.

It's largely an irrelevent question. I was just curious.
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#13 Stephen Williams

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Posted 12 June 2005 - 12:35 PM

Well, even if you experienced reciprocity failure, you can still expose correctly. It just means that if you meter for a 2 second exposure at f8, for instance, you cannot count on f5.6 being correct at 4 seconds or f4 being correct at 8 seconds. You need to re-meter for each change in iris or shutter. Hence- no reciprocity.

It's largely an irrelevent question. I was just curious.

<{POST_SNAPBACK}>

Hi,

I exposed correctly the printer lights were spot on. Had I overexposed 1 stop my printer lights would have been high!

Stephen Williams DP

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#14 Christian Tanner

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Posted 14 June 2005 - 10:34 AM

@david: thanx
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#15 John Pytlak RIP

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Posted 14 June 2005 - 10:52 AM

Do you gents (or ladies) in the motion picture world have to deal with reciprocity failure? If so, at what point? I seem to recall in still photography, it occured (or became noticeable) when you exposed a frame for a number of seconds.

<{POST_SNAPBACK}>

The published data for each Kodak film usually includes a discussion of the film's reciprocity characteristics, and any compensation needed for very short or very long exposures. For example, here's the data for 5218:

http://www.kodak.com....4.4.14.4&lc=en

Reciprocity Characteristics
You do not need to make any filter corrections or exposure adjustments for exposure times from 1/1000 to 1/10 of a second. If your exposure is in the 1 second range, it is recommended that you increase your exposure by 2/3 stop and use a KODAK Color Compensating Filter CC 10R. If your exposure is in the 10-second range, it is recommended that you increase your exposure by 1 stop and use a KODAK Color Compensating Filter CC 10R.

Here are all the Kodak color negative films:

http://www.kodak.com...0.1.4.4.4&lc=en
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