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sensitometry - underlighting


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#1 Christian Tanner

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Posted 07 June 2005 - 12:57 PM

Hi guys!

This is a tricky one - so please forgive me if the following sounds a little ...odd.

First, a statement:
"Wheter I use a lot of light and close down the apperture, or use little light and open it up, influences my depth of field and just my depth of field. Because either way the same amount of light reaches the film and sets the scenes brightness (assuming the shot is correctly exposed) into the straight area of the sensitometric curve".

So, here's the question: What's wrong with this statement?
(I shot the first two rolls of my last 35mm film at 1.4 and two thirds - but lit my grayscale at around 4. I'm 120% sure that I exposed both correctly, but ended up with an underexposed film - about 2/3 of a stop).

Help! Please.

Cary
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#2 John Pytlak RIP

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Posted 07 June 2005 - 01:21 PM

Hi guys!

This is a tricky one - so please forgive me if the following sounds a little ...odd.

First, a statement:
"Wheter I use a lot of light and close down the apperture, or use little light and open it up, influences my depth of field and just my depth of field. Because either way the same amount of light reaches the film and sets the scenes brightness (assuming the shot is correctly exposed) into the straight area of the sensitometric curve".

So, here's the question: What's wrong with this statement?
(I shot the first two rolls of my last 35mm film at 1.4 and two thirds - but lit my grayscale at around 4. I'm 120% sure that I exposed both correctly, but ended up with an underexposed film - about 2/3 of a stop).

Help! Please.

Cary

<{POST_SNAPBACK}>


I'm sorry, but I am not understanding exactly how you exposed the film. :blink: Are you saying you exposed the film at T/1.4 plus 2/3 stop, and then lit the gray scale differently for a lens opening of T/4? Which scene was underexposed?

Here is an incident light exposure table:

http://www.kodak.com...t/h2/ilit.shtml

Please state how you exposed the film, giving the film Exposure Index, Illumination Level (footcandles) and lens opening used.
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#3 Christian Tanner

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Posted 07 June 2005 - 01:37 PM

I exposed most of my shots at "2" and "1.4 2/3" - correctly for the keylight. (No under or overlighting). So that my skintone is correctly exposed.
I lit and exposed my grayscale for a 4.
According to my teacher, this caused the problem. (Unfortunately, no explanation could be given at this point).

All the scenes where underexposed.

Unfortunately, I can't give any indication on footcandles.
Does that answer your question?
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#4 John Pytlak RIP

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Posted 07 June 2005 - 01:43 PM

I exposed most of my shots at "2" and "1.4 2/3" - correctly for the keylight. (No under or overlighting). So that my skintone is correctly exposed.
I lit and exposed my grayscale for a 4.
According to my teacher, this caused the problem. (Unfortunately, no explanation could be given at this point).

All the scenes where underexposed.

Unfortunately, I can't give any indication on footcandles.
Does that answer your question?

<{POST_SNAPBACK}>


If you are using the gray scale to help monitor and control tone scale, why are you lighting and exposing it differently than your scenes?
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#5 David Mullen ASC

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Posted 07 June 2005 - 03:41 PM

Well, if the gray scale was lit to an f/4 and exposed at an f/4 -- and then the following scene was lit to an f/2.0 and exposed at an f/2.0, then the density on both the gray scale and scene should have been normal or the same at least.

So somewhere something was misexposed, probably the scene (it's pretty hard to light a gray scale flat and then meter it incorrectly! Which is the whole point, shooting the gray scale is supposed to be dummy-proof on your part and the colorist's part, easy to figure out what the exposure should be.)

It shouldn't matter if the stop on a gray scale shot separately was at a different stop if it was exposed correctly at that stop.

Besides, if the scene and the gray scale were lit to the same level and he exposed the scale at f/4 and the scene at f/2.0, then the scene would be brighter-looking than the card, not the other way around.
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#6 IvanKane

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Posted 08 June 2005 - 03:21 PM

Thanx for the replies guys.
I came to the very same conclusion.

But here's the thing: Some (I'd like to call them "well established") DP's told me that exposing a greycard in a totaly different light level than a scene, can and does cause problems in exposure - even if both are correctly exposed. (Yet they failed to give me a clear explanation to that phenomena and refered to sensitometry).
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#7 Christian Tanner

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Posted 08 June 2005 - 03:29 PM

sorry about the confusion: the post just above this one is actualy mine
...school's computer...
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#8 John Pytlak RIP

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Posted 08 June 2005 - 09:32 PM

Thanx for the replies guys.
I came to the very same conclusion.

But here's the thing: Some (I'd like to call them "well established") DP's told me that exposing a greycard in a totaly different light level than a scene, can and does cause problems in exposure - even if both are correctly exposed. (Yet they failed to give me a clear explanation to that phenomena and refered to sensitometry).

<{POST_SNAPBACK}>


If you use a different light level for the gray scale than the scene, you are just adding an additional variable. If the light source is actually different, you could even have a difference in color temperature.
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#9 Christian Tanner

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Posted 09 June 2005 - 02:48 AM

it seems that we all agree on the same thoughts.
but I'm still unsure wheter I got the idea of "the films sensitometry" or not.
Here's another - hopefully better - example.

It is common knowledge to overrate (overexpose) a low-key scene to bring the blacks into the straight line of the curve. If printed down (on the interpos.) the blacks will look "richer" this way.
I get the first bit: Since my scene is a low-key scenario, (little highlights, lots of shadows), I overexpose to bring these shadows out the toe area.
BUT: When printed down again - that particular filmstock will have a toe area as well - and by printing it down again my blacks will be at the exact same spot as in the beginning?
...
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#10 Stephen Williams

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Posted 09 June 2005 - 03:07 AM

All the scenes where underexposed.

<{POST_SNAPBACK}>



What were the printer lights? or are you looking at a dark video transfer?

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#11 John Pytlak RIP

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Posted 09 June 2005 - 05:28 AM

it seems that we all agree on the same thoughts.
but I'm still unsure wheter I got the idea of "the films sensitometry" or not.
Here's another - hopefully better - example.

It is common knowledge to overrate (overexpose) a low-key scene to bring the blacks into the straight line of the curve. If printed down (on the interpos.) the blacks will look "richer" this way.
I get the first bit: Since my scene is a low-key scenario, (little highlights, lots of shadows), I overexpose to bring these shadows out the toe area.
BUT: When printed down again - that particular filmstock will have a toe area as well - and by printing it down again my blacks will be at the exact same spot as in the beginning?
...

<{POST_SNAPBACK}>


Kodak VISION Color Intermediate Film 2242 has more than enough latitude to capture all of the scene information contained on your negative. However, you should make a timed IP. One of my first projects at Kodak was to develop the Laboratory Aim Density (LAD) system to help labs consistently place all of the information on the straight line portion of the intermediate film during duplicating:

http://www.kodak.com....4.11.8.6&lc=en

http://www.film-tech...1262&category=3
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#12 Sam Wells

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Posted 09 June 2005 - 09:10 AM

If the "blacks" are on the straight line they're not black !

-Sam
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#13 Christian Tanner

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Posted 09 June 2005 - 09:40 AM

@ stephen: unfortunately I don't know. The Lab gave me a wrong labreport first - and missplaced the right one...
I was able to see the actual rushes - not just a telecine.

@SamWells: Yes - you're right. But as I understood:
If one overexposes ones low-key scene (to bring the blacks into the straight line of the curve and make it actually grey) and let the lab print it down again (by overlight the grayscale as well), the blacks suppose to turn out "richer" then the normal way (exposing correctly from the start).
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#14 Patrick Neary

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Posted 09 June 2005 - 09:56 AM

...but you're assuming that all of your blacks are just at the fringe of the bottom end of the film's range, when in fact the darkest areas of your scene might probably require qrotesque amounts of overexposure to bring them into the 'straight line'...

like Sam sez, black is black- it's an area of no exposure.

Edited by PatrickNeary, 09 June 2005 - 10:01 AM.

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#15 Christian Tanner

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Posted 10 June 2005 - 11:51 AM

thanx guys for your help.
obviously I have too many questions running. (and in the wrong topic as well...).
I'll try to organize myself and open a new threat.
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#16 David Mullen ASC

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Posted 10 June 2005 - 11:58 AM

I think our question to you is: is the actual scene underexposed (i.e. misexposed) or just printed too dark? Did the printer lights change from the scale to the scene? Because they shouldn't have, so if the printer lights are the same for both the gray scale and the scene.... AND the scale is printed correctly... AND the following scene is too dark... then you underexposed the scene (or overexposed the gray scale).

If they DID change the printer lights for the scene (which they shouldn't have) and they are higher than that used for the gray scale, then THEY printed the scene darker than it should have been, since they shouldn't have done anything but keep the same lights as for the scale.

If you exposed the gray scale correctly (lit it in flat, white light, etc.) it shouldn't matter if the stop used was different from that of the scene if the stop you used for the scene was also correct for that scene.

I've been shooting gray scales for a long time; it's not necessary to use the same stop as is being used for the scene if you are using the same ASA rating for the scene and the scale and are exposing correctly for both.

The film doesn't care what f-stop you used, only how you exposed the scene for the correct density.

If you light and shoot one gray scale to f/2.8 and then you light and shoot another gray scale at f/5.6, they would both look the same to the film since both were exposed correctly for their respective light levels.
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#17 Adam Frisch FSF

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Posted 10 June 2005 - 03:13 PM

I'm guessing - you used a spotmeter to measure the grey scale, right?
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#18 Yusuf Aslanyurek

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Posted 12 June 2005 - 05:15 AM

I'm guessing - you used a spotmeter to measure the grey scale, right?

<{POST_SNAPBACK}>


no, he used colormeter.
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#19 Riku Naskali

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Posted 12 June 2005 - 09:07 AM

I think Adam meant what was used to measure the exposure on the greycard? I'm sure it couldn't have been a color meter...
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#20 J. Lamar King

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Posted 13 June 2005 - 01:53 AM

Maybe it's that I just came off a 14 hour day and I can't get my head around it but it seems obvious that you lit the card and exposed it for f/4 and then did not open the iris to f/2 for the scene and it was underexposed.
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