sensitometry - underlighting

Christian Tanner · June 7, 2005

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

John Pytlak RIP · June 7, 2005

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/US/en/motion/support/h2/ilit.shtml

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

Christian Tanner · June 7, 2005

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?

John Pytlak RIP · June 7, 2005

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?

David Mullen ASC · June 7, 2005

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.

IvanKane · June 8, 2005

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).

Christian Tanner · June 8, 2005

sorry about the confusion: the post just above this one is actualy mine

...school's computer...

John Pytlak RIP · June 9, 2005

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.

Christian Tanner · June 9, 2005

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?

...

Stephen Williams · June 9, 2005

All the scenes where underexposed.

<{POST_SNAPBACK}>

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

Stephen Williams Cameraman

www.stephenw.com

John Pytlak RIP · June 9, 2005

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/US/en/motion/support/....4.11.8.6&lc=en

http://www.film-tech.com/warehouse/warevie...1262&category=3

Sam Wells · June 9, 2005

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

-Sam

Christian Tanner · June 9, 2005

@ 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).

Patrick Neary · June 9, 2005

...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 June 9, 2005 by PatrickNeary

Christian Tanner · June 10, 2005

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.

David Mullen ASC · June 10, 2005

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.

Adam Frisch FSF · June 10, 2005

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

Yusuf Aslanyurek · June 12, 2005

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

<{POST_SNAPBACK}>

no, he used colormeter.

Riku Naskali · June 12, 2005

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...

J. Lamar King IMPOSTOR · June 13, 2005

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.

Christian Tanner · June 14, 2005

let me clarify as good as I can:

- I used an incident meter. (the seconic zoommaster. I prefer the dome rather than the flat disc to measure gray scales by the way, since I usualy use two "micers" with heavy difussion at a 45° angle, right and left the grayscale, to get an even exposure).

- Unfortunately there is very little information in terms of printer lights. First the lab missplaced the lab sheets, then they gave me the ones for the next role. And even IF they're gonna find them eventualy, they only provide one set of printer lights because they only grade role by role.

I agree with David. The printer lights could clear out all these questions quite easily. Since these informations seem to be lost, I think I just have to live with it.

The main reason why I asked the question of "underlighting" in the first place was because of the discussions that followed.

For instance - and that's in fact a new question - there was the topic of overrating a lowkey scene as the DP's common practice:

I heard AND red about the technique of overexposing a low-key scene. A low-key scene with all its shadows can become very grainy (often refered as dirty blacks). To prevent that, DP's overexpose it (by just overrate their ASA on their light meter) by 1 to 2 stops. This brings the shadows, sitting in the toe area of the film, into the straight line of the curve.

The lab ought to print that down again.

The result ought to be a low key scene with "richer" shadows without nasty grain.

And here's the question: If the lab prints the scene down again, doesn't the interpos have a curve as well - and doesn't the shadow areas end up in the same area (the toe) as they started with in the first place?

Hopefully, that didn't add to the confussion...

John Pytlak RIP · June 14, 2005

I heard AND red about the technique of overexposing a low-key scene. A low-key scene with all its shadows can become very grainy (often refered as dirty blacks). To prevent that, DP's overexpose it (by just overrate their ASA on their light meter) by 1 to 2 stops. This brings the shadows, sitting in the toe area of the film, into the straight line of the curve.
The lab ought to print that down again.

The result ought to be a low key scene with "richer" shadows without nasty grain.

And here's the question: If the lab prints the scene down again, doesn't the interpos have a curve as well - and doesn't the shadow areas end up in the same area (the toe) as they started with in the first place?

Hopefully, that didn't add to the confussion...

<{POST_SNAPBACK}>

Yes, slightly overexposing a color negative will increase shadow detail and give "richer" blacks in the final print/transfer.

When a lab makes a master positive, they usually incorporate the scene-to-scene timing from the approved answer print. So any over or underexposure of the negative is compensated by the printer exposure, placing all the scene information on the straight line portion of the color intermediate film. Here is Laboratory Aim Density (LAD) method that I developed to simply this procedure:

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

http://www.kodak.com/US/plugins/acrobat/en...ort/h61/h61.pdf

Christian Tanner · June 17, 2005

So any over or underexposure of the negative is compensated by the printer exposure, placing all the scene information on the straight line portion of the color intermediate film. Here is Laboratory Aim Density (LAD) method that I developed to simply this procedure:

I tried to formulate this in an itelligent way and I failed. So:

How can that be John?

Don't the shadows ALWAYS live in the toe area of the sensitometric curve?

John Pytlak RIP · June 17, 2005

I tried to formulate this in an itelligent way and I failed. So:
How can that be John?

Don't the shadows ALWAYS live in the toe area of the sensitometric curve?

<{POST_SNAPBACK}>

On a camera negative image, shadows normally "live" in the toe area.

On a positive image (e.g., master positive), shadows are placed on the upper part of the curve.

A duplicating film like Kodak VISION Color Intermediate Film 2242 has enough latitude to get all of the scene information from a timed negative on the straight line portion of the film's characteristic curve -- nothing significant should fall on the toe or shoulder. That's what LAD was designed to do:

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

http://www.film-tech.com/warehouse/warevie...1262&category=3

Stuart Brereton · June 29, 2005

I heard AND red about the technique of overexposing a low-key scene. A low-key scene with all its shadows can become very grainy (often refered as dirty blacks). To prevent that, DP's overexpose it (by just overrate their ASA on their light meter) by 1 to 2 stops.

<{POST_SNAPBACK}>

You don't get 'dirty shadows' from underexposing. Shadow areas only become grainy when you try to Print them UP.

If you shoot a grayscale and your scene correctly exposed at t4, and your shadow areas are falling around t1 or t1.4, those shadow areas will reproduce normally when you print to your grayscale. If you suddenly decide that you don't want the scene as dark as you've exposed it, and you try to Print Up, then your shadows can start to show grain

DP's sometime underrate film stock (overexposing it) by a 1/3 of a stop or more, as this helps to reduce grain generally, not just in the shadows.