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Calculating T (F) Stop


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#1 Corey Bringas

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Posted 24 February 2007 - 08:43 PM

Hi,

I know there is a way to do this. I am trying to figure out what film stock I want and if I will be able to get enough light out of my lamps with my desired stock. I'm trying to figure out if I am using (for example) a 500w par can @ 10 ft distance from subject shooting @ 24fps what T(F) stop will I get? I know I can figure this out through tests, which I plan on doing, but I would like to calculate it beforehand for a rough estimate.
Thank you,
Corey
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#2 Jonathan Bowerbank

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Posted 24 February 2007 - 09:52 PM

You don't even have to shoot a test, just set the light up and meter it at 10'
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#3 Paul Bruening

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Posted 24 February 2007 - 09:58 PM

Harry Box has the lamp tables in his book, Set Lighting Technician's Handbook.
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#4 Markus Kloiber

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Posted 24 February 2007 - 11:23 PM

Arri Germany has a Photometric calculator on the website.
you can use this as a point to start with.

also, for Par 64 related fixtures go to http://www.ultralightmfgco.com/
set up your lamps and take your readings from different distances.
a friend of mine did that with every lamp available at a rental house.
Well,this reality and the website is just the IDEAL LAMP.


the ASC manual lists the footcandles you need to get an exposure of let's say, T4. when i look it up, it would say:
ASA 100/24fps: 200 footcandles --->T4
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#5 David Mullen ASC

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Posted 24 February 2007 - 11:35 PM

"lightyourvision", please edit your Display Name to a real first and last name, as per the forum rules. You can do that under My Controls, thanks.

I memorized the old rule "100 ASA and 100 footcandles at 24 fps = f/2.8" but you could also memorize the same rule but with 200 footcandles and f/4.0, it's the same thing.

From that, you can figure out an unknown value if you know the others, i.e. at 200 ASA and 100 footcandles at 24 fps = f/4.0, etc.

This is why if you learn that some old 3-strip Technicolor movie in the late 1930's, which were usually shot at f/2.8 on interior stages (basically wide-open), was using around 1,000 footcandles, then the effective ASA must have been near 10.
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#6 Sam Kim

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Posted 25 February 2007 - 03:00 AM

Are foot candles still used for measurement?

"lightyourvision", please edit your Display Name to a real first and last name, as per the forum rules. You can do that under My Controls, thanks.

I memorized the old rule "100 ASA and 100 footcandles at 24 fps = f/2.8" but you could also memorize the same rule but with 200 footcandles and f/4.0, it's the same thing.

From that, you can figure out an unknown value if you know the others, i.e. at 200 ASA and 100 footcandles at 24 fps = f/4.0, etc.

This is why if you learn that some old 3-strip Technicolor movie in the late 1930's, which were usually shot at f/2.8 on interior stages (basically wide-open), was using around 1,000 footcandles, then the effective ASA must have been near 10.


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#7 Evan Pierre

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Posted 25 February 2007 - 03:48 AM

Are foot candles still used for measurement?


yes, its usually a matter of preference. Most people that I know of use f/stops though
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#8 Markus Kloiber

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Posted 25 February 2007 - 11:47 AM

yes, its usually a matter of preference. Most people that I know of use f/stops though



f/Stops are used for Depth of Field Calculations.
T/Stops are used for exposure calculations.
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#9 Sam Kim

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Posted 27 February 2007 - 01:03 PM

f/Stops are used for Depth of Field Calculations.
T/Stops are used for exposure calculations.


I've never heard anyone explain it that way before. I've always just led myself to believe that if you have the option on the lens go by the T.


i've still never seen anyone who's of a younger generation mention footcandles but if evan says so.
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#10 Kyle Geerkens

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Posted 01 March 2007 - 09:53 AM

Ive been led to believe that F stops and T stops are almost identical. I don't shoot film and thus sounds stupid right now, but this has been bugging me for some time. in video and HD i have no concern for T-stops...

The F is determined from the T stop with some math formula correct?
ie. aperature / ASA? i dont know
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#11 John Holland

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Posted 01 March 2007 - 11:23 AM

T.stop = transmission ie the lens maker will determine the about of light, well the glass of the lens absorbes , f stop is really the amount of light that is measured on the scene /subject . So a f 2.8 zoom , is a T 2.2 . sounds as clear as mud doesnt it , means really you open uo lens to let a bit more light in .
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#12 Sam Wells

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Posted 01 March 2007 - 01:54 PM

T.stop = transmission ie the lens maker will determine the about of light, well the glass of the lens absorbes , f stop is really the amount of light that is measured on the scene /subject . So a f 2.8 zoom , is a T 2.2 . sounds as clear as mud doesnt it , means really you open uo lens to let a bit more light in .


Hey John, I've always wanted an f 2.8 T 2.2 zoom ! an f 2.0 T 1.4 even more :D

Do you about something that I don't ? (maybe this is the secret of the RED zoom lens ????)

I think ya postraned the numbers...

-Sam
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#13 John Holland

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Posted 01 March 2007 - 02:03 PM

Yep Sam as i hit reply thought sod it , wish i hadnt bothered , please everyone ignore me . :)
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#14 Troy Warr

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Posted 01 March 2007 - 02:22 PM

Ive been led to believe that F stops and T stops are almost identical. I don't shoot film and thus sounds stupid right now, but this has been bugging me for some time. in video and HD i have no concern for T-stops...

The F is determined from the T stop with some math formula correct?
ie. aperature / ASA? i dont know

As I understand it, an f-number indicates the ratio of a lens's entrance pupil to its effective focal length - i.e. it's a purely geometrical ratio that doesn't take the actual light transmission properties of the lens into account.

The t-number is more accurate and realistic, as it better indicates the actual light transmission capabilities of a given lens. For example, t/2.5 would be equivalent to an ideal lens (zero loss of light) at f/2.5. It may realistically be at the same point on an aperture ring as f/2.8 because the t-number will always be the same or lower than the f-number - i.e. no light is ever "gained" through a lens, but some of the best lenses will lose very little.

Edited by Troy Warr, 01 March 2007 - 02:23 PM.

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#15 Kyle Geerkens

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Posted 01 March 2007 - 02:24 PM

so even with the mistakes... am i correct state that the F will always be equal to or higher than the T stop (higher # i mean) and the T stop is actually the natural F number minus the amount of light the lens absorbes. thus being a more exact representation of what hits the film?
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#16 John Holland

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Posted 01 March 2007 - 02:31 PM

If your light meter says f.4 for instance ,set the lens to t.4 .
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#17 Sam Wells

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Posted 01 March 2007 - 03:22 PM

so even with the mistakes... am i correct state that the F will always be equal to or higher than the T stop (higher # i mean)


The f number will be lower (unfortunately :( ) as say Cooke 9-50mm zoom: f 2.2 T 2.5

The difference will typically diminish as a lens is stopped down because less air to glass surface area is being used to form the image.

-Sam
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#18 Troy Warr

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Posted 01 March 2007 - 04:07 PM

For example, t/2.5 would be equivalent to an ideal lens (zero loss of light) at f/2.5. It may realistically be at the same point on an aperture ring as f/2.8 because the t-number will always be the same or lower than the f-number - i.e. no light is ever "gained" through a lens, but some of the best lenses will lose very little.

Er, yeah, just realized that I switched that up - sorry. Sam's got it right.
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#19 Kenny N Suleimanagich

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Posted 12 March 2007 - 07:18 PM

I've never heard anyone explain it that way before. I've always just led myself to believe that if you have the option on the lens go by the T.
i've still never seen anyone who's of a younger generation mention footcandles but if evan says so.

Let's hope footcandels dont go out, possibly the most crucial measurement. I wish my meter came with a footcandel meter. The main thing is, F-stop is a measurement of the opening of the lens based on the speed of the film, whereas footcandles at how much light a light emits at a certain distance. Think of it as footcandles=constant reading, f-stop = based on film speed.
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#20 Markus Kloiber

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Posted 24 March 2007 - 11:42 AM

so even with the mistakes... am i correct state that the F will always be equal to or higher than the T stop (higher # i mean) and the T stop is actually the natural F number minus the amount of light the lens absorbes. thus being a more exact representation of what hits the film?


THE T-stop will be a higher Number cause Light is Lost inside The Lens.

Imagine putting a Lightmeter behind a Lens. The Stop it would indicate with the lens set at a specific F-stop, would be the T-Stop, light that would hit the film plane.
Whatsoever, the difference is only "big" where lot's of glass is needed...e.g. big zoom or telephotolenses
Lens manufacturers have their depth of field charts calculated with the lens specific T-stops, since the diameter opening for T2.0 will be slightly wider than for F2.0
hope this helps.
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