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THe relationship between film speed and Stops of light


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#1 Jeremy Hunt

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Posted 08 January 2010 - 06:24 AM

Hi

I was unable to find any information on this.

What is the relationship between the filmspeed and the F/stops on the lens?

If i have a 200asa stock but i rate it at 130, does that mean im effectivley underexposing by 2/3rds of a stop?

What about if i hae a 500asa stock, and i rate it at 430, is this also 2/3rds of a stop underexposed?

What im asking is that are the changes in the film speed equal across all speeds in terms of exposure

-Jeremy
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#2 Phil Rhodes

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Posted 08 January 2010 - 07:29 AM

> If i have a 200asa stock but i rate it at 130, does that mean im effectivley underexposing by
> 2/3rds of a stop?

No, you'd be overexposing by about that much. Considering some people recommend aiming half a stop over anyway, that's not that significant.

> What about if i hae a 500asa stock, and i rate it at 430, is this also 2/3rds of a stop
> underexposed?

Again you'd be overexposing, but by a fairly insignificant amount.

One F-stop is twice as much light; by rating 200ASA stock at 100ASA, you're overexposing by one stop. You'd need to rate your 500ASA stock at 250ASA to do the same.

This is one reason why faster film sometimes doesn't help that much. 500ASA stock is way grainier than 250, but it only gains you one stop of exposure. This goes for digital cameras, too, which get far noisier.

P
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#3 Jeremy Hunt

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Posted 08 January 2010 - 08:26 AM

One F-stop is twice as much light; by rating 200ASA stock at 100ASA, you're overexposing by one stop. You'd need to rate your 500ASA stock at 250ASA to do the same.


Thank you very much Phil, sorry for the muddled way i asked this question, but this is what im most intrested in. How do you work out the difference in stops between each film speed?

For instance, i want to pull some ektachrome 64t by one stop, so what asa should i put on the light meter?

-Jeremy

Edited by Jeremy Hunt, 08 January 2010 - 08:27 AM.

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#4 Adrian Sierkowski

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Posted 08 January 2010 - 09:38 AM

32 asa. You double or half the film speed.
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#5 Jeremy Hunt

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Posted 08 January 2010 - 09:43 AM

32 asa. You double or half the film speed.


THanks very much Adrian, so very obvious really 0_0
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#6 K Borowski

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Posted 08 January 2010 - 11:46 AM

Jeremy, there is no such thing as "130" unless you are talking about fractions of a stop smaller than 1/3.

The ASA (now the most commonly-used number of ISO) progression, which arbitrarily rounds from what should be "128" to "125" in the interests of keeping numbers simple, goes like this:

100, 125, 160, 200, 250, 320, 400, 500, 640, 800, add a zero and keep going. . .


You can also subtract a zero and go in the other direction (80 downward), but sometimes the numbers get rounded/truncated when you get below a speed of ASA 20 (12 instead of 12.5, 6 instead of 6.4).

Each number in this arbitrary progression is a third of a stop faster or slower than the one before.
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#7 Jeremy Hunt

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Posted 08 January 2010 - 12:04 PM

Jeremy, there is no such thing as "130" unless you are talking about fractions of a stop smaller than 1/3.

The ASA (now the most commonly-used number of ISO) progression, which arbitrarily rounds from what should be "128" to "125" in the interests of keeping numbers simple, goes like this:

100, 125, 160, 200, 250, 320, 400, 500, 640, 800, add a zero and keep going. . .


You can also subtract a zero and go in the other direction (80 downward), but sometimes the numbers get rounded/truncated when you get below a speed of ASA 20 (12 instead of 12.5, 6 instead of 6.4).

Each number in this arbitrary progression is a third of a stop faster or slower than the one before.


Thanks very much Karl

I picked 130 becuase of this post on Roger Deakins website

"I will usually use a 200 rated stock for daylight interiors which is tungsten based and therefore becomes 130 with correction. If I am pressed for light I will use a 500 ASA stock." www.Rogerdeakins.com

THanks very much to you all for this information.
-Jeremy
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#8 David Mullen ASC

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Posted 08 January 2010 - 12:29 PM

Thanks very much Karl

I picked 130 becuase of this post on Roger Deakins website

"I will usually use a 200 rated stock for daylight interiors which is tungsten based and therefore becomes 130 with correction. If I am pressed for light I will use a 500 ASA stock." www.Rogerdeakins.com

THanks very much to you all for this information.
-Jeremy


Well, that's a bit odd, most people would round it off to 125 ASA as the speed being a 2/3-stop drop from 200 ASA, just based on what numbers you can input into a light meter, but maybe Roger's meter is different than mine. Doesn't really matter though, 130 or 125.
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#9 K Borowski

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Posted 08 January 2010 - 12:40 PM

Did Roger write that?

Either he made a typo, or he is using a very precise correction factor that is in an interval of less than a third of a stop, so somewhere between 125 and 160.

Since 125 is rounded from 128, I don't know what fraction of a stop more than 125, 130 would be.


I'd suggest you not over-complicate it, though, by worrying about anything smaller than third of an F/stop.

There's certainly no "430". If you wanted to overexpose (also, confusingly, called under-rating) 500 stock by a third it'd be 400. If you wanted to underexpose it by 1/3, you'd rate it at 640. The numbers in the ISO/ASA system increase ARITHMETICALLY, which means every time you use a film that is one stop FASTER than another, you DOUBLE the speed.

A 250-speed film is HALF the speed of a 500-speed stock. So, switching from a 32- to 64-speed stock gains you the same amount as switching from a 3200- to 6400-speed stop, even though in one the number has only increased by 32 points, whereas in the other it has increased by 3200.

You're only gaining a stop in either case.




Don't feel bad that you were confused by this, though. I remember thinking the same thing when I started out.

The DIN system (the second number in a hybridized ISO rating, i.e. ISO 100/21°) is a logarithmic numerical rating. The 21° rating means the same speed as 100, but instead of doubling to 200, with DIN, the number increases to 24°. So each single-digit increase in DIN represents a third of a stop.

Maybe it is easier for you to think about speed increases logarithmically, because an increase of "3°" is a stop no matter where you are on the scale.



Don't take this article as gospel; they F-ed up the "12,800" "25,600" speed ratings and up because they didn't round to "12,500" etc., but this Wikipedia article shows the equivalences in DIN, ASA, and the former GOST system of the Soviet Block, so it'd be good for you to see the relationships between the different systems: http://en.wikipedia....wiki/Film_speed

Edited by Karl Borowski, 08 January 2010 - 12:44 PM.

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#10 Jeremy Hunt

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Posted 08 January 2010 - 01:58 PM

Did Roger write that?

Either he made a typo, or he is using a very precise correction factor that is in an interval of less than a third of a stop, so somewhere between 125 and 160.

Since 125 is rounded from 128, I don't know what fraction of a stop more than 125, 130 would be.


I'd suggest you not over-complicate it, though, by worrying about anything smaller than third of an F/stop.

There's certainly no "430". If you wanted to overexpose (also, confusingly, called under-rating) 500 stock by a third it'd be 400. If you wanted to underexpose it by 1/3, you'd rate it at 640. The numbers in the ISO/ASA system increase ARITHMETICALLY, which means every time you use a film that is one stop FASTER than another, you DOUBLE the speed.

A 250-speed film is HALF the speed of a 500-speed stock. So, switching from a 32- to 64-speed stock gains you the same amount as switching from a 3200- to 6400-speed stop, even though in one the number has only increased by 32 points, whereas in the other it has increased by 3200.

You're only gaining a stop in either case.




Don't feel bad that you were confused by this, though. I remember thinking the same thing when I started out.

The DIN system (the second number in a hybridized ISO rating, i.e. ISO 100/21°) is a logarithmic numerical rating. The 21° rating means the same speed as 100, but instead of doubling to 200, with DIN, the number increases to 24°. So each single-digit increase in DIN represents a third of a stop.

Maybe it is easier for you to think about speed increases logarithmically, because an increase of "3°" is a stop no matter where you are on the scale.



Don't take this article as gospel; they F-ed up the "12,800" "25,600" speed ratings and up because they didn't round to "12,500" etc., but this Wikipedia article shows the equivalences in DIN, ASA, and the former GOST system of the Soviet Block, so it'd be good for you to see the relationships between the different systems: http://en.wikipedia....wiki/Film_speed


THanks very much Karl

A great help, really informative.

-Jeremy
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#11 K Borowski

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Posted 08 January 2010 - 03:31 PM

THanks very much Karl

A great help, really informative.

-Jeremy


Glad I could be of service Jeremy.


Feel free to PM or respond back on this thread with any further questions you may have.

You know what's funny though, my response to your question prompted me to clean up the wikipedia article on film speeds, correcting "12,800" to 12,500 as it should be. I'm actually surprised that someone hasn't tried to re-edit my four film speed edits to their original appearance. That *must* be proof-positive that I know what I'm talking about, a Wikipedia correction that isn't immediately undone ;-)

Learning film speeds is just like learning a foreign language. You just have to learn the semantics and a reference point to your method of communication, before you understand. After that it is just common sense. . .

Edited by Karl Borowski, 08 January 2010 - 03:34 PM.

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