# How to determine how much film is on a roll

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### #1 Craig Knowles

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Posted 09 June 2010 - 07:19 PM

I have a whole bunch of rolls of S16mm film from a project of undetermined length. The rolls themselves are all of varying length -- short ends joined together at the lab during processing into rolls of varying sizes. I need to figure out how much film I have in total.

Is there an equation which relates the radius of a roll to its length?

That is, if a particular roll is 12 inches in diameter, is there any formula that tells me I have roughly, say, 700 feet of film on the roll?

Any thoughts would be appreciated.

Thanks!
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### #2 Keith Walters

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Posted 09 June 2010 - 10:12 PM

I have a whole bunch of rolls of S16mm film from a project of undetermined length. The rolls themselves are all of varying length -- short ends joined together at the lab during processing into rolls of varying sizes. I need to figure out how much film I have in total.

Is there an equation which relates the radius of a roll to its length?

That is, if a particular roll is 12 inches in diameter, is there any formula that tells me I have roughly, say, 700 feet of film on the roll?

Any thoughts would be appreciated.

Thanks!

The problem is, your answer is going to depend on how tightly the film is wound on the roll.
If you have access to a set of reasonably accurate digital scales, you could weigh a short length of film (say 10 feet) and then divide one-tenth of that into the total weight, minus the weight of the core, to give an answer in feet. That would probably be as accurate a way as any.
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### #3 Karel Bata

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Posted 10 June 2010 - 06:05 AM

Sounds like a nice idea, but " and then divide one-tenth of that into the total weight"

That will always give you an answer of 10.

And what units..? Methinks you typed that too quickly.

EDIT Ah! I geedit. Very good.

Edited by Karel Bata, 10 June 2010 - 06:06 AM.

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### #4 Chris Millar

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Posted 10 June 2010 - 04:44 PM

We've used immersion techniques on occasion.

Dunk the feed side of your magazine into onion soup until it stops bubbling.

This only works if:

light trap efficacy / onion soup flux capacitance = 1 make sense ?

You were counting the bubbles right ?

Footage of film = 1/amount of bubbles (in hectares)

If you didn't count the bubbles you may as well throw away the remaining film as its ruined
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### #5 Craig Knowles

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Posted 10 June 2010 - 09:18 PM

If you have access to a set of reasonably accurate digital scales, you could weigh a short length of film (say 10 feet) and then divide one-tenth of that into the total weight, minus the weight of the core, to give an answer in feet. That would probably be as accurate a way as any.

That is a very, very good idea. Thanks very much!
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### #6 Dominic Case

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Posted 13 June 2010 - 02:54 AM

Back in the 70s in the laba I worked in, all print stock short ends were managed on a butcher's scale. If you had (say) a 900 ft negative to print, the stock control man would go through his box of short ends and find one that seemed about right, and weigh it out to you. Short end returns were managed the same way.

There used to be a wonderful brass ruler device, with a round knob on the end that fitted into the middle of a core. It was engraved with footages along both edges - corresponding to a 2" or 3" core. Some may still be around, or it's easy to make one: but they aren't as convenient in the dark.

The equation that relates roll radius to film length isn't terribly complicated - in fact I think I provided it in this forum a year or so ago if someone cares to search: the footage is proportional to the area of the edge of the roll, which is proportional to the square of the radius. But you have to account for the size of the core, and the thickness of the film (polyester is slightly thinner than acetate - not much, but if it's 10% thinner, your calculation will be 10% out).
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