Hello, thanks to whoever reads this and may be able to give me an answer. Being new to learning cinematography I have been going over the ACM 9th edition. This may be a stupid question, but in the formual for figuring out footcandles needed to expose a certain ASA the formual is

E=25 fsquared

over

St

In working the formual it obviously works, but can anybody tell me what the 25 represents. I have looked all over and I just can seem to figure it out.

Again, this may be a stupid question, but if anybody can give me an answer that would be great.

# Technical Question

Started By Chris Jordan, Mar 23 2008 08:28 AM

4 replies to this topic

### #1

Posted 23 March 2008 - 08:28 AM

### #2

Posted 23 March 2008 - 11:38 AM

I don't know that formula.

I just use the old rule (for a camera running at 24 fps with a 1/48th shutter speed):

From that you can figure out most variations of ASA or footcandles or f-stops.

I just use the old rule (for a camera running at 24 fps with a 1/48th shutter speed):

**100 fc at 100 ASA = f/2.8**From that you can figure out most variations of ASA or footcandles or f-stops.

### #3

Posted 24 March 2008 - 02:49 AM

I'm not familiar with that formula or with the terms you use (E and st).

But the relationship between f/stop, film speed and footcandles is of exactly that sort.

The film speed (ASA rating) was originally calculated in an extraordinarily complicated way based on the exposure needed to reach a density a certain amount more than the minimum density of the film, when it was processed to a certain gamma, and the moon was at a particular angle in the sky etc.

As it turns out, there is then a simple relationship betwen the film speed and exposure settings on a camera. In proportional terms, doubling the footcandles, doubling the film speed, or halving the square of the f/stop all have the same effect that is, twice as much light on the film.

But it needs a constant, or a conversion factor to skip from the ASA system to the camera settings. It turns out to be 25. It doesn't mean anything more than saying a kilogram is 2.24 pounds.

But the relationship between f/stop, film speed and footcandles is of exactly that sort.

The film speed (ASA rating) was originally calculated in an extraordinarily complicated way based on the exposure needed to reach a density a certain amount more than the minimum density of the film, when it was processed to a certain gamma, and the moon was at a particular angle in the sky etc.

As it turns out, there is then a simple relationship betwen the film speed and exposure settings on a camera. In proportional terms, doubling the footcandles, doubling the film speed, or halving the square of the f/stop all have the same effect that is, twice as much light on the film.

But it needs a constant, or a conversion factor to skip from the ASA system to the camera settings. It turns out to be 25. It doesn't mean anything more than saying a kilogram is 2.24 pounds.

### #4

Posted 24 March 2008 - 08:32 AM

Thanks for your help. I believe that answers my question.

### #5

Posted 24 March 2008 - 11:55 AM

I just use the old rule (for a camera running at 24 fps with a 1/48th shutter speed):

100 fc at 100 ASA = f/2.8

As David say this formula could be your best friend. If you don't have near you and you need this guide numbers in only a few minutes you can fill and make your Incident Key Light table... Skills in math is your key...

Xavier