ASA/exposure ratings

Ryan K · January 12, 2005

Can one of you gents furnish me with a scale with which to interpret different ASA ratings into their respective differences in exposure? For instance, if I were working with a 250D stock and I wanted to consistantly overexpose it by a stop what would the revised rating be?

Wendell_Greene · January 12, 2005

Rate it at 125 ASA

John Pytlak RIP · January 12, 2005

Here are the normally used film exposure indices, in 1/3 stop increments:

16

20

25

32

40

50

64

80

100

125

160

200

250

320

400

500

640

800

1000

Note the pattern of numbers in this series that are the 1-stop increments (each is about twice the previous):

16-32-64-125-250-500-1000

20-40-80-160-320-640

25-50-100-200-400-800

So, if I have an EI500 film, and want to set my meter to overexpose by a stop, I set the meter to a 1-stop lower index (250).

If I want to overexpose by only 1/3 stop, I set the meter to 400.

To overexpose by 2/3 stop, set the meter to 320.

See the pattern?

Greg Gross · January 12, 2005

Just remind yourself though to be aware of light changing. It may change

enough to effect your look,mood,color.

Greg Gross

Ryan K · January 12, 2005

You are veritable founts of knowledge.

Justin Hayward · January 13, 2005

I?m curious about frame rate. Isn?t doubling it is one stop? 30 to 60 is one stop. 60 to 120 is one stop. What about 500 to 1000? This can?t be one stop? right? I?m missing something.

David Mullen ASC · January 13, 2005

I?m curious about frame rate. Isn?t doubling it is one stop? 30 to 60 is one stop. 60 to 120 is one stop. What about 500 to 1000? This can?t be one stop? right? I?m missing something.

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Yes, 1000 ASA is one-stop faster than 500 ASA. 2000 ASA is one-stop faster than 1000 ASA, etc.

Justin Hayward · January 13, 2005

You?re right. I understand, but (for some reason) I can?t wrap my brain around it. I just have to trust my meter.

It?s just interesting to me.

Dominic Case · January 13, 2005

One stop IS double or half the exposure.

You can vary a lot of things to get the effect of one stop. For reasons to do with maths, physics, and the way the various controls are defined, they don't all follow the same scale.

So EI (ASA) ratings go in steps of doubling for each stop: 64, 125, 250, 500 etc.

Frame rates go in steps of doubling for each stop of light lost: 24, 48, 96 etc and for higher speeds, 250, 500, 1000 etc

Shutter speeds on still cameras, the same: 1/30, 1/60, 1/125, 1/250, 1/500 etc

F/stops follow a different scale, but still, one stop is half the exposure, half the amount of light. The scale goes by the square root of 2: so f/2, f/2.8, f/4, f.6, f/8 etc, in one stop steps.

ND filters follow a different scale again: not a factor, but adding .30 for each stop: so ND0.30, ND0.60, ND0.90 etc. (the filters are named by dropping all the zeros and decimal points and other drivel, so ND3, ND6 etc. An ND9 is worth 3 stops, and is the same as an ND3 added to an ND6.