Converting Exposure

Sivanesan · January 19, 2004

Hi,

Is there any formulae for converting the exposure of Film Camera to a Still Camera:

For example, my Exposure Meter Says:

Film Speed: 200 ASA

Shutter Angle: 180 Degrees

Shutter Speed: 1/48

FPS: 24

and my Exposure Meter says it needs f11 for the above............

What would be the Exposure for a still Camera for the following:

Film Speed: 200 ASA

Shutter Speed: 1/125

Thanks in advance.

Regards

Sivanesan

Mitch Gross · January 19, 2004

Film Speed: 200 ASA
Shutter Angle: 180 Degrees

Shutter Speed: 1/48

FPS: 24

and my Exposure Meter says it needs f11 for the above............

What would be the Exposure for a still Camera for the following:

Film Speed: 200 ASA

Shutter Speed: 1/125

Well you can eliminate what is essentially some repetitious info to help clarify. 24fps with a 180 degree shutter is what gives you a shutter speed of 1/48th. So now you're down to two pieces of info: 200 ISO (ASA is no longer used but is the same thing) and 1/48th of a second exposure time. Each stop of light is a halving of exposure (or doubling depending which direction you go). Halve the time of exposure and you've also halved the exposure itself, so that is the one-to-one relationship between exposure time and aperture.

So if a 200 ISO film at 1/48th of a second requires an F11 for proper exposure, you need to do some simple math of the shutter speed to find the correct exposure on your still camera. 48 x 2.6 equals 125 (close enough). So the difference in exposure time is 2.6 times. Put another way, 1/125th is 2.6 times shorter an exposure time than 1/48th, so you need to open your lens aperture by 2.6 stops to allow the proper amount of light in. If correct aperture at 1/48th is F11, then correct exposure at 1/125th is F4 & 1/3.

Changing filmstock will have a similar effect. 100 ISO film is one stop less sensitive to light than 200 ISO film (it is half as sensitive).

Hope this is clear. Simple math once you understand the relationship.