Quick question about exposure.

Hey I got a quick question that I've always wondered but never really took the time to figure it out.

Let's say I have a scene and the proper settings to expose are 30 seconds at f4. Now let's say that I want the shutter to be open for 60 seconds. How would I properly adjust the aperture to match doubling the shutter? This is all assuming the lighting hasn't changed and is consistent over time.

If I wanted to double the shutter from 30 to 60 seconds, would I also double the aperture from 4 to 8, or would doubling the exposure equal 1 stop of aperture going from 4 to 5.6?

Thank you for taking the time to read!

Here's another example if my phrasing doesn't make sense. So I was shooting a sunset the other day where at 30 seconds, f4, ISO 100 - the exposure was perfect but I wanted streakier clouds. How would I know what aperture to set if I wanted to BULB shoot at 60 seconds, or 120 seconds, and so on. Is there a simple way to know how far to adjust aperture when doubling, tripling, etc. the exposure time?

JM Photos wrote in post #17667078
Hey I got a quick question that I've always wondered but never really took the time to figure it out.

Let's say I have a scene and the proper settings to expose are 30 seconds at f4. Now let's say that I want the shutter to be open for 60 seconds. How would I properly adjust the aperture to match doubling the shutter? This is all assuming the lighting hasn't changed and is consistent over time.

If I wanted to double the shutter from 30 to 60 seconds, would I also double the aperture from 4 to 8, or would doubling the exposure equal 1 stop of aperture going from 4 to 5.6?

Whole f-stop numbers are multiples of the square root of 2 (approximately 1.414).

One "stop" of adjustment of the aperture (f-stop) such as the difference between f/5.6 and f/8 or between f/4 and f/5.6 halves or doubles (depending on which way the change is, of course) the resulting exposure.

Doubling or halving f-stop values (such as f/4 to f/8) results in two stops of adjustment (1/4 or four times the exposure).

Adjusting shutter speeds or ISO values affects the exposure in a direct relationship to the numbers.

Bottom line - a 30-second exposure at f/4 would be the equivalent of a 60-second exposure at f/5.6, assuming the ISO value (or film "speed") did not change.

A little history: When doing very long exposures at low light levels with film, there's a thing called reciprocity that gets in the way. The exposures at very low light levels require additional time because of the way film responds.

SkipD wrote in post #17667116
Whole f-stop numbers are multiples of the square root of 2 (approximately 1.414).

One "stop" of adjustment of the aperture (f-stop) such as the difference between f/5.6 and f/8 or between f/4 and f/5.6 halves or doubles (depending on which way the change is, of course) the resulting exposure.

Doubling or halving f-stop values (such as f/4 to f/8) results in two stops of adjustment (1/4 or four times the exposure).

Maybe more than you wanted to know: This is based on the area of the aperture which is a circle, & the only variable is the radius (f-stop) squared.
So f/4 squared is 16. F/8 squared is 64.
64/16 = 4 times the exposure when you go from f/4 to f/8, as Skip said. Now you can figure the exposure change between any two f-stops.

JM Photos wrote in post #17667078
Hey I got a quick question that I've always wondered but never really took the time to figure it out.

Let's say I have a scene and the proper settings to expose are 30 seconds at f4. Now let's say that I want the shutter to be open for 60 seconds. How would I properly adjust the aperture to match doubling the shutter? This is all assuming the lighting hasn't changed and is consistent over time.

If I wanted to double the shutter from 30 to 60 seconds, would I also double the aperture from 4 to 8, or would doubling the exposure equal 1 stop of aperture going from 4 to 5.6?

Thank you for taking the time to read!

Here's another example if my phrasing doesn't make sense. So I was shooting a sunset the other day where at 30 seconds, f4, ISO 100 - the exposure was perfect but I wanted streakier clouds. How would I know what aperture to set if I wanted to BULB shoot at 60 seconds, or 120 seconds, and so on. Is there a simple way to know how far to adjust aperture when doubling, tripling, etc. the exposure time?

Process:

Shutter is 30 seconds.
Aperture is F4.
ISO 100.

Shutter to 60 seconds (+1 stop of light).
Aperture becomes F5.6 (-1 stop of light).
ISO 100.
And exposure stayed the same, but you gained more time (for more blur of the clouds).

A stop of light is a stop of light regardless of where it comes from in the exposure triangle.

Very best,

or if you dont want to math in your head, get a chart of stops and adjust the aperture and shutter speed that way.

Perfect, that explains it!

30 seconds at f/4, 60 seconds at f/5.6, 120 seconds at f/8, 240 seconds at f/11, 480 seconds at f/16, and 960 seconds at f/22.

JM Photos wrote in post #17667482
Perfect, that explains it!

30 seconds at f/4, 60 seconds at f/5.6, 120 seconds at f/8, 240 seconds at f/11, 480 seconds at f/16, and 960 seconds at f/22.

You got it.

Or, you can leave it at F8 or F11, and add ND filters to drop stops of light and stay in the best range of aperture for sharpness while still opening up that shutter for a long time. Especially in bright day light, to open up more possibilities.

Example:

F22 at ISO 100, with 13 stops of ND filter stacked, allowed for an 8 minute (480 seconds) exposure in broad day light to do what you were talking about (blurring clouds).

IMAGE LINK: https://flic.kr/p/wUwV​8E