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Old Mar 29, 2003, 8:41 PM   #1
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Default Basic Exposure Question

I need some clarification.

Lets say I have an initial exposure of f/5.6, 1/125. I know that +1 f-stop of that would be f/4, 1/125, and -1 f-stop would be f/8, 1/125. The f/4 would be +1 f-stop because I am opening the aperture iris one stop larger and f/8 would be 1 f-stop because I'm closing the aperture iris one stop smaller.

If I change the shutter speed instead of changing the aperture, then would f/5.6, 1/60 be +1 f-stop, since I'm letting in more light, and f/5.6, 1/250 is -1 f-stop since I'm letting in less light?
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Old Mar 29, 2003, 9:37 PM   #2
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A one stop change doubles/halves the amount of light reaching the film/detector. You can double/halve the light by halving/doubling the shutter speed or by changing the f/stop number by the square root of two. Square root of two instead of double because it is the area of the opening that determines the amount of light and the f/number is a measure of the radius.

pi-Rsquared and all that.

I've never figured out which way the plus or minus works so have just refered to opening or closing one stop instead.
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Old Mar 29, 2003, 9:52 PM   #3
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Default Re: Basic Exposure Question

Quote:
Originally Posted by gmlasam
I need some clarification.

Lets say I have an initial exposure of f/5.6, 1/125. I know that +1 f-stop of that would be f/4, 1/125, and -1 f-stop would be f/8, 1/125. The f/4 would be +1 f-stop because I am opening the aperture iris one stop larger and f/8 would be 1 f-stop because I'm closing the aperture iris one stop smaller.

If I change the shutter speed instead of changing the aperture, then would f/5.6, 1/60 be +1 f-stop, since I'm letting in more light, and f/5.6, 1/250 is -1 f-stop since I'm letting in less light?
Please refer to the Exposure Mode section of my Nikon Coolpix 4500 for the details. Although these pages are for the Coolpix 4500, the theory and techniques are all the same. The difference is the way of setting various modes and doing exposure compensation.

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 user guide
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Old Mar 30, 2003, 3:28 AM   #4
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This may be going beyond the question a little, but you also got to think of what you're shooting...yes it's like a pan balance if you close down the lens and slow down the shutter speed, or open up the lens and speed up the shutter (i.e. F5.6-1/60 & F2.8-1/250)you'll get the same overal exposure because although you gave it two stops less light through the aperture, you also gave it two stops more light by leaving the shutter open longer...

BUT the shutter speed and aperture affects the picture in other ways, for instance the F-stop you choose affects Depth of Field (DOF)...that means what is in (or out of ) focus behind and in front of the subject. Also your shutter speed affects if shooting a fast moving object (or panning the camera) if the picture will be frozen or blurred. It depends on what you're shooting...that's what shutter priority and aperture priority is for; you concentrate on one, and the camera compensates with the other.

There's tons of photography books on the subject, and check out this site for playing with Depth of Field, http://www.photonhead.com/exposure/simcam.htm
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Old Mar 30, 2003, 5:34 AM   #5
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Yes, you are exactly correct.

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