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# Confused about F-number

Started Jul 10, 2002 | Discussions thread
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Confused about F-number

I recently got an F707 and I am trying to understand how it works.

According to what I've read, the F-number is a ratio of focal length to aperture diameter.

F-number = f/d

f= focal distance in mm
d= aperture diameter mm

I want to know the actual aperture diameter when its set at F-8, so here's what it calculates to:

Example 1: Focal length 9.7mm divided by F-number 8 yields a diameter of 1.21mm

Example 2: Focal length 48.5 divided by F-number 8 yields a diameter of 6.06mm

But all I have done is zoom from full wide (example 1) to full telephoto (example 2). How can the aperture diameter change so dramatically when I haven't touched the aperture setting?

What am I doing wrong?

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