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Dumb Question of the Day...
Old 02-05-2007, 06:53 PM   #1 (permalink)
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Where do f-stop numbers come from?

Doc
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Re: Dumb Question of the Day...
Old 02-05-2007, 07:54 PM   #2 (permalink)
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www.uscoles.com/fstop.htm
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Re: Dumb Question of the Day...
Old 02-05-2007, 08:25 PM   #3 (permalink)
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It's actually a ratio of the width of the opening compared to the distance from the film / ccd
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Re: Dumb Question of the Day...
Old 02-06-2007, 04:44 AM   #4 (permalink)
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Reason I'm asking is - I was reading an article (which I am trying to locate) the other day on exposure values and the inverse square law. The article said that originally, f-stop numbers relate to the distance a light source may be moved closer to, or further from, a target to either double or halve the amount of light falling on the target. For example, if a light was 4 feet from a target, and you wanted to double the amount of light falling on the target, you would move the light to a position 3.5 feet from the target. To halve the amount of light falling on the target, you would move the light to a position 5.6 feet from the target. To halve the amount of light again, the distance would be 8 feet, and so on. From what I recall the article also said the original term was f-steps instead of f-stops and that the numbers were guides for establishing lighting ratios for portraiture. Interesting, eh? Has anyone ever heard of this explanation?

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Old 02-06-2007, 11:04 AM
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Re: Dumb Question of the Day...
Old 02-06-2007, 04:45 PM   #5 (permalink)
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Doc,

The link in the other post tells it like it is. However, your explanation reminds me of the T-stop. Used to be used a lot in cinematography, wherein the T (transmission factor) was a function of light vs. distance.
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Re: Dumb Question of the Day...
Old 02-07-2007, 12:36 AM   #6 (permalink)
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Quote:
Originally Posted by trainstation View Post
I have a background in math & physics and have never had a problem understanding this. I have had problems trying to explain it to others. That is the best written explanation of these concepts I have ever seen. Thanks for posting the link.

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Re: Dumb Question of the Day...
Old 02-07-2007, 05:39 AM   #7 (permalink)
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Several weeks ago we had a speaker at our local camera club who brought along part of his large collection of antique cameras and other items of photographic interest.

One of the items he brought was a little wooden box containing a collection of Aperture Stops!

This early system was for the old glass plate cameras with a fixed aperture lens. They were small metal plates that fitted into a slot behind the lens to stop the light. Each plate had a different size hole cut in it at different fractions of the lens focal length.

The numbers relate to the ratio of the diameter of the hole and the focal length of the lens as explained above. So an f8 plate would have a hole diameter 1/8 the focal length of the lens, and the f4 plate would have a hole diameter 1/4 the focal length of the lens.

I am sure he explained where the f bit came from as well, but I can't remember.
I will ask at the camera club this week to see if anyone else can recall.
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Re: Dumb Question of the Day...
Old 02-07-2007, 02:08 PM   #8 (permalink)
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That's the way the Lens Baby II lens works. You have these little metal disks with a hole and you drop one of them in for the aperture you want. I also had an old soft focus lens that used plastic disks in the same way (Sima).
Things keep changing but coming around again.

And many will also ask, what does the F in Fstop stand for. Well there are many stories. Some try to say it was from Ansel Adams, but the term was used in the 1800's, so the Adams idea won't work. I think the F simply stands for Focal length. After all if you want to know the size of the hole you're shooting through, its focal length divided by the fstop setting. So if we have a 100mm lens and we're at F2, then then size of the hole is 50mm (F/2) or (100/2). It used to be you always saw the / being included in fstops. Now days, we don't see it included and so we forget it's a ratio to describe the size of the hole (aperture).

Cheers,
rfs
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Re: Dumb Question of the Day...
Old 02-07-2007, 02:55 PM   #9 (permalink)
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Quote:
Originally Posted by DocSmith View Post
Where do f-stop numbers come from?

Doc
well, if a mummy f-stop and a daddy f-stop love each other very much, then...
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Re: Dumb Question of the Day...
Old 02-10-2007, 05:17 PM   #10 (permalink)
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Yeah, those are actually known as Waterman stops. When I was at the Navy photo school (lo these many years) the senior guys would always send some green kid upstairs 4 floors to get a box of f-stops (kinda like going after 50 feet of shoreline). They tried that stuff with me and I asked "You mean Waterman stops?". That shut 'em down for a while until they could come up with better gags to try on me.
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