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07-29-2009, 03:21 PM   #1
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f-stops....what do they stand for???

Sorry for my ignorance but I was just cleaning Ruby, Charlie, Fat Albert and Pop and it dawned on me that even though I know what f-stops mean in a practical sense...eg.f1.8 open aparture = faster shutter speeds and less DOF etc. versus f13 =slower shutter speeds and lots of DOF.
But then I asked myself...."1.8 what?...13 what?
What do the numbers actually mean? What are they a measurement OF?
I am annoyed that I don't actually know.
Please advise before my kids ask me.
Jack.

07-29-2009, 03:31 PM   #2
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Here is more than you ever want to know:
F-number - Wikipedia, the free encyclopedia
In case you are in a hurry...in layman's terms, it is the focal length divided by the diameter of the aperture.

Steve
07-29-2009, 03:41 PM   #3
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QuoteOriginally posted by stevebrot Quote
Here is more than you ever want to know:
F-number - Wikipedia, the free encyclopedia
In case you are in a hurry...in layman's terms, it is the focal length divided by the diameter of the aperture.

Steve
Thankyou and you were right.
Its much more than I would EVER want to know f/\# = N = \frac fD \

Cheers
07-29-2009, 04:37 PM   #4
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QuoteOriginally posted by jackbullet Quote
Thankyou and you were right.
Its much more than I would EVER want to know f/\# = N = \frac fD \

Cheers
then how can some zoom lenses have a constant f# throughout it's FL without changing the diameter of the aperture?

07-29-2009, 04:42 PM   #5
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QuoteOriginally posted by mtroute Quote
then how can some zoom lenses have a constant f# throughout it's FL without changing the diameter of the aperture?
DOH!!!! because, F/N = F-.8xFN multiplied by factor zoom!!!! Thats why!!!
07-29-2009, 06:22 PM   #6
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QuoteOriginally posted by stevebrot Quote
Here is more than you ever want to know:
F-number - Wikipedia, the free encyclopedia
In case you are in a hurry...in layman's terms, it is the focal length divided by the diameter of the aperture.

Steve
Again, a very succinct explanation!
07-29-2009, 06:40 PM   #7
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QuoteOriginally posted by mtroute Quote
then how can some zoom lenses have a constant f# throughout it's FL without changing the diameter of the aperture?
The actual calculation uses the "affective" aperture. My understanding is that constant aperture zooms have a mechanism that is able to modify the affective aperture at shorter focal lengths. I know that sounds like a bit of a cop-out, but that is the explanation I was given.

Steve
07-30-2009, 06:28 PM   #8
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QuoteOriginally posted by mtroute Quote
then how can some zoom lenses have a constant f# throughout it's FL without changing the diameter of the aperture?
If you remove a zoom lens from your camera and look through the front of it while holding it up against a white background and rotating the zoom ring, you'll see that the apparent diameter of the white circle decreases as you zoom toward a wider angle of view. This is the "effective aperture", and it's what's used in the f/ratio calculation.

07-30-2009, 07:00 PM   #9
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QuoteOriginally posted by mtroute Quote
then how can some zoom lenses have a constant f# throughout it's FL without changing the diameter of the aperture?
It is a function of how the aperture blades are located within the lens and the light path.

Constant aperture zooms are more expensive to make, as a rule (especially in telephoto zooms) , because to offer the constant (and low) F number through out the whole range, usually drives the front element diameter up. At the end of the day, no matter where you put the diaphram on the lens, a 50 mm (for example) diameter front element will only ever be an F4 200mm lens (note here front element is the real front element diameter not the filter diameter).

During the mid 1980's as TTL flash became more dominant, the need for constant aperture zooms went away, TTL metering could cope with the loss of lght from zooming, and with TTL flash, the sensor saw light off the film plane directly, so the need for constant aperture zooms went away. Since variable aperture lenses were cheaper, and offered at the short end, faster lenses, they became popular. It is a case where camera technology allowed lenses to become less expensive.
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