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10-06-2010, 02:40 PM   #16
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I saw somewhere that only certain f-stops are actually 'full-stops' and that the ones between these are 3rd's of stops or half stops... can anyone confirm and if so explain?

The way I understood it to work was that these particular 'full-stops' marked a doubling of light allowed to hit the sensor as the aperture moves down towards being 'wide-open'. Is this anywhere near being correct?

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Dave

10-06-2010, 04:14 PM   #17
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I'll have a stab. Someone correct me where I'm wrong.

F stops are defined as focal length divided by iris diameter. So F/1 in a 50 mm focal length lens would have an iris of 50mm. As the formula for area of a circle is a function of the square root, halving the amount of light means dividing the former number by the square root of 2 - roughly 1.4 (actually 1.4142135)

Therefore to half the amount of light from F/1 (focal length divided by one), one would multiply 1 x 1.414 = F/1.414 but known as F/1.4

To halve it again multiply 1.414 x 1.414 = 1.9999396 close enough to 2

to halve again 2 x 1.4 = 2.8

and so on

You question was about half and 1/3 stops of light.
this is true, they exist the multiplication factor changes


As an aside, a 'stop' was a block of wood used in early cameras to hold the iris open at various apertures. They literally 'stopped' the iris from closing fully. Different sized blocks of wood were used fro different apertures.
10-06-2010, 05:23 PM - 1 Like   #18
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QuoteOriginally posted by DaveHolmes Quote
I saw somewhere that only certain f-stops are actually 'full-stops' and that the ones between these are 3rd's of stops or half stops... can anyone confirm and if so explain?

The way I understood it to work was that these particular 'full-stops' marked a doubling of light allowed to hit the sensor as the aperture moves down towards being 'wide-open'. Is this anywhere near being correct?

Cheers
Dave
The "Full Stops" (2, 2.8, 4, 5.6 etc.) are considered the full stops. Yes, as you move from 2 to 2.8, you half the light (double it going from 4 to 2.8).

Likewise, there is a halving of the light intensity when moving from f3.5 to f5 and is a "full stop" change, although these are not considered "full stops". Mostly we use the term full stops for two reasons, first, the full stops are based off starting on f1 and then halving the light at each succeeding stop. Second, using this method of the "full stops" becomes very easy to remember. Start with f1.4 and f2 and then double every other one (f1.4, f2, f2.8, f4, f5.6, f8, f11(.2), f16, f22(.4), f32, f45 (actually 44.8) and so on.

Using the in between stops is only important when you are doing the calculations and reporting EXIF data. Otherwise you are using the information from your meter to get your exposure or going to the closest full f stop number and tweeking the exposure from there. What's 1/3 of a stop between friends anyway!

Regards,
10-06-2010, 07:04 PM   #19
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QuoteOriginally posted by fekish Quote
ok thanks...

Because i was reading comments, and they were saying continuously "stop down" and i wasn't understanding what they meant!

So in general by stopping down, usually gives sharper images? At least that's what i understood from some other comments
Yes and no for the sharper images. Each lens has it's 'sweet spot' that has the sharpest image for that lens - which are normally found in lens reviews. But, depending on what you want, a razor sharp image sometimes isn't the best thing (think depth of focus and such) - especially when you are trying to make your photos into art.

10-06-2010, 07:54 PM   #20
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QuoteOriginally posted by DaveHolmes Quote
I saw somewhere that only certain f-stops are actually 'full-stops' and that the ones between these are 3rd's of stops or half stops... can anyone confirm and if so explain?
Wizofoz and BigDave already covered it, but I'll add that the wikipedia article for f-stop has some handy charts of the 1/2 and 1/3 stops. I found them useful when learning about new lenses and answering questions like "how much faster is an f/1.4 over a f/4?".

F-number - Wikipedia, the free encyclopedia
10-07-2010, 10:31 AM   #21
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Another approach to "What's the difference between f-stops?" is monetary. Consider typical prices of used manual lenses at 1/2-stop increments in USD:

55/2.0 - $10
55/1.7 - $25
55/1.4 - $75
55/1.2 - $400
55/1.0 - $10k (ha)
10-08-2010, 01:24 AM   #22
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QuoteOriginally posted by RioRico Quote
Another approach to "What's the difference between f-stops?" is monetary. Consider typical prices of used manual lenses at 1/2-stop increments in USD:

55/2.0 - $10
55/1.7 - $25
55/1.4 - $75
55/1.2 - $400
55/1.0 - $10k (ha)
That was nice. When I was interested in telephoto lens and was about to buy one, difference between f/4.0 and f/2.8 was over 150$.
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