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F Stops explained
This should help many people..

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Comments (17)

Grablife wrote:.

For those just starting out..

Http://blog.patyuen.com/lessons/photography-f-stops-explained/.

I got to the 2nd sentence before I found an error:.

"The f is an abbreviation for the term factor...".

Actually, the "f" represents the focal length of the lens..

Http://en.wikipedia.org/wiki/F-numberhttp://blog.epicedits.com/...06/16/so-you-think-you-know-what-an-f-number-is/http://dictionary.reference.com/browse/f-numbers.

The "formula" is f-number = FL / D.

[where "D" represents the diameter of the aperture].

Thus, it's correctly written as f/2.8 or f:2.8...it's a ratio. The "f" represents the FL of the lens. Even if the Nikon School writes otherwise... .

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #1

There are different definitions of what exactly the F stands for in F-Stop. By your own definition it makes little sense if it stands for Focal length. If the F stand for Focal length, then the formula f=f/d makes little sense. How can the Focal length=Focal length/Diameter?.

What's important to understand is that F-Stop is a ratio of Focal length to Diameter. For all I care, it can be call Fidget..

Chuxter wrote:.

Grablife wrote:.

For those just starting out..

Http://blog.patyuen.com/lessons/photography-f-stops-explained/.

I got to the 2nd sentence before I found an error:.

"The f is an abbreviation for the term factor...".

Actually, the "f" represents the focal length of the lens..

Http://en.wikipedia.org/wiki/F-numberhttp://blog.epicedits.com/...06/16/so-you-think-you-know-what-an-f-number-is/http://dictionary.reference.com/browse/f-numbers.

The "formula" is f-number = FL / D.

[where "D" represents the diameter of the aperture].

Thus, it's correctly written as f/2.8 or f:2.8...it's a ratio. The"f" represents the FL of the lens. Even if the Nikon School writesotherwise... .

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'.

Http://blog.patyuen.comhttp://www.FashionSF.com..

Comment #2

Grablife wrote:.

Not necessarily.

When you turn the aperture ring you set the aperture. What are you setting? You're setting the diameter of the aperture opening. What are you setting it to? Setting the aperture to f/2 or f:2 means the focal length divided by 2, so for a 50mm lens youre setting the aperture opening to a diameter of 50 / 2...or 25mm. If you set the aperture to f/4 then youve set the aperture opening to a diameter of 50 / 4...or 12.5mm..

Thats why the "f" means focal length...

Comment #3

Hi,.

FWIW, the letter "f" can stand for a lot of things but in optics it stands for the focal length and has done so for a good many years/centuries..

And to explain that each one is the previous one multiplied by 1414 etc is daft. Better to show that 1, 14, 2, 28, 4, 56 etc when squared simply double. Like.

1 = 1; 14 = 2; 2 = 4; 28 = 8; 4 = 16; 56 = 32 and so on..

The figures get rougher along the way but that's just to fit the engraving on the lens. (If I could find the squiggle that means "approximately" in maths, then I would use it but I don't even know what type face we're using here. I expect someone will tell me... ).

Regards, David..

Comment #4

Grablife wrote:.

There are different definitions of what exactly the F stands for inF-Stop. By your own definition it makes little sense if it stands forFocal length. If the F stand for Focal length, then the formula f=f/dmakes little sense. How can the Focal length=Focal length/Diameter?.

Read the formula again?.

The "formula" is f-number = FL / D.

[where "D" represents the diameter of the aperture].

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #5

David Hughes wrote:.

Hi,.

FWIW, the letter "f" can stand for a lot of things but in optics itstands for the focal length and has done so for a good manyyears/centuries..

And to explain that each one is the previous one multiplied by 1414etc is daft. Better to show that 1, 14, 2, 28, 4, 56 etc whensquared simply double. Like.

1 = 1; 14 = 2; 2 = 4; 28 = 8; 4 = 16; 56 = 32 and so on..

The figures get rougher along the way but that's just to fit theengraving on the lens. (If I could find the squiggle that means"approximately" in maths, then I would use it but I don't even knowwhat type face we're using here. I expect someone will tell me... ).

I think it's ~ Arial/Helvetica/Swiss... .

Charlie DavisNikon 5700, Sony R1, Nikon D300HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/'Experience: Discovering that a claw hammer will bend nails.Epiphany: Discovering that a claw hammer is two tools...'..

Comment #6

OP is correct..

According to the Wikipedia article a poster referenced, the f (the f-number) is dimensionless (since you're dividing the FL by the diameter, both expressed in mm), hence a "factor", a "ratio", a "whatever you want to call it to understand it represents the amount of light passing through the diaphragm.".

We commonly talk of setting the aperture to f/5.6, max. aperture of f/1.4, stop down aperture to f/16, etc..

On a lens of fixed focal length, say 50mm, we can set f to be from a range, say from f/1.4 to f/16, and all we're doing is making the aperture hole larger or smaller, i.e. varying the amount of light that goes through the lens. FL stays at 50mm...

Comment #7

My wong wrote:.

We commonly talk of setting the aperture to f/5.6, max. aperture off/1.4, stop down aperture to f/16, etc..

Yes we do. But when expressed (correctly) as f/5.6 the letter f represents the focal length. Hence for a 50mm lens, when setting the aperture to f/5.6, we are setting it to a diameter of 8.8mm..

So the f-number itself (5.6 in this case) is dimensionless, but the aperture f/5.6 has the dimension of length, the same as the focal length.Regards,Peter..

Comment #8

Chuxter wrote:.

Grablife wrote:.

For those just starting out..

Http://blog.patyuen.com/lessons/photography-f-stops-explained/.

I got to the 2nd sentence before I found an error:.

"The f is an abbreviation for the term factor...".

Actually, the "f" represents the focal length of the lens..

Nope, it doesn't..

The f in f/5.6 represents the focal length, i.e. it is the first letter of "focal length"..

The f in f-number (or f-stop) represents the ratio between the focal length and the aperture. Or to put it another way, the f in f-number is simply the first letter in "f/5.6". Pedantic, confusing, but true..

However, I don't believe the link in the OP when it says that the f stands for 'factor'. I think that is one of those things that someone has made up after the fact...

Comment #9

Complete nonscence is written below. That is what these forums are for though I guess.jules.

Graystar wrote:.

Grablife wrote:.

Not necessarily.

When you turn the aperture ring you set the aperture. What are yousetting? You're setting the diameter of the aperture opening. Whatare you setting it to? Setting the aperture to f/2 or f:2 meansthe focal length divided by 2, so for a 50mm lens youre settingthe aperture opening to a diameter of 50 / 2...or 25mm. If you setthe aperture to f/4 then youve set the aperture opening to adiameter of 50 / 4...or 12.5mm..

Thats why the "f" means focal length..

Why can't you blow bubbles with chewing gum?..

Comment #10

???.

In what way is it nonsense? Graystar's post is correct, subject to the clarification that it is the f in f/2 that means focal length. (Other posts have discussed the f in f-number.).

Makes a change from discussing the f in ridiculous I guess..

JulesJ wrote:.

Complete nonscence is written below. That is what these forums arefor though I guess.jules.

Graystar wrote:.

Grablife wrote:.

Not necessarily.

When you turn the aperture ring you set the aperture. What are yousetting? You're setting the diameter of the aperture opening. Whatare you setting it to? Setting the aperture to f/2 or f:2 meansthe focal length divided by 2, so for a 50mm lens youre settingthe aperture opening to a diameter of 50 / 2...or 25mm. If you setthe aperture to f/4 then youve set the aperture opening to adiameter of 50 / 4...or 12.5mm..

Thats why the "f" means focal length..

Why can't you blow bubbles with chewing gum?..

Comment #11

Sherwoodpete wrote:.

So the f-number itself (5.6 in this case) is dimensionless, but theaperture f/5.6 has the dimension of length, the same as the focallength..

Hi,.

Years since I did number and dimension theory but ratios are numbers and not dimensions. I think; I'm better at queue theory... F'instance f would be inches or millimetres and the aperture's dia would be inches or millimetres too..

Regards, David..

Comment #12

Steve, Maybe I'm misunderstanding what's being written. But If I use, say, a 400mm lens and set it to F2 then the opening (according exactly to what's stated below) is 400/2=200mm. Hmmmn, 200mm that's about 8 inches. I've never seen a 400mm lens that big, have you?That's why I think that explanation, and formula is nonsence.Jules.

Steve Balcombe wrote:.

???.

In what way is it nonsense? Graystar's post is correct, subject tothe clarification that it is the f in f/2 that means focal length.(Other posts have discussed the f in f-number.).

Makes a change from discussing the f in ridiculous I guess..

JulesJ wrote:.

Complete nonscence is written below. That is what these forums arefor though I guess.jules.

Graystar wrote:.

Grablife wrote:.

Not necessarily.

When you turn the aperture ring you set the aperture. What are yousetting? You're setting the diameter of the aperture opening. Whatare you setting it to? Setting the aperture to f/2 or f:2 meansthe focal length divided by 2, so for a 50mm lens youre settingthe aperture opening to a diameter of 50 / 2...or 25mm. If you setthe aperture to f/4 then youve set the aperture opening to adiameter of 50 / 4...or 12.5mm..

Thats why the "f" means focal length..

Why can't you blow bubbles with chewing gum?.

Why can't you blow bubbles with chewing gum?..

Comment #13

Infact, to clarify I you guys are confusing the diameter of the lens and the focal length of the lens. It's the diameter you dive by 2 for an F2 setting.Jules.

JulesJ wrote:.

Steve, Maybe I'm misunderstanding what's being written. But If I use,say, a 400mm lens and set it to F2 then the opening (accordingexactly to what's stated below) is 400/2=200mm. Hmmmn, 200mm that'sabout 8 inches. I've never seen a 400mm lens that big, have you?That's why I think that explanation, and formula is nonsence.Jules.

Steve Balcombe wrote:..

Comment #14

JulesJ wrote:.

Infact, to clarify I you guys are confusing the diameter of the lensand the focal length of the lens. It's the diameter you dive by 2 foran F2 setting.Jules.

JulesJ wrote:.

Steve, Maybe I'm misunderstanding what's being written. But If I use,say, a 400mm lens and set it to F2 then the opening (accordingexactly to what's stated below) is 400/2=200mm. Hmmmn, 200mm that'sabout 8 inches. I've never seen a 400mm lens that big, have you?That's why I think that explanation, and formula is nonsence.Jules.

Jules:.

The "f-number" (in this case 2) represents the ratio of the focal length to the diameter of the aperture of the lens. As was pointed out in an earlier post, this is expressed as.

FL / D = 2.

That equation can be rewritten as.

FL / 2 = D.

That is, the diameter of the aperture is equal to the focal length divided by the f-number, in this case 2. So yes, Steve and the previous poster were correct in stating this..

I don't know if you have ever seen a 400 mm f/2 lens (I certainly haven't, but the thought makes me drool), but if you had, it would have a physical diameter of about 200 mm or greater, in order to accomodate an internal aperture of about 200 mm. To test this out, look at 400 mm lenses that are available; eg. http://www.pictureline.com/products/488/Canon_EF_400mm_f2.8L_IS_USM/..

The 400 mm f/2.8 lens would need an aperture of about 400 / 2.8 = 142 mm, or about 5.6 inches; the actual external diamater is 6.4 inches..

I understand that the effective aperture is not always exactly the same as the physical aperture inside the lens, but I'll let someone more qualified than me comment on that..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #15

JulesJ wrote:.

Steve, Maybe I'm misunderstanding what's being written. But If I use,say, a 400mm lens and set it to F2 then the opening (accordingexactly to what's stated below) is 400/2=200mm. Hmmmn, 200mm that'sabout 8 inches. I've never seen a 400mm lens that big, have you?That's why I think that explanation, and formula is nonsence.Jules.

How about a 500mm f/2.8 lens. It would need an aperture diameter of 500/2.8 = 178mm or about 7 inches..

Does this lens appear to contradict that? Its the Sigma APO 200-500mm F2.8 EX DG..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

Http://www.dpreview.com/news/0703/07030805sigma200500mm.asp.

Regards,Peter..

Comment #16

JulesJ wrote:.

Steve, Maybe I'm misunderstanding what's being written. But If I use,say, a 400mm lens and set it to F2 then the opening (accordingexactly to what's stated below) is 400/2=200mm. Hmmmn, 200mm that'sabout 8 inches. I've never seen a 400mm lens that big, have you?.

The front of the lens must be at least as large as the largest (effective) aperture opening..

Youve never seen a 400mm lens that big because the fastest is f2.8. However, on a Canon 400mm f/2.8 IS USM lens, the diameter is 163mm, which is 400 / 2.8 = 143mm, plus 20mm for the housing..

Http://www.usa.canon.com/...categoryid=154&modelid=7319#ModelTechSpecsAct..

Comment #17

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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