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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 31 Jan 2014 (Friday) 19:38
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Does f/ number value go up when you "stop down"?

 
skilsaw
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Jan 31, 2014 19:38 |  #1

There is so much jargon around aperture.
For example: Faster lens means larger aperture means smaller aperture value.

Does"The aperture goes up" mean "The value goes down and the aperture gets bigger"?

Does "Stopping down" mean "The aperture value goes up and the aperture gets smaller?"

Does "Increase your aperture" mean "the f/ value goes up and the aperture gets smaller?"

I'm confused.:confused:




  
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chris_holtmeier
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Jan 31, 2014 19:51 |  #2

Because it's a ratio.

Example: a 50mm lens at F2 has a 25mm aperture. That same 50mm at F4 has a 12.5mm aperture.



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archer1960
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Jan 31, 2014 19:56 |  #3

skilsaw wrote in post #16653953 (external link)
There is so much jargon around aperture.
For example: Faster lens means larger aperture means smaller aperture value.

Does"The aperture goes up" mean "The value goes down and the aperture gets bigger"?

Does "Stopping down" mean "The aperture value goes up and the aperture gets smaller?"

Does "Increase your aperture" mean "the f/ value goes up and the aperture gets smaller?"

I'm confused.:confused:

I've never seen the term "aperture goes up", so I'm not sure what they would mean by that.

"Stopping down" *always* means making the hole smaller, therefore the f-number gets larger. I've never seen an exception to this definition, though it's certainly possible. The common term for the opposite adjustment is to "open up" your aperture.

"Increase your aperture" *usually* means open it up, make the hole larger, and the f-number smaller.


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xarqi
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Jan 31, 2014 19:57 |  #4

You're right. It's confusing. The main reason for the confusion is that people use the word "aperture" when they mean "f ratio".

"Aperture" is the diameter of the entrance pupil of a lens.
"f ratio" is the ratio of the focal length to the actual aperture, as controlled by the iris diaphragm, thus f/something. For example, f/2 means the aperture is half the focal length.
That "something", "2" in this case, is often called the "f number".




  
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Snydremark
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Jan 31, 2014 20:05 |  #5

Yes.

"Stopping down" is the act of making the physical opening smaller.


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Nick3434
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Jan 31, 2014 20:30 |  #6

xarqi wrote in post #16653982 (external link)
You're right. It's confusing. The main reason for the confusion is that people use the word "aperture" when they mean "f ratio".

Guilty as charged:D

I think if I started saying "shot this at F ratio 8" it would confuse more people though:lol:


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xarqi
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Jan 31, 2014 20:43 |  #7

Nick3434 wrote in post #16654036 (external link)
Guilty as charged:D

I think if I started saying "shot this at F ratio 8" it would confuse more people though:lol:

True. It's common enough now that as long as someone is comfortable with the concepts involved, the meaning is usually clear. Let's face it: in most discussions "aperture" will mean f ratio, unless it really is a technical discussion about the physical diameter of the iris/entrance pupil. Where it is confusing is for those who are not yet sufficiently familiar with the field to make the necessary allowances for "loose talk".

The topic where confusion is most likely I think is that of DoF, where "aperture" can sometimes be used in the true technical sense, and other times in the sense of f ratio.




  
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tzalman
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Feb 01, 2014 07:38 |  #8

Is a f22 aperture smaller than a f2 aperture?
Does a shutter set to 2 let in more light than a shutter set to 8000?
Is wide the opposite of long?
Is anything left after a 100% crop?
Is clean the opposite of noisy?
Is sharp the opposite of soft?
Why is my Canon only 72 dpi and my buddy's camera is 300 dpi?
If I converted my Raw, why is it still the same?
Does a 16 bit image have 48 bits?
If I drag my shutter, won't it get dirty?
Is Street Photography about asphalt or cobblestones?


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archer1960
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Feb 01, 2014 07:56 |  #9

tzalman wrote in post #16654869 (external link)
Is a f22 aperture smaller than a f2 aperture?
Does a shutter set to 2 let in more light than a shutter set to 8000?
Is wide the opposite of long?
Is anything left after a 100% crop?
Is clean the opposite of noisy?
Is sharp the opposite of soft?
Why is my Canon only 72 dpi and my buddy's camera is 300 dpi?
If I converted my Raw, why is it still the same?
Does a 16 bit image have 48 bits?
If I drag my shutter, won't it get dirty?
Is Street Photography about asphalt or cobblestones?

YES!


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nightcat
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Feb 01, 2014 08:03 |  #10

The word that confuses me here is "value"?




  
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Wilt
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Feb 01, 2014 08:53 |  #11

Beginners always get confused because they think about 'magnitude of the number', rather than thinking about 'size of the diaphram opening'.

Since the f/number = FL/Aperture, a 50mm FL with 25mm diaphram opening is f/2 aperture, with 12.5mm opening is f/4 aperture, etc.
What is engraved on the lens is '1/f', which refers to the size of the opening... and

  • we see '1/2' (f/2 lens) = FL/2 = 25mm diaphram opening size;
  • '1/4' = 12.5mm opening.


Hence the confusion...25mm is bigger than 12.5mm, and 'bigger aperture' makes sense. Yet the number '2' is smaller than the number '4'; so FORGET THE MAGNITUDE OF THE NUMBER!

That the shutter speed 4000 lets in less light than the shutter speed 1000 is not confusing for most folks. The f/number is similar to the shutter speed in the sense that the larger number lets in less light than the smaller number. Both the shutter speed ('1000' or '4000') and the f/number happen to be the denominator in the fraction, so when both numbers get bigger the fraction becomes smaller...bigger number, less light.

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trx125
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Feb 01, 2014 10:04 |  #12

So say I bought a lens that is 600mm focal length and aperture of f/.5, that means the iris is 1200mm? :)


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archer1960
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Feb 01, 2014 10:10 |  #13

No, it means the aperture (big end) of the lens is 1200mm in diameter. And if you have the half-million dollars a lens like this would cost, you wouldn't care about the technicalities of it!


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Feb 01, 2014 10:11 |  #14

trx125 wrote in post #16655215 (external link)
So say I bought a lens that is 600mm focal length and aperture of f/.5, that means the iris is 1200mm? :)

Now you understand why that lens would be incredibly difficult on the wallet and heavy to carry around!


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amfoto1
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Feb 01, 2014 10:35 |  #15

It's actually f/aperture value, though it's often just abbreviated to f or / (50mm f1.4, or 50/1.4)

You can call it a ratio or a fraction or a division problem... doesn't really matter, same result:

"f" = focal length

/ = "divided by"

50mm lens at f/1.4 = 50/1.4 = 36mm aperture.

50mm lens at f/2.8 = 50/2.8 = 18mm aperture.

50mm lens at f/5.6 = 50/5.6 = 9mm aperture.

50mm lens at f/16 = 50/16 = 3.125mm aperture.

The larger the divisor, the smaller the result.

(Note: some numbers are rounded off above.)

The beauty of this is that f5.6 on a 50mm focal length lens will give identical results to f5.6 on another 50mm lens or on any other focal length lens. By "same result" I mean that the same amount of light will be delivered to the film/sensor.

A 100mm lens at f5.6 will have approx. an 18mm aperture. Compare with size of the 50mm lens' aperture above.

And, the other important factor to remember is that each full stop difference either doubles or halves the amount of light allowed throught the lens.

f1.0 is twice as bright as f1.4, which is twice as bright as f2.0, which is twice as bright as f2.8, which is twice as bright as f4.0, etc.


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Does f/ number value go up when you "stop down"?
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