Beginner f numbers.

sam51

sam51

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sam
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hi all, is there something wrong,
I have a canon 550d, when I put my canon 18-55 ef-s 3.5-5.6 is 11 the f stop only goes down to 5.6 I can increase these numbers,
I thought that as it is a 3.5 to 5.6 I can go to 3.5 I have tried all settings including manual.
can someone please advise thanks sam.
 
As I understand it you can only get the higher F number (5.6) when using the longest extent of the zoom. If you wind the lens in to 18mm the F stop should come down to 3.5.
Regards
Dom.
 
sam51 said:
hi all, is there something wrong,
I have a canon 550d, when I put my canon 18-55 ef-s 3.5-5.6 is 11 the f stop only goes down to 5.6 I can increase these numbers,
I thought that as it is a 3.5 to 5.6 I can go to 3.5 I have tried all settings including manual.
can someone please advise thanks sam.
Click to expand...
Your lens will open to 3.5 at 18mm, but at 55 it'll only open as far as 5.6.
 
When the F number on a ZOOM lens reads like a range, then you have different maximum apertures at the two ends of the zoom range.

18-55mm f3.5-5.6

at 18mm you have a maximum aperture of 3.5
at 55mm you have a maximum aperture of 5.6
somewhere in the middle you have a maximum aperture of 4 lol


17-35mm f2.8-4

at 17mm you have a maximum aperture of 2.8
at 105mm you have a maximu aperture of 4
I suppose somewhere in the middle you have a maximum aperture of 3.5


If the F number on a ZOOM lens reads like single number, you have a consistent maximum aperture at any focal length.

17-50mm f2.8

at 17mm you have a maximum aperture of 2.8
at 50mm you have a maximum aperture of 2.8
anything in between can still be f2.8

et cetera.
 
As above. Constant aperture zooms - which do not vary with the zoom range - are available but are generally more expensive. It's worth paying for this if you can afford it, and it's important to you.
 
KingRoach said:
When the F number on a ZOOM lens reads like a range, then you have different maximum apertures at the two ends of the zoom range.

18-55mm f3.5-5.6

at 18mm you have a maximum aperture of 3.5
at 55mm you have a maximum aperture of 5.6
somewhere in the middle you have a maximum aperture of 4 lol


17-35mm f2.8-4

at 17mm you have a maximum aperture of 2.8
at 105mm you have a maximu aperture of 4
I suppose somewhere in the middle you have a maximum aperture of 3.5

.
Click to expand...

that should be minimum not max ..
 
donut said:
that should be minimum not max ..
Click to expand...

Common debate about whether we should refer to the number as a number, or to the number as a reference to the opening. Everybody has a say on this unfortunately.

The aperture is the opening. It can be measured in diameter or circumference. It can have different shapes. It often refers to the visible size not the real size. Therefore it can have a unit, such as millimeters.

The F number is a ratio (focal length to visible opening). It does not have a unit.

Love physics.

If you want to be this accurate, the phrase should be "maximum/minimum F NUBMER of 3.5" not "max/min aperture of 3.5".
I think is is a worthier correction than how you arrange your numbers in ascending or descending order. But as you see people, and manufacturers, often refer to the F number as an Aperture value (Av), so if you want to think of it as an opening, maximum is the biggest hole, which means the smallest number. Keeps the language consistent with the common logic of the photographic community.
 
minimum f number maximum aperture , so the minimum aperture of the lens would be f3.5
 
I try to use the terms low f/number (eg f/2) and high f/number (eg f/16). They are the least ambiguous I can think of.
 
I think that that's the first time I've heard the maximum size of opening on a lens defined as the minimum aperture. I've always seen it stated that the maximum aperture is the opening that lets through the most light.

Given that the f number is a ratio or fraction (and that's a debate in its own right) it's easy to remember how the sizes go when you express it as e.g. f/4, f/16. All you have to recall is that you'll have a bigger portion if you take 1/4 of a cake rather than 1/16th.
 
OK - but I still think 1/4 is bigger than 1/16th :D
 
donut said:
that should be minimum not max ..
Click to expand...

You could well be right but why confuse matters :) its got to be easier for the op to understand that the aperture @ f3.5 is the maximum amount of light the lens lets in
shrug2.gif


Its post like this that certainly confused me when I first started :confused:
 
Phil-D said:
You could well be right but why confuse matters :) its got to be easier for the op to understand that the aperture @ f3.5 is the maximum amount of light the lens lets in
shrug2.gif


Its post like this that certainly confused me when I first started :confused:
Click to expand...

and i also said ( minimum f number maximum aperture , so the minimum aperture of the lens would be f3.5 )
its not that hard really
 
donut said:
and i also said ( minimum f number maximum aperture , so the minimum aperture of the lens would be f3.5 )
its not that hard really
Click to expand...

No, again, you are right, its not hard, but contradicting Noahs post for no reason, when the op is trying to learn the basics, is just confusing the matter surely? :)
 
An aperture is a hole, thus the maximum aperture is the biggest hole. Simple. And it hardly takes one minute to learn how the numbers correlate with the hole sizes.

I suppose that long ago we could turn our aperture rings and see the blades opening or closing, which made it more immediately obvious. These days it seems that such things are more isolated behind an electronic - mechanical interface. Ho hum.
 
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KingRoach said:
Common debate about whether we should refer to the number as a number, or to the number as a reference to the opening. Everybody has a say on this unfortunately.

The aperture is the opening. It can be measured in diameter or circumference. It can have different shapes. It often refers to the visible size not the real size. Therefore it can have a unit, such as millimeters.

The F number is a ratio (focal length to visible opening). It does not have a unit.

Love physics.

If you want to be this accurate, the phrase should be "maximum/minimum F NUBMER of 3.5" not "max/min aperture of 3.5".
I think is is a worthier correction than how you arrange your numbers in ascending or descending order. But as you see people, and manufacturers, often refer to the F number as an Aperture value (Av), so if you want to think of it as an opening, maximum is the biggest hole, which means the smallest number. Keeps the language consistent with the common logic of the photographic community.
Click to expand...

The f number is f/3.5 - it's a fraction.

so maximum aperture = maximum f number
 
donut said:
and i also said ( minimum f number maximum aperture , so the minimum aperture of the lens would be f3.5 )
its not that hard really
Click to expand...
This is where you contradicted yourself really, not the first part of the sentence.


st599 said:
The f number is f/3.5 - it's a fraction.

so maximum aperture = maximum f number
Click to expand...
Thank you for repeating what I said. fraction or ratio, call it what you wish.

Anyway, can we drop this before the OP drops on photography? I think we should all shut up and let the guy play with his camera. He'll figure things out in better ways than we can phrase our arguments. :p
 
donut said:
well you'd better tell nikon that they're wrong then
http://imaging.nikon.com/lineup/dslr/basics/19/02.htm
Click to expand...
I think you misunderstood the Noink site, when they're saying 'minimum' they're actually talking about the low numbers, not small apertures.

No-one calls it a minimum f no (as you'd understood it), if we all stick to the universally understood 'maximum aperture' no one gets confused.
 
The f number is the denominator in a fraction, it is not a fraction.
If the f number is 8 ( f/8 ) and the focal length is 50mm then the aperture* diameter is fifty eighths of a millimetre.

*The f-number actually relates to the diameter of the entrance pupil; not the true aperture. But the difference between apertures and entrance pupils doesn't really matter unless you're a lens technician or optical scientist.
 
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The aperture is the size of the hole through the lens.
The f number is NOT the size of the aperture.
The f number is the ratio of aperture size to focal length.

They can be worked out using the equation:
F number = Focal length / Aperture Size.
As the focal length changes the aperture size must also change if we wish to keep the same f number.

For any given focal length, having a bigger aperture will give you a smaller f number.
Therefore giving the maximum aperture as the minimum f number is correct.

Back to the O/P:
If a lens is unable to make the aperture progressively larger as you zoom in, ratio between the focal length and aperture changes giving a different f number.
 
Andy_S said:
The aperture is the size of the hole through the lens.
The f number is NOT the size of the aperture.
The f number is the ratio of aperture size to focal length.

They can be worked out using the equation:
F number = Focal length / Aperture Size.
As the focal length changes the aperture size must also change if we wish to keep the same f number.

For any given focal length, having a bigger aperture will give you a smaller f number.
Therefore giving the maximum aperture as the minimum f number is correct.
Click to expand...
If you wanted to be fully accurate, the f number is the ratio of entrance pupil to focal length; not aperture to focal length.

F = focal length/entrance pupil diameter

But that's probably overcomplicating things for photographers' purposes unless they get really excited about the optical physics.
 
And if you really want to be accurate:

the relative aperture is written as a fraction of the focal length e.g. f/4, f/5.6, f/8
the f number scale is written 4, 5.6, 8
the German standard means that German lenses have the maximum aperture as given by the relative aperture expressed as a ratio e.g. 1:4, 1:5.6, 1:8
the term f stop is actually undefined so it can mean just what you want it to mean (officially, anyway)

The relative aperture is defined only for infinity focus; but some lens designs don't give the same reduction of relative aperture as others when focused closer.

The fact that the relative aperture depends on the diameter of the entrance pupil means that you don't get the same relative aperture when you reverse a lens (you might get a small difference or a whopping great one depending on the lens design).
 
thanks guys, but half of you have lost me, the rest went over my head, I understand the small f number the more light you let in,
so better for low light photos, for us beginners it is easier to keep it simple,
so tiny steps, all the best sam.
 
sam51 said:
thanks guys, but half of you have lost me, the rest went over my head, I understand the small f number the more light you let in,
so better for low light photos, for us beginners it is easier to keep it simple,
so tiny steps, all the best sam.
Click to expand...
And that's pretty much all you need to know. There's quite a lot of physics surrounding the numbers and it can be quite interesting, but whether it make you a better photographer is debatable*. You just need to know how it will affect your pictures.




* Actually it's not debatable...It won't! (Some folks are happy to debate anything though.) :exit:
 
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sam51 said:
thanks guys, but half of you have lost me, the rest went over my head, I understand the small f number the more light you let in,
so better for low light photos, for us beginners it is easier to keep it simple,
so tiny steps, all the best sam.
Click to expand...

A lot of it lost me too :D

Don't forget also that the smaller the f number the shallower the dof. This is very good for learning about dof, just put in the appropriate details, camera, focal length, ect, :)
 
I think it is quite difficult for someone learning what the differences are with aperture as to me it works backwards.
Let me explain,
If you think of the aperture as the size of a hole the larger the F number the smaller the hole and the smaller the F number the bigger the hole.
I know this isn't scientific but for me it the message across
 
Provided you always either write it or think of it as f/4 etc. it's crystal clear if you already know that 1/4 is bigger than 1/16th. When I started in photography it was always (so far as I can remember) written that way; and if you actually wanted to know how big the hole was, you just substituted the focal length of the lens for the f in f/8 (say) and got the diameter directly.

The only thing that did confuse me was the odd sequence of whole stops, until it was pointed out that the amount of light getting through depends on the area of the hole, and that in turn depends on the square of the diameter of the hole; hence doubling and halving the amount of light required a square root of two (1.4) change in the number, and hence the sequence f/1, f/1.4, f/2, f/2.8 and so on.
 
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