Aperture number: What does it mean?

[prodigy2k7]

Wikipedia says:

"More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane."

So, im assuming there is some complex calculation or something that has to do with the "cone angle", whatever that is....

Does anyone have any information on what exactly the numbers mean? Where did they come from etc. Is it from some mathematical formula?

Any information at all! thanks

 

[reg]

I'm sure someone else will come in here with WHAT it's compared to because I forget, but it's a measure of a fraction of the opening of the lens.

f/2 = one half of the full possible opening.
etc.

I wanna say that it's based on the same diameter as the filter threads but I don't remember offhand.

 

[icassell]

Here ya go:

http://en.wikipedia.org/wiki/Aperture

 

[reg]

Yeah, he clearly references the wikipedia article in the OP so that doesn't help at all.

:roll:

 

[John_Olexa]

http://www.kevinwilley.com/l3_topic01.htm

 

[christopher walrath]

The aperture number on your lens is the denominator of a fraction that provides the aperture's diameter (or whatever Helen B wants to call it). f/2 = d (where f equals lens' focal length and d equals aperture diameter). So you have a 50mm lens. f/2 (50/2) = a 25mm diameter aperture. f/4 = a 12.5mm aperture diameter. f/8 = a 6.2mm aperture diameter.

 

[reg]

Ask Helen.

Best.
Answer.
Ever.

 

[epatsellis]

chris, repeat 50 times, entrance pupil, entrance pupil

The f.l / entrance pupil (or effective aperture, with apparent mag. by the front element.) is the f. number.

erie

 

[brianne5499]

Simply Put

the bigger the number, the smaller the opening...so the less light the that can enter = a longer exposure time

and of course, it's exactly the opposite at the other end.

 

[prodigy2k7]

I wasn't going for simply put
Thanks! I finally get it now

"
It's all relative

A 400mm lens set at f/4 has a pupil diameter of 100mm.
A 100mm lens, also set at f/4, has a pupil diameter of 25mm.
"
http://www.kevinwilley.com/l3_topic01.htm

So, when talking about a zoom, such as EF 70-200mm F/4 USM, there is a zoom but constant f/4 does that mean the f/4 stats the same but as you zoom the actual physical size changes such at 70/4 = 17.5mm and 200/4 = 50mm. That sounds weird to me...

 

[Helen B]

So, when talking about a zoom, such as EF 70-200mm F/4 USM, there is a zoom but constant f/4 does that mean the f/4 stats the same but as you zoom the actual physical size changes such at 70/4 = 17.5mm and 200/4 = 50mm. That sounds weird to me...

This is one of the reasons that it is important to understand the concept of the entrance pupil. As already mentioned by erie, the f-number is equal to the focal length divided by the diameter of the entrance pupil. This may be written

N = f/d

The entrance pupil is the image of the actual physical aperture (let's call it the iris) as seen from the front of the lens - ie it is the image of the iris formed by the lens elements in front of the iris. If there are no lens elements in front of the iris, the iris and the entrance pupil are one and the same.

What that means is that a zoom lens can have a variable entrance pupil diameter without the iris changing physical size. It may change position within the lens, as may the elements in front of it. These position changes affect the size of the image. In some lenses the iris itself may actually change diameter as the lens is zoomed as well as the magnification changing. The two effects may combine to produce a constant f-number.

There is also an exit pupil - the image of the iris, as seen from the back of the lens. The ratio of the exit pupil diameter to the entrance pupil diameter is known as the pupil magnification.

The entrance pupil is also the place from which the lens 'sees' the world outside - the centre of perspective of the lens. It is where the lens and camera combination should be rotated for a stitched panorama.

There are a few threads on the issue, here is a link.

Within that thread is the definition the ISO use:

ISO 517:2008 Photography - Apertures and related properties pertaining to photographic lenses - Designations and measurements

f-number
the reciprocal value of the relative aperture

relative aperture of a photographic lens
twice the numerical aperture where the numerical aperture is the sine of the semi-angle subtended by the exit pupil at the focal plane
NOTE For photographic applications, the relative aperture is equivalent (within 1/3 stop) to the ratio of the diameter of the entrance pupil to the focal length.

entrance pupil
image of the aperture stop as viewed from a point in the object space on its optical axis (the image of the aperture stop formed by the front elements of the lens)

aperture stop
physical stop that limits the cross-section of the light beam that can pass through the lens to reach the centre of the on-axis image

The definition of relative aperture hints at the theory behind all this. The exit pupil is where the image is 'projected' from.

Best,
Helen

 

[manaheim]

och. my brain hurts.

*manaheim bookmarks another helen post and plans to come back and read later*



Thanks, Helen.

 

[Bifurcator]

As already mentioned by erie, the f-number is equal to the focal length divided by the diameter of the entrance pupil. This may be written

N = f/d

Where "N" is the focal ratio or f-number, "f" is the focal length, and "d" is the diameter. Right?

I think it's useful to point out (if I myself get the terminology right) that the "f" in "f-number" stands for focal-ratio and is the ratio "N" result in the above equation.

 

[Moglex]

Where "N" is the factor or f-number, "f" in the focal length, and "d" is the diameter. Right?

I think it's useful to point out (if I myself get the terminology right) that the "f" in "f-number" stands for factor and is the factor number "N" result in the above equation.

I have always read that it stands for 'Focal ratio'.

The 'stop' in f-stop being the detente introduced to make it easier to select the specific f-numbers that halve or double the exposure. Prior to these detentes the aperture control would move smoothly with no tactile feedback.

 

[Helen B]

...

The 'stop' in f-stop being the detente introduced to make it easier to select the specific f-numbers that halve or double the exposure. Prior to these detentes the aperture control would move smoothly with no tactile feedback.

'Stop' predates the use of indents at whole steps (or half steps or third steps) - it refers to the 'stopping' of light by the 'aperture stop'. There is also a 'field stop' - the general optical term for the gate, or film gate.

Best,
Helen