no you have it backward.

the numbers ar not fractions, they are ratios. if they were written out it would look like this

1:2.8; 1:4.0; 1:5.6; 1:8.0 why waste ink on all those ones and colons?

they represent the relation of the effective area(opening) of the lens to the focal length.

so in a sense the do represent fractions but a shorthand style going the other way.

"Very technical lingo" This is quite an amazing debate. Here is what I have found regarding the f-stop: From Wikipedia, the free encyclopedia: "F number" In optics, the f-number (sometimes called focal ratio, f-ratio, or relative aperture[1]) of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography. Further on they discuss the f-stop in terms of fractions: Fractional stops..On modern cameras, especially when aperture is set on the camera body, f-number is often divided more finely than steps of one stop. Steps of one-third stop (1/3 EV) are the most common, since this matches the ISO system of film speeds. Half-stop steps are also seen on some cameras. As an example, the aperture that is one-third stop smaller than f/2.8 is f/3.2, two-thirds smaller is f/3.5, and one whole stop smaller is f/4. The next few f-stops in this sequence are f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, etc.

Hi

Something else about apertures.... you'll get a sharper digital photo at f/11 than you will at f/22... due to the way the pixels are arranged.

 

Rick

 Just for a little clarity: Consider the f-stop # - f/2.8; the slash does not indicate a fraction or ratio. In fact it is irrelevant and is used simply to separate the letter "f" from the number "2.8".

The f-stop # (where f-stop is indicated by f/ and # is indicated by 2.8) = Focal length of the lens/diameter of the aperture. As you can see, for the same focal length, if the aperture gets bigger (more light) the f-stop gets smaller. This also accounts for the varying f-#s on many zoom lenses. Consider: the aperture on these lenses  has it's maximum diameter at its lowest focal length. When you increase the focal length (zoom in) the f-stop also increases. That is why many zoom lenses are listed like this: 75-200mm f/2.8-f/4.

Don't worry if it is a fraction or ratio because it doesn't really matter. (actually it's both since they are virtually the same thing) okay, if you want to get technical, the f-stop is a number that represents the ratio of the focal length to the aperture diameter AND that ratio is, in fact, a fraction as are all ratios.

No matter what focal length lens you use; the same f-stop # will let in the same amount of light. The confusion come into the frey because many people (and manuals) will tell you to "set your aperture to 2.8" when they really mean "set your f-stop to 2.8"

 

Just my 2 cents.

Rick Sammon:

...due to the way the pixels are arranged.

 Rick

 

 

Not really Rick. Without getting into the physics of it,the simple explanation is that the smaller diameter aperture reduces the amount of scattered light.

Mike

ImageQuest:

Rick Sammon:

...due to the way the pixels are arranged.

 Rick

 

 

Not really Rick. Without getting into the physics of it,the simple explanation is that the smaller diameter aperture reduces the amount of scattered light.

Mike

in a word, "diffraction". in longer terms, light will actually bend around an edge. Optics and the associated physics is a very interesting and sometime confusing subject. It all goes to the "wave/particle" nature of light. particles dont bend, but waves do!

but the way digital image sensors are made may also add a factor.

BUT no math!! 

Light doesn't bend around an "edge", it bends when the medium through which it travels changes density (from air into water or air into glass). I think you meant refraction (the bending of light), not diffraction (otherwise known as the rainbow effect, but it's more than just that).

We can control exactly how light bends by using curved glass (aka a lens) therby creating a focal point. The light bends at a different angle at the edges of the lens than it does toward the center which creates several focal points instead of a single point. This is what causes the DOF phenomenum.

Our focal plane is a located in a single fixed location and what is "in focus" ( and therefore our DOF) is determined by the amount of variance in the focal points. With a large variance (caused by a large aperture) we get a lot of OOF light hitting our focal plane (sensors or film) When we focus on near or far objects we actually change what part of the variance is focused on the focal plane.

When we use a small aperture (say, f/18), we limit the light to entering the center of the lens and block the light that hits the outside of the lens. The disparity of the angle of light is then less. The focal variance is less and we get closer to a pinpoint at the focal plane making a bigger DOF.

It stands to reason that the digital sensors have nothing to do with DOF because the same thing happens when you use film. It's all optics.

 

ImageQuest:

It stands to reason that the digital sensors have nothing to do with DOF because the same thing happens when you use film. It's all optics.

 Iphoto, Your not missing anything. We lose sharpness (soften the image) at extremely small apertures due to the diffusion of the light through the tiny hole of the aperture (doesn't happen at larger apertures). It is the same affect as when we put a diffuser on our flash/studio lights to soften the light but not quite as noticeable.

 Rick was saying that it was due to something about the way the pixels are arranged or something. The Cmos/JFED/CCD has nothing to do with either DOF or the softening at extremely small apertures. That being said; the "chip" can/does have an affect on the image based on the # of pixels it contains. More pixels can collect finer details and allow for much better enlargments (the film equivelent would be in the Gigapixels range). HOWEVER (and this is the big difference), the effect due to the chip is uniform at ALL focal lengths and apertures.

My original post which makes the comment that smaller apertures minimize scattered light was misleading. I knew what I was saying (wanted to say), but I didn't write it accurately so the physics explanation was more to clarify my own statement.

Mike

Mike,

the light does not diffuse with the smaller aperture, it diffracts.

diffusion is light scattering(uncontrolled refracting) due to interference(such as water vapor in air) in the medium.

you have also confused diffraction, (bending around an edge) with refraction ( bending through a medium/density change).

diffraction can cause an interference pattern in an image;

On film it might show up as halos around point light sources, in an array of sensors, that pattern will have a different, and possibly more deliterious apperance.

 all these words mean something quite specific, check your physics text or a dictionary. 

 

drh681:

diffraction can cause an interference pattern in an image;

Exactly, and is why I asked my question above.

 

Just joined and am reading this discussion with interest.  Not a pro, but a serious hobbyist, I have been shooting film (and now digital) for over thirty years, mostly various Nikon equipment.

Recently, I was trying to explain this very thing to a friend who has a nice, but pretty much automatic, digital point and shoot camera and really was not getting anywhere.  He wanted results more like I and others were getting, and proper use of f-stop/aperture was something he needed to understand.  The going was was tough until I pulled out an old manual aperture lens and plopped it onto my DSLR.  We found that the entire f-stop/aperture number was made much easier to grasp by using the manually stopped lens.  He could directly see the effect of changing the aperture from 1.4 to 32:  higher stop number on the aperture ring = smaller hole = less light & generally greater depth of focal field; lower number=lots of light & very thin field of focus.  I did explain the numbers as being ratios (fractions if you prefer), which does stand the whole "lower number/higher number" issue on its' head, but seeing made the difference in comprehension.

Understanding the math has its' place, certainly, but knowing how to apply the proper technique in actual practice is the real goal.  Sounds to me like, generally speaking, most everyone had it right, aside from a bit of confusion.  Mostly the same ideas stated from differing viewpoints.  And thats what makes communication a wonderful thing, is it not?

drh681:

Mike,

the light does not diffuse with the smaller aperture, it diffracts.

diffusion is light scattering(uncontrolled refracting) due to interference(such as water vapor in air) in the medium.

you have also confused diffraction, (bending around an edge) with refraction ( bending through a medium/density change).

diffraction can cause an interference pattern in an image;

On film it might show up as halos around point light sources, in an array of sensors, that pattern will have a different, and possibly more deliterious apperance.

 all these words mean something quite specific, check your physics text or a dictionary. 

 

 

Not trying to get yet more technical here, but to add a hair of precision to the terminology here, diffraction does not cause a bending of light.  Instead, it occurs when the hole the light passes through has a small spacing relative to the wavelengths of light passing through.  Strongest diffraction occurs with the aperture size approaching the wavelength size.  When this happens, the you get the phase of the wave (peaks vs. troughs) of light adding to one another (peaks in step) or canceling each other out (peak and trough aligned).  The result is banding patterns of light rather than the full area illuminated.  See the wikipedia link here. About 1/2 way down, you'll see a bit on diffraction by a circular hole.  This is what applies to photography.  Note that the only two parameters that get to change are the wavelength - and that is not usually a variable unless you are nuts like me and shoot some IR - and the absolute aperture (radius in the equation) of the hole.  About 2/3 the way down on the page is what I do for a living: x-ray diffraction (image with Bragg's Law in the caption). 

The largest source of diffraction in the digital camera would be the microlenses that sit on top of the pixels and the pixels themselves... if the sensor was then some distance beyond them.  Their sizes are still an issue, but when that is coupled with smaller apertures of lens diaphrams, then you essentially get three sources of light diffraction and attenuation.  A common misconception though is that f/22 or so is always bad.  Since the above responses have all pointed out that the aperture is relative to the focal length, small apertures on wide angle focal lengths are far worse than the same f-stop number on telephoto lenses, at least in terms of diffraction:  the absolute size of the aperture hole is what matters, and it is very different when comparing wide and tele lenses.  f/32 on a 16mm focal length gives you a 0.5mm diameter (0.25mm radius) for the aperture.  f/32 on a 300mm lens gives you almost a 10mm diameter hole (~5mm radius).

Diffraction is a significant issue for film or digital when the diameter drops to ~2mm or less.

Jim 

 DRH... I know... I have no excuse or idea why I wrote diffusion instead of diffraction?!? Can I claim slow fingers and fast brain?!? Thanks for straightening that out!

I am going to claim that in trying to not get too heavy into the physics in explaining DOF and f-stops, I watered it down too much and confused my fingers! Well, at least, that's my story and I'm sticking to it!

Jim, Thanks for saying elegantly, most of the things trying to be said here. I would only argue that the diffraction at the sensor micro lens, while present, was

A.  insignificant and

B. Uniform for every lens/aperture combo (as I stated earlier)

and therefore not a factor when discussing f-stops, DOF  and the softening at high f-stops. But I'm not an expert, just an old man trying to go back to school after 25 years and relearn physics and chemistry and such. BTW: Optics is next semester.

Anyway, in my head I had it right, it just got garbled coming out of my fingers, sorry about that, I'll try to do better next time.

I think I'll go take a picture of my dog.

all this makes my head spin...

It is nice to have one of these discussions without the snarling that can happen at some other forums.