GrahamS Loc: Hertfordshire, U.K

1 & 2 are essentially the same thing. The iris is a mechanism used to alter the effective diameter of the lens. It is the effective diameter of the lens, (the entire lens, if there is no iris diaphgram or the diameter of the iris opening if there is) that is used to calculate the "f" number.

potmead Loc: 191miles North of London, England

I have now measured my other lenses and i have discovered that on the telephoto lenses the 'aperture' as calculated by dividing the focal length by the 'f'' number is greater than the diameter of the lens, ie

300mm f5.6 = 53.6
Diameter = 51mm

400mm f6.3 = 63.5
Diameter = 59mm

And i have found the following link that explains this 'Entrance Pupil' or Aperture.

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/stop.html

So, from this i understand that the 'Aperture' is something which can't be attributed to any dimension on the lens but is calculated to be:-

"Size of the physical aperture that would be required to pass the extreme ray if the lens were not present"

I now understand...

While this does not answer the question regarding the accuracy of the f-stop or why anyone would need or want a lens with a 'wide' aperture, it does at least for me, clear up the calculation of f-stops.

We must trust the lens makers to calculate f-stop to at least some degree of accuracy to maintain consistency in exposure measurements, i suspect that meter inaccuracies/incorrect use to be more of a factor in exposure variations than f-stops.

Graham

CaptainC Loc: Colorado, south of Denver

I think that is where we are!

GrahamS Loc: Hertfordshire, U.K

Some take the long way around.

potmead Loc: 191miles North of London, England

Some like to understand.

I have learned something new today.

photoninja1 Loc: Tampa Florida

To see the circle of confusion, look above!:)

photoninja1 Loc: Tampa Florida

Sorry. The Devil makes me do it.

Nikonian72 Loc: Chico CA

Your own reference refutes your statement: http://en.wikipedia.org/wiki/Entrance_pupil "In an optical system, the entrance pupil is the optical image of the physical aperture stop, as 'seen' through the front of the lens system. No mention is made about size of front element lens determining f/stop, which is solely a function of iris diameter (pupil, as in your own eye) and focal length.

CaptainC Loc: Colorado, south of Denver

Oh good grief! I have been taking images for 50 years and never worried about this. I knew how to use a meter and now I use a meter AND check the histogram.

Of course I know engineers who are OCD about this stuff. Mediocre photographers but they know all about entrance pupils and T-Stops. :-)

GoofyNewfie Loc: Kansas City

Reminds me of a time we got slide film to process from someone. It came with a page-long letter explaining what the scenes contained, camera, lens and a rather tedious section about testing the battery output for the meter..."battery is supposed to provide 1.5 volts, but upon returning home I tested it and only had 1.45 volts. Please process accordingly" We ran it normal.

potmead Loc: 191miles North of London, England

Douglas
It appears i have been given misleading information in the past, believing that f-numbers are purely a function of focal length and lens diameter.

It seems the calculation is more complicated than i thought.

If you view the link by GrahamS, this shows a simple lens which uses this formula, f-number = f/D
This is what i was taught 40 years ago.

But this is not true for compound lenses, this uses f-number = f/Aperture

The 'Aperture' figure is a calculated diameter which is dependant on various things including number of elements in front of the iris, position of the iris in the lens, type of glass etc. and is not related to the diameter of the front element or the physical diameter of the iris, but is the 'Apparent' diameter of the iris.

Cliff
I can live without knowing this, i didn't know about this until today, it will not improve my photography, i am not OCD, but, as nobody was able or willing to explain my error, i looked it up, and shared what i found. For Interest only.

PS i am an Engineer and have been taking photographs for 35 years, Mediocre? That is for others to judge.

Graham

CaptainC Loc: Colorado, south of Denver

Graham - Remember - it WAS just a joke.

Ray and JoJo Loc: Florida--Tenneessee

The circles of confusion are calculated using the formula:

CoC = (CoC for 35mm format) / (Digital camera lens focal length multiplier)

The focal length multiplier for a camera is specified by the manufacturer, or is calculated using the formula:

Multiplier = (35mm equivalent lens focal length) / (Actual lens focal length)

NOW lets make sence of this discussion and what we have brough from. All are corect, a simple lenes f/stop is simple if complex lenes this is the rub.

With all that said my question is at night a light taken at f/22 will produce a 6 pointed star at f/32, same light, it now has 8 points I have never heard or read where this is explained???

PhotoArtsLA Loc: Boynton Beach

Nope.

There is only one Circle of Confusion, based on film or sensor size. It defines actual image sharpness.

All lenses focus light as cones. The cone, if sliced straight across, is a circle. As the image attains focus, that cross section circle gets very, very small. The point at which, for the given format best sharpness occurs, is the "Circle of Confusion." It generally occurs at the point the light rays cross.

The Circle of Confusion changes based on imaging size. In the olde days, there were different Circles of Confusion for 35mm, 6x6cm, 4x5 inch, 8x10 inch... and so on film formats.

As you stop a lens down, depth of field can be defined by how the ever narrowing cone (due to smaller cones shaped by the iris,) and its cross sections, attain the Circle of Confusion over a greater range. In other words, the distance, both in front of, and behind the film plane, that the Circle of Confusion is achieved, defines the depth of field in physical terms.

Further, the quality of a lens can be defined by its ability to attain the Circle of Confusion. Some just can't, and these are often referred to as "soft lenses." When the maximum sharpness reaches the Circle of Confusion, the lens might be called "tack sharp." The eye opening experience of going from a $500 Tokina zoom to the $1,700 top flight manufacturer's version, where the images suddenly POP with image quality vastly better than the third party lens, is all about the Circle of Confusion. Cheaper lenses distort the Circle of Confusion or simply, at their best, don't quite achieve true focus.

Still further, in the movie business, we actually adjust our lens mounts (their distance to the film plane) to maximize performance of certain lenses. These are microscopic adjustments, but make a huge difference in some cases. It's all about the Circle of Confusion.

GrahamS Loc: Hertfordshire, U.K

:thumbup:

All of the above is covered in an easy to understand way in Michael Langford's bookl "Basic Photography" which, together with Langford's "Advanced Photography" are the two books used as reference textbooks in most of the colleges where I have lectured.
Here is the Waterstones link:
http://www.waterstones.com/waterstonesweb/products/michael+langford/anna+fox/richard+sawdon+smith/langford27s+basic+photography/7141301/?gclid=CMSNhv-viLUCFQ7LtAods1UA6A

It is also available from Amazon. Of all the photography books, this is one I strongly recommend. It will become a life-time reference on your bookshelf.