Right you are, Jim.
I vaguely recalled that f-stops express a ratio of some sort so I took my
ignorance to the Web and found the following at
http://www.thepeaches.com/photography/Basics.htm :
--------
"Apertures are expressed in ratios; the ratio refers to the focal length of the
lens over the effective optical diameter of the lens. It’s this part of the
definition where the aperture picks up its other name 'F-Stop.' I’m guessing
the 'f' stands for 'focal', but I’m not sure. In photography, 'aperture' and
'f-stop' are pretty much interchangeable terms. An aperture might be expressed
as '1:2.8' in deference to this ratio definition, but it’s more commonly called
'2.8' or 'f2.8'.
"The available apertures of the lens on a typical camera are: 2.8, 4, 5.6, 8,
11, 16 and 22. You might notice that every other number is double / half the
next. (that is, 11 is roughly twice 5.6). Each aperture or f-stop lets in half
as much or twice as much light as the adjacent one. The reason the numbers are
so odd (as compared to the shutter speed scale) is that these ratios are
expressions of circular area... Since circular area is determined by the
formula of 'pi times radius squared', you get twice the area by increasing the
diameter by a factor of the square root of 2.
"The maximum aperture on many cameras is 2.8. This setting lets in the most
light, and is sometimes called 'wide open' or the 'fastest aperture.' The
minimum aperture is 22. This lets in the least light, and is called 'stopping
down' or the 'slowest aperture.'
"In addition to controlling the amount of light that passes through the lens,
the aperture controls something called 'depth of field.' In technical terms,
depth of field is the expansion of the plane of focus into a zone of
'acceptable sharpness.' A wide open aperture like 2.8 has very narrow depth of
field, while a 'slow' aperture like 22 has very broad depth of field."
---------
I my case, I'm using a FujiFilm FinePix 4900Zoom. The lens has the following
imprinted around its circumferencial ring: "SUPER-EBC FUJINON LENS 1:2.8 - 3.1
OPTICAL 6X ZOOM f=7.8 - 46.8mm". Now, it didn't take much to figure out that
six times 7.8 gives 46.8, but I figure that the "1:2.8 - 3.1" are the "wide
open" f-stop values for wide-angle (it 'zooms out' a bit) and "zoomed-in" (max
optical telephoto). Assuming that the f/11 value is for its "normal" position,
then indeed I get the equivalent of something smaller than an f/11 aperture as
I zoom in to fill the frame with the shell (using close-up lenses, usually).
Thanks for pointing that less-than-obvious (to us amateurs, anyway) fact.
Aloha,
Bob Dayle
+++++++++++++++++++++++++++++++++++++++++++++++
Quoting Jim Miller <[log in to unmask]>:
>>The depth-of-focus problem on close-ups is always a hassle. The answer is a
high f-stop number (i.e., very small aperture). Unfortunately, digital cameras
don't do well in this area. I had to search around for a camera with f/11 AND
full exposure control. I would have liked to have had a couple of stops lower
(higher f-stop numbers) but that wasn't going to happen, apparently.
>>
> Hi Bob,
>
> Using a digital camera, which writes data using a CCD (well, almost all use
> one
> or possibly three), an f-stop of f11 is about the rough equivalent of
> about f22 using a
> 35mm camera. It all depends upon the size of the CCD. For a digital
> camera, f
11 is
> a great setting! My Nikon only goes to f9 and that's plenty of depth.
> I can get into
> more on this topic if people are interested.
>
> Aloha,
> Jim
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