"dennis@home" wrote in message
eb.com...
On 25/09/2015 16:27, whisky-dave wrote:
Why call it a stop one have such strange stop
numbers....... why does f5.6 let in twice that of f8

f number is a measure of aperture *diameter* relative to lens focal length.
As diameter is doubled (eg f8 - f4) area of aperture (and hence amount of
light let in) increases by factor of four. So you multiply or divide an f
number by square root of two = 1.4 to go from a given aperture to one that
lets in half/double the amount of light. That's why one stop (a doubling or
halving of aperture) is a strange number:

- start at f1
- multiply by 1.4 to get f1.4
- multiply by 1.4 to get f2
- multiply by 1.4 to get f 3.5
- multiply by 1.4 to get f4
- multiply by 1.4 to get f5.6
- multiply by 1.4 to get f8
- multiply by 1.4 to get f11
- multiply by 1.4 to get f16
- multiply by 1.4 to get f22

Its the same as a film camera, where do you think it differs?
Why do you even think it might differ?

My thoughts exactly. I can't work out whether he really doesn't know or is
winding us all up. Film and digital cameras have more in common that maybe
he realises. Matters of basic optics are common to both. Even a pinhole
camera has a sort of f number: diameter of pinhole divided by length from
pinhole to plane where film is placed. I proved this by making two pinhole
cameras from different sized cocoa tins. Same pinhole (ie same piece of
tinfoil with pinhole made in it, transplanted from one camera to the other)
gave same darkness of negative on photographic paper if I adjusted exposure
time in proportion to ratio of lengths of camera to compensate for change in
f number.

1 stop is a halving/doubling of the amount of light getting through
the lens (eg f 5.6 - f8) or a halving/doubling of the shutter
speed. Maybe 1/3 stop should be expressed as 0.33 recurring, as
you say :-)

with shutter speeds even digital camera, you select 1/125 or 1/250
why not have the dial set to 187ms exposure ?

You can on some, why do you restrict yourself to such limited steps?
what advantage does 1/250 give you over 1/180 that is the flash sync speed
on my film SLR?

Yes if you set the camera on aperture priority and look at EXIF data of the
resulting photos taken in various lighting conditions, you'll see a variety
of unusual shutter speeds - whatever the meter judges is correct; likewise
for aperture when in shutter priority. I *think* most film cameras with
aperture priority also do this - it's just a matter of varying the spacing
between the two curtains of the focal plane shutter which needn't move in
discrete jumps corresponding to 1/125, 1/250, 1/500 etc, although usually in
shutter priority this is all you can set and then lens then stops down to
some obscure aperture like f4.75 or f7.9 according to exactly what the meter
says.

Why does the image on my LCD look the same irrespected of the
aperature and shutter speed I set.

Because you have a cr@p camera and can't set the image to look correct at
a guess. On mine if you stop down, the image darkens when you do DoF
preview just like an old fashioned camera.