On Jan 29, 17:31, Sean 'Captain Napalm' Conner wrote:
> So, a picture taken at f16 (smallest apature) at 8 (1/8 second shutter
> speed) will let in the same amount of light as f11 at 15, f8 at 30,
[...]
> Now, what's the difference between f16/8 and f1.4/1000?
> Field of focus, or that part of the picture that is sharply in focus.
That's the most obvious difference, and the only one you'd need to consider
for a static subject with a camera on a tripod, at "normal" shutter speeds.
As Tony pointed out, a moving subject will be subject to greater blurring
at slow shutter speeds. For a hand-held camera, there will also be some
evidence of blurring due to the unsteadiness of the operator; for a normal
35mm SLR in competent hands you'll typically start to see this at speeds
below 1/125s. As Tony(?) also mentioned, at *really* small apertures (f/22
or smaller) the definition falls off, even though the depth of field
increases.
There is another difference, of particular importance to colour film, due
to what's know as "reciprocity failure". At very short and very long
exposures, the "double the aperture, double the speed" rule breaks down,
and you find that you need longer exposures than you might expect (because
of the particulate nature of the film emulsion, and the activation energy
of the silver halides and dye sensitisers in it). For B/W film, that just
means it seems slower outside the normal shutter speed range. Astronomers
are well aware of this, for exposures of a minute or so, speed can easily
drop by a factor of 2.
Worse still, colour film is actually made up of three or more layers
(typically 6 layers, plus a dye layer), and they aren't all the same.
Although they're balanced for "normal" eposures, reciprocity failure
begins to show itself at different times for the different layers. The
effect is that colour balance can be wildly different at very short or very
long exposure times.
> Also, film is more sensative to light than paper is (about an order of
> magnitude). Indoor light is cooler (redder) than outdoor light (bluer)
and
> there, you can balance the colors either in the film (special film for
> indoor use), filters on the camera (let's see if I remember here---a red
> filter for indoor use will cut down the reds and let more blue through,
thus
> increasing the tempurature of the scene if you ahve outdoor film and are
> trying to use it indoors) or during the print processing..
Also, ordinary B/W film is "panchromatic" -- sensitive to most of the
visible colour range (and also to UV, which is why most professionals tend
to put a UV or "skylight" filter on every lens as a matter of course).
Paper and process film is usually "orthochromatic" -- insensitive to reds
-- or "blue-sensitive", which is why darkroom safelights are usually
yellow.
By "indoor light" I assume you mean tungsten lighting? Yes, it's redder,
but you need a *blue* filter to compensate, if you're using "daylight" film
(which is what most ordinary colour film is). These filters usually have a
filter-factor of about 2 to 4, ie they cut the effective speed of the film
by between 2 times and 4 times.
--
Pete Peter Turnbull
Dept. of Computer Science
University of York
Received on Sat Jan 30 1999 - 07:57:00 GMT