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 Ockham's Razor 04-12-2007 02:58 PM

ISO question

Does increasing the ISO setting on a digital camera do the same thing as
opening the shutter? If so, is there any info on the shutter stop
equivalent for a given increase in ISO?

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

 Jim 04-12-2007 03:29 PM

Re: ISO question

"Ockham's Razor" <Mencken@pdx.net> wrote in message
news:Mencken-BA97BC.07583412042007@sn-ip.vsrv-sjc.supernews.net...
> Does increasing the ISO setting on a digital camera do the same thing as
> opening the shutter? If so, is there any info on the shutter stop
> equivalent for a given increase in ISO?
>
> --
> "When fascism comes to America, it will be wrapped in the flag and
> carrying a cross."
> Sinclair Lewis

A camera is a camera is a camera....

Figure it out for yourself. The exposure time is:

T = N*N/(S * B)
Where T = shutter speed in seconds
N = focal ratio (i. e. f stop)
S = ISO speed
B = Bightness of illumination in candelas per square foot.

You will find this equation lots of places. A couple are Michael
Covington's bool "Astrophotograpy for Amateurs" and Rudolf Kingslake's book
"Lenses in Photography".
Kingslake's book is long out of print.

Note: A candela is merely a closely specified version of a candle. If you
search the internet for "candela", you will find more information than you
ever imagined.

Jim

 Marvin 04-12-2007 05:20 PM

Re: ISO question

Ockham's Razor wrote:
> Does increasing the ISO setting on a digital camera do the same thing as
> opening the shutter? If so, is there any info on the shutter stop
> equivalent for a given increase in ISO?
>

In digicams, the ISO setting does not affect the exposure
settings. It changes the amplification (i.e., gain) in

 Matt Ion 04-12-2007 06:09 PM

Re: ISO question

Ockham's Razor wrote:
> Does increasing the ISO setting on a digital camera do the same thing as
> opening the shutter? If so, is there any info on the shutter stop
> equivalent for a given increase in ISO?

The ISO setting in digital cameras affects how sensitive the camera is to light
(or more technically, it adjusts how much the signal from the sensor is
boosted). It's intended to correspond to the ISO rating of film, which is a
rating of how sensitive a film is to light (higher ISO = more sesitivity).

In both digital and film, there are always trade-offs: higher ISO means you need
less light, but you get more "noise" in the image and less contrast; wider
aperture allows more light, but reduces depth of focus; slower shutter allows
more light, but increases the chance of motion blur.

There is a direct correlation when calculating the effect of the three. Each
doubling of ISO doubles the sensitivity to light (ISO 200 to 400, for example).
One full "stop" on the aperture also means a doubling or halving of the light
allowed (f/4 passes twice as much light as f/5.6 - you'll note that every
doubling or halving of the aperture number is actually two stops of exposure,
eg. f/4 to f/8). So at a given shutter speed, 200 ISO at f/4 will give the same
exposure as 400 ISO at f/5.6, as will 800 ISO at f/8, or 100 ISO at f/2.8.

 M-M 04-12-2007 06:17 PM

Re: ISO question

In article <sBuTh.64610\$6m4.5332@pd7urf1no>,
Matt Ion <soundy106@gmail.com> wrote:

> So at a given shutter speed, 200 ISO at f/4 will give the same
> exposure as 400 ISO at f/5.6, as will 800 ISO at f/8, or 100 ISO at f/2.8.

That was a good explanation.

But what do the aperture numbers represent? 2.8-what? Or is it
1/2.8-something? And what is the relationship between the numbers?

I believe f/64 represents one millimeter of aperture. Is there a way to
measure the diameter of the opening to determine the f/#?

I've always wondered.

--
m-m

 Ed Ruf 04-12-2007 06:32 PM

Re: ISO question

On Thu, 12 Apr 2007 14:17:06 -0400, in rec.photo.digital M-M
<nospam@ny.more> wrote:

>But what do the aperture numbers represent? 2.8-what? Or is it
>1/2.8-something? And what is the relationship between the numbers?
>
>I believe f/64 represents one millimeter of aperture. Is there a way to
>measure the diameter of the opening to determine the f/#?

f is the focal length of the specific lens you are considering. So
consider the 70or 80-200mm f/2.8 lenses. The max aperture of the lens
is then 200mm / 2.8 = 71.4mm.
-
Ed Ruf (Usenet2@EdwardG.Ruf.com)
http://edwardgruf.com/Digital_Photog...ral/index.html

 Ken Lucke 04-12-2007 06:55 PM

Re: ISO question

<nospam@ny.more> wrote:

> In article <sBuTh.64610\$6m4.5332@pd7urf1no>,
> Matt Ion <soundy106@gmail.com> wrote:
>
> > So at a given shutter speed, 200 ISO at f/4 will give the same
> > exposure as 400 ISO at f/5.6, as will 800 ISO at f/8, or 100 ISO at f/2.8.

>
> That was a good explanation.
>
> But what do the aperture numbers represent? 2.8-what? Or is it
> 1/2.8-something? And what is the relationship between the numbers?
>
> I believe f/64 represents one millimeter of aperture. Is there a way to
> measure the diameter of the opening to determine the f/#?

f/# == (focal length) / (diameter of entrance pupil)

--
You need only reflect that one of the best ways to get yourself a
reputation as a dangerous citizen these days is to go about repeating
the very phrases which our founding fathers used in the struggle for
independence.
-- Charles A. Beard

 David Dyer-Bennet 04-12-2007 07:09 PM

Re: ISO question

M-M wrote:
> In article <sBuTh.64610\$6m4.5332@pd7urf1no>,
> Matt Ion <soundy106@gmail.com> wrote:
>
>> So at a given shutter speed, 200 ISO at f/4 will give the same
>> exposure as 400 ISO at f/5.6, as will 800 ISO at f/8, or 100 ISO at f/2.8.

>
> That was a good explanation.
>
> But what do the aperture numbers represent? 2.8-what? Or is it
> 1/2.8-something? And what is the relationship between the numbers?

It's a pure number, no units -- a ratio, in fact. It's the focal length
of the lens over the aperture size (in complex modern designs it's a
*bit* more complicated than that, but for the simplest case, a
single-element lens, that's accurate). That's why it's most correctly
written as f/2.8.

The "standard" sequence of f-numbers, which are all one stop apart, have
the ratio sqrt(2) between any two adjacent values, which makes sense
(given the formula for the area of a circle). There are some rounding
glitches, though :-).

1.4 2 2.8 4 5.6 8 11 16 22 32 45 64

 Westy 04-12-2007 07:30 PM

Re: ISO question

On Thu, 12 Apr 2007 07:58:34 -0700, Ockham's Razor <Mencken@pdx.net>
wrote:

>Does increasing the ISO setting on a digital camera do the same thing as
>opening the shutter? If so, is there any info on the shutter stop
>equivalent for a given increase in ISO?

ISO 200 is 1 stop greater than ISO100

ISO 400 is 1 stop greater than ISO200

ISO 800 is 1 stop greater than ISO400

You get the picture?

 Bill Funk 04-12-2007 08:00 PM

Re: ISO question

On Thu, 12 Apr 2007 13:20:56 -0400, Marvin <physchem@cloud9.net>
wrote:

>Ockham's Razor wrote:
>> Does increasing the ISO setting on a digital camera do the same thing as
>> opening the shutter? If so, is there any info on the shutter stop
>> equivalent for a given increase in ISO?
>>

>In digicams, the ISO setting does not affect the exposure
>settings. It changes the amplification (i.e., gain) in

Well, it *should* affect the exposure.
Doubling the ISO means half the light is needed for proper exposure.
Thus, if proper exposure at ISO 100 is f/4 @ 1/100 sec shutter speed,
doubling the ISO to 200 would let you go up one f/stop or halve the
shutter speed (or a proper combination of the two).

--
THIS IS A SIG LINE; NOT TO BE TAKEN SERIOUSLY!

Hillary Clinton visited a veteran's hospital
in Syracuse Tuesday and promised better care
for wounded veterans. She's wounded her share.
Whenever the Commander-in-Chief used to walk
into the Rose Garden with a black eye, you
knew he was having a good week.

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