'Stop' history and usage

I was wondering how the word stop came to mean cut the light in half?
Maybe someone could provide me with a link to that story.

Also I am a little confused about how to use it. For example, if I
close the iris one stop that reduces the aperture area by half. But if
I cut the time the shutter is open by half does that constitute a stop?
What about ISO (indirectly) and ND filters, are their reductions in
light referred to as stops too?

Thanks,
Ron

wrote in message
oups.com...

I was wondering how the word stop came to mean cut the light in half?
Maybe someone could provide me with a link to that story.

Also I am a little confused about how to use it. For example, if I
close the iris one stop that reduces the aperture area by half. But if
I cut the time the shutter is open by half does that constitute a stop?

Yes

What about ISO (indirectly)

Doubling or halving ISO equals one stop.

and ND filters, are their reductions in
light referred to as stops too?

Can be but more usually as 4x 8x etc... 8x = 1/8 light = 3 stops (2,4,8)
Working things out in 'stops' makes exposure calculations easy - for the
same EV (exposure value) you can close aperture -1 stop and double exposure
time (+1 stop) OR double ISO value (+1 stop)...
It's just what the calculator on the back of a light meter is doing....

Thanks,
Ron


Guy

Ron, I'm not sure about the origination of the word "stop" for describing
the amount of light striking the film or digital sensor. Perhaps it's a
reference of the mechanical "stops" that use to be used?

Regardless, understanding "stops" is almost counter intuitive but, once
understood, makes life easier. Some basics: The "f" stops used to
described aperture settings (you used the term "iris" -- same difference)
has been colloquialized to whole numbers but in actuality are
fractions/ratios. The fraction describes the amount the aperture is opened
compared to the potential maximum opening. For example: an aperture
setting of "f/8" means the aperture is opened 1/8th of the total area
theoretically possible. An aperture setting of f/11 is a smaller opening
that is 1/11th of the maximum. Thus the larger the number, the smaller the
opening. Geometrically, when you go from f/8 to f/11 (1/8th to 1/11th), you
are halving the total open area of the aperture -- in other words, half the
amount of light is coming through the lens. The plus side is you gain depth
of field - a whole other discussion.

(FYI -- If an aperture on a lens could be opened as wide as the opening on
the camera mount, then it would be an aperture of "f/1.0" or 1/1.) While
technically possible to make such a lens, there are VERY few lenses that
can, it simply is not economically very feasible nor needed in day to day
photography. And, for more control by the photographer, you can open/close
the aperture in other than full stops -- generally 1/3 or 1/2 steps in
between full stops.)

Thus the aperture controls how much light comes through the lens. This
amount of light coming through the aperture is inversely related in a 1:1
ratio to the amount of time the shutter is open on the camera. When a
shutter speed is changed from 1/500th of a second to 1/250th of a second,
you are doubling the amount of time the shutter is open. -- twice the amount
time the light will strike the film/sensor.

Thus, the combination of aperture setting and shutter speed determines the
amount of light exposing the recording medium.. And there is a theoretical
perfect "exposure value" level (EV) for the light sensitivity level of the
film or sensor for a given amount of available light. This sensitivity
level is represented by the "ISO" (International Standards Organization)
number -- an agreed upon standard within the industry to described light
sensitivity of a recording medium.

And, cleverly, this ISO rating is related to the aperture and shutter
measurements. An ISO of "100" is exactly twice as sensitive to light as an
ISO of "50". An ISO of "200" is twice as sensitive as "100" and 4 times as
sensitive as "50", etc. These ISO numbers represent one whole "stop" of
sensitivity as well although seldom used as a descriptor because once the
light sensitivity of the recording medium is known -- the ISO -- then the
amount of light needed for a good exposure is known. The aperture and
shutter speed allows the photographer quite a bit of lattitude to meet this
exposure requirement.

Didn't say that well -- here's another shot at the explanation: the ISO
speed is the arena a photographer works in -- the aperture and shutter speed
allows movement within that boundary. Hmmm, not much better -- I'm sure
some one else has a better way of explaining it.

In each of the examples above, the change in settings would be described as
a "one stop" change. Going from f/8 to f/11 is stopping down one stop
(halving the area of the opening of the aperture). Going from a shuttter
speed of 1/500th to 1/250th is opening up one stop (doubling the time the
shutter is open). Going from an ISO setting of 100 to a setting of 200 is
"opening up" one stop (doubling the senstivity by one stop).

The combination of all the above allows a photographer to have a lot of
latitude in how he wants a subject captured. I.E., you would want a high
speed ISO (more sensitive) for shooting in low light or indoors without a
flash (the trade-off, more grain in the print). You would need a relatively
wide open aperture for low light and a low ISO film to attain an acceptable
level of exposure. You would need a slow shutter speed in those lighting
conditions......bottom line, the correlation between aperture, shutter
speed, and ISO is most easily described in "stops" and the industry long ago
worked out what these relationships are.

Clear as mud?


wrote in message
oups.com...

I was wondering how the word stop came to mean cut the light in half?
Maybe someone could provide me with a link to that story.

Also I am a little confused about how to use it. For example, if I
close the iris one stop that reduces the aperture area by half. But if
I cut the time the shutter is open by half does that constitute a stop?
What about ISO (indirectly) and ND filters, are their reductions in
light referred to as stops too?

Thanks,
Ron

You might find some good answers to your general questions here ...

http://en.wikipedia.org/wiki/Photography

HTH

wrote in message
oups.com...

I was wondering how the word stop came to mean cut the light in half?
Maybe someone could provide me with a link to that story.

Also I am a little confused about how to use it. For example, if I
close the iris one stop that reduces the aperture area by half. But if
I cut the time the shutter is open by half does that constitute a stop?
What about ISO (indirectly) and ND filters, are their reductions in
light referred to as stops too?

Thanks,
Ron

I found this link through Google. It's pretty informative.

http://www.uscoles.com/fstop.htm

Mike Rooney wrote:
The fraction describes the amount the aperture is opened
compared to the potential maximum opening. For example: an aperture
setting of "f/8" means the aperture is opened 1/8th of the total area
theoretically possible. .

(FYI -- If an aperture on a lens could be opened as wide as the opening on
the camera mount, then it would be an aperture of "f/1.0" or 1/1.)

A minor correction: The denominator of the fraction is the focal
length of the lens, not the "potential maximum opening" or the diameter
of the camera mount. In very simple terms:

diameter of aperture / focal length of lens = f stop value.

And if you think about the following facts, it all becomes clear.
1. The amount of light admitted is a function of the AREA of the
opening
2. The area of the opening is a function of the SQUARE of the
diameter.

Cheers,
Mike.

"blumesan" wrote in message
ups.com...

diameter of aperture / focal length of lens = f stop value.

And if you think about the following facts, it all becomes clear.
1. The amount of light admitted is a function of the AREA of the
opening
2. The area of the opening is a function of the SQUARE of the
diameter.

The amount of light is proportional to the area of the aperture but
inversely proportional to the focal length

50mm F/2 has a 25mm effective aperture diameter
100mm F/2 has a 50mm effective aperture diameter and 4 times the aperture
area of the 50mm F/2.

Both have the same brightness.

However the 100mm lens only collects light from 1/4 of the scene area that
the 50mm lens sees at the same distance and therefore is the same brightness
with 4 times the aperture area.

The brightness is proportional to the square of the aperture, but inversely
proportional to the square of the focal length since the solid angle of view
is proportionally smaller with the increase in focal length.

Ron, I'm not sure about the origination of the word "stop" for
describing the amount of light striking the film or digital sensor.
Perhaps it's a reference of the mechanical "stops" that use to be
used?

I'd bet on it. The term is pretty standard when considering other
things that one can set via some sort of continuous mechanical device
that has various places where it "clicks" into a particular position.

The "f" stops used to described aperture settings (you used the term
"iris" -- same difference)

Note, however, the word "iris" really isn't in common use to describe
this, and that fact led to some confusion, I think, on another recent
thread, where it wasn't clear if the poster was referring to the iris of
the eye or the aperture blades of the lens.

---------------
Marc Sabatella


Music, art, & educational materials
Featuring "A Jazz Improvisation Primer"
http://www.outsideshore.com/

wrote:

I was wondering how the word stop came to mean cut the light in half?
Maybe someone could provide me with a link to that story.

Seek and ye shall find at Google. In short (and probably wrong
sequence) there were once upon a time plates with holes in them called
"stops" because they "stopped" light from getting in. Holes were (at
some point) graduated in doublings (halvings) of area and eventually
became even ratios of the focal length.


Also I am a little confused about how to use it. For example, if I
close the iris one stop that reduces the aperture area by half. But if
I cut the time the shutter is open by half does that constitute a stop?
What about ISO (indirectly) and ND filters, are their reductions in
light referred to as stops too?

The term "stop" usually applies to the aperture, but for the sake of
compositional trades one could say "I traded a stop of speed for more
depth of field and another stop of ISO (DSLR's) to get the speed back."

The above could (for a given exposure) translate as (eg):

f/2.8 -- f/4 so 1/125 -- 1/60 then

ISO 100 -- ISO 200; 1/60 back to 1/125.

Cheers,
Alan.

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