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ISO speed value meaning



 
 
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  #1  
Old July 18th 07, 08:08 AM posted to rec.photo.digital
Desert Dweller
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Posts: 27
Default ISO speed value meaning

I understand a faster ISO means the exposure will be created faster,
thus requiring less light and enabling faster shutter speeds.

What does the 50 in ISO50 mean? What does the 800 in ISO800 mean? etc.

--
DD
  #2  
Old July 18th 07, 11:19 AM posted to rec.photo.digital
Ray Paseur[_3_]
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Posts: 102
Default ISO speed value meaning

Desert Dweller wrote in news:YVini.38$fK1.17
@newsfe12.phx:

I understand a faster ISO means the exposure will be created faster,
thus requiring less light and enabling faster shutter speeds.

What does the 50 in ISO50 mean? What does the 800 in ISO800 mean? etc.

--
DD


Without getting too detailed, the ISO number is a statement of the
amount of light sensitivity. It is linear (unlike f/stop) so ISO 200
means twice as much light will be recorded as ISO 100. ISO and f/stop
(aperture) and shutter speed combine to give you creative control over
the depth of field, stop action, etc.

You may have heard of the "sunny 16" exposure rule: On a sunny day, at
ISO 100, you have enough light for f/16 at 1/100 second exposure. It
follows that at ISO 50 with 1/100 second, you can only achieve f/11 -
half the light. If your camera (or film) is set to ISO 400 and you want
f/16, you need to make your shutter speed 4x faster.

More on Sunny 16 he
http://en.wikipedia.org/wiki/Sunny_16_rule


A general "Ohm's Law" of photographic exposure relates these three
variables: ISO speed, shutter speed, and aperture. If you fix any two
of these variables, the third is determined. Knowing which two to fix
is the creative aspect of photography.

From the tone of your questions to this NG, I would guess that you are
just starting out in creative control of photographs. You might want to
consider a course in photography at a local park or community college.
They can teach you all the basics in short order, faster than you can
learn from any book. With digital cameras, you can shoot all day long
and not worry about the cost of film!

have fun,
Ray

  #3  
Old July 18th 07, 11:43 AM posted to rec.photo.digital
Alex Monro
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Posts: 79
Default ISO speed value meaning

Desert Dweller wrote:

I understand a faster ISO means the exposure will be created faster,
thus requiring less light and enabling faster shutter speeds.

What does the 50 in ISO50 mean? What does the 800 in ISO800 mean? etc.

See http://en.wikipedia.org/wiki/Film_speed

A google on "ISO film speed" found this as the first hit.

  #4  
Old July 18th 07, 09:03 PM posted to rec.photo.digital
Enric Martinez
external usenet poster
 
Posts: 5
Default ISO speed value meaning

Ray Paseur wrote:
Desert Dweller wrote in news:YVini.38$fK1.17
@newsfe12.phx:

I understand a faster ISO means the exposure will be created faster,
thus requiring less light and enabling faster shutter speeds.
[...]


Lovely, I think I will love this group

Thanks people for sharing the knowledge with us humble beginners.

Regards.

  #5  
Old July 19th 07, 03:00 AM posted to rec.photo.digital
Roger N. Clark (change username to rnclark)
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Posts: 1,818
Default ISO speed value meaning

Ray Paseur wrote:
Desert Dweller wrote in news:YVini.38$fK1.17
@newsfe12.phx:


I understand a faster ISO means the exposure will be created faster,
thus requiring less light and enabling faster shutter speeds.

What does the 50 in ISO50 mean? What does the 800 in ISO800 mean? etc.

--
DD



Without getting too detailed, the ISO number is a statement of the
amount of light sensitivity. It is linear (unlike f/stop) so ISO 200
means twice as much light will be recorded as ISO 100.


Half as much light. As you go up is ISO, less light
contributes to the image. That's why exposure times decrease.
If you needed more light, exposures times would be longer.

ISO and f/stop
(aperture) and shutter speed combine to give you creative control over
the depth of field, stop action, etc.

You may have heard of the "sunny 16" exposure rule: On a sunny day, at
ISO 100, you have enough light for f/16 at 1/100 second exposure. It
follows that at ISO 50 with 1/100 second, you can only achieve f/11 -
half the light. If your camera (or film) is set to ISO 400 and you want
f/16, you need to make your shutter speed 4x faster.

More on Sunny 16 he

http://en.wikipedia.org/wiki/Sunny_16_rule



A general "Ohm's Law" of photographic exposure relates these three
variables: ISO speed, shutter speed, and aperture. If you fix any two
of these variables, the third is determined. Knowing which two to fix
is the creative aspect of photography.


If you really mean aperture, then yu need another parameter:
focal length. Aperture determines how much light can be collected,
focal length spread it out. You probably meant f/ratio.
Use of "aperture" in place of f/ratio is becoming
misued too often.



From the tone of your questions to this NG, I would guess that you are
just starting out in creative control of photographs. You might want to
consider a course in photography at a local park or community college.
They can teach you all the basics in short order, faster than you can
learn from any book. With digital cameras, you can shoot all day long
and not worry about the cost of film!


Some more info:

See table 1 at:
http://www.clarkvision.com/imagedeta...photons.and.qe

and below tabel 2, equations 3 and 4 shows the relationship between
ISO and lux. Note the constant used in the equations varies
by manufacturer (and has for decades).

Roger

Roger
 




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