Corelation between ISO and Shutter Speed ?

Started Feb 6, 2013 | Discussions thread
Hugowolf
Forum ProPosts: 11,309
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Re: Not quite
In reply to gollywop, Feb 7, 2013

gollywop wrote:

Vernon D Rainwater wrote:

Lenni Vilen wrote:

Silver Herenaillor wrote:

Of what i heard every ISO stops divide the shutter speed by two. Little example :
For a correct exposure of the same subject,
At ISO 100 SS is 1/8,
At 200 Ss is 1/15
At 400, 1/30,
800, 1/60,
Etc...
Is that accurate ?

Not quite: an important point to remember is that the ISO-number is not part of what exposure is, but instead it is just a parameter for a brightening function (which may be done in hardware of firmware or both, or it may be just a piece of metadata).

So if we keep the aperture constant and the ambient light constant, changing the shutter speed changes the exposure. Changing the ISO does nothing to the exposure by itself.

In practise, if you're a JPG-shooter, setting the ISO to allow for desired output image brightness is what one should do (though one should first set the desired aperture and shutter speed - to maxmize the amunt of light the sensor captures one should set these to be as large and as long as possible for a given shot).

If however you were to shoot in RAW and convert to JPG in a computer, the strategy regarding ISO is more complicated as is it often beneficial to choose a lower ISO than what the camera would choose for you.

(Regarding JPG image brightness doubling the ISO typically brightens the image by approximately the same amount doubling the exposure time or aperture would do, but compared to the exposure related parameters doubling the ISO offers lower image quality.)

I hope this is of help.

The short and specific other post is absolutely correct and I would suggest to completely disregard all the long definition/description and you will do just fine. The short post (by Hugowolf) is how it has been since the beginning -- for the first camera and that principle has not changed --

The first camera didn't have ISO, nor did any film cameras. Film had speeds (ASA, ISO, whatever), but not cameras. Cameras had exposures, SS and f-stops.

And I would argue that film did not have 'speeds', if you want to push the insane on a beginner.

Just use the KISS method (Keep it Simple, without the last letter S definition) and you will be OK.

The "first post" is not "absolutely correct," even though it may lead to the results the OP has in mind. Lenni is right, and you are adopting the KISAW method (Keep it Simple and Wrong).

If the OP is only interested in using his camera by rote, he can well ignore Lenni's comment. If he is interested in learning what he is doing with his camera, he would do well to give Lenni's comment the consideration it deserves.

I did say “Pretty much”, when answering the question “Corelation between ISO and Shutter Speed”, and it was only later that the original poster mentioned ‘exposure’ as a way of expressing what would be very difficult to describe in other terms.

What term would you prefer that the OP use?

And your sprawling cats help the OP in what way?

Brian A

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