How many stops from 160 ISO to 320 to 640 etc

[carstenw]

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I am pretty happy about not having to deal with 1/3 stop increments (like Zeiss lenses), but I would like 1/2 stop ISO increments, just like with the aperture and shutter speed.

 

Iron (flatline; interesting pseudonym...),

 

yes, as others have said, halving or doubling ISO values is also a stop, and as has been mentioned, with digital it is likely more exact than with film. The fact that the ISO range is anchored at 160 rather than 100 doesn't change the proportional steps between the values.

 

In fact, ISO numbers are meant to be given as for example 100/21, where the 100 is the ASA value (American Standards Association) and 21 the DIN value (Deutsche Industrie Norm). The DIN values never made it very far outside of Germany though, so most people drop them. The DIN values were on a logarithmic scale, I believe, like dB, so an increase in the value by 3 would be a doubling in sensitivity, rather than a doubling of the value...

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[scott kirkpatrick]

Arrghh... I think I'm being misunderstood.

 

My question is does changing the ISO from 160 to 320 mean exactly one stop difference?

 

[answered already] yes, of course, 2X ISO means that the system is one stop more sensitive. But the more interesting question is why aren't we offered 1/2 stop, 1/3 stop, or continuous increments. My guess is because the corrections are done after the pixel luminosity values have been digitized. Then you increase the signal by 2 with a simple shift of one bit in the high direction. Any multiplication by other than a power of 2 takes real work, and introduces a real time delay at a point in the data chain where there may not be general purpose hardware.

 

scott

[ron110n]

Ron, no it isn't. One stop slower than f4 is f5.6, not f8 (4 x 2).

 

Yes I realize that Steve when I was in my car leaving for work, duh

Then I saw that you corrected it when I reached work. Thanks!

 

-Ron