ISO question:

[soocom1]

First, i am new to the forum so hello to all.

A question that has been bothering me.

As an old school film shooter from years past and who has been a Canon owner for more than 7 years now a question that keeps bothering me.

Why has Canon not produced or written firmware to allow the cameras to shoot at a 25, 12, or even 6 ISO.

Have worked int he past to ISO's as low as 2 BTW and was wondering why is it always that the digitals (with some exceptions like my 1Ds) stops at 100 ISO? Why not 50, 25 or 12?

[williammcewen]

My 5dmkII will shoot at ISO 50 through the ISO Expansion function in the Custom Menu.

[Peter]

Maybe not the answer, but it is an interesting reading http://www.clarkvision.com/articles/iso/

http://www.magiclantern.fm/forum/index.php?topic=10111.0

[RobertTheFat]

Part of the answer could be that below a certain point, lower ISO could mean more noise. All else equal, the lower the ISO setting, the longer the exposure that is needed, and there is a component of noise that is essentially proportional to the length of the exposure.

[ebiggs1]

Now the real reason.  

Digital cameras, ISO sensitivity is a measure of the camera's ability to capture light. Almost the same as film ISO or ASA numbers. Digital cameras convert the light that falls on the image sensor into electrical signals for processing. Here is where ISO and film ISO have nothing in common.  ISO sensitivity is raised by amplifying the signal.  Doubling ISO sensitivity doubles the electrical signal, halving the amount of light that needs to fall on the camera sensor to for optimal exposure.  When ISO sensitivity is raised from ISO 100 to ISO 200 and the aperture is left unchanged, the same exposure will be achieved with a shutter speed twice as fast. 

What this really means is you are trying to compare film 'grain' to an electric signal.  You can not!  You can increase the signal to the amplifer circuits in the camera untill they are overloaded.  IE, noise or grain, if you will. It is actually no different with your stero.  If you increase the volume enough the sound will be loud but will be distorted.  Right?  Same, same. If I decrease the volume at some point you won't be able to hear it at all. It might not be distorted but you still can't hear it.  Right?  Same, same.

In digital the only thing that remained were the numbers because most film guys would be comfortable with them.

[TCampbell]

As an alternative, you can use ND filters to reduce the amount of light reaching the sensor without having to alter the aperture (so you won't impact other attributes you might want to preserve... such as depth of field.)

[ebiggs1]

Tim is right.  ND filter allow you to reduce the signal strength the sensor makes with out distorting it.  The signal has to be electronicly altered if you use the 50 ISO rating.  It is really no different than changing the exposure in Photoshop by 1 stop.  Same, same.