Archived posting to the Leica Users Group, 2006/02/08

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Subject: Digital ISO, was Re: [Leica] I have given up
From: jonathan at openhealth.org (Jonathan Borden)
Date: Wed Feb 8 13:47:52 2006
References: <22776246.1139338750534.JavaMail.root@elwamui-milano.atl.sa.earthlink.net> <43E90565.5000507@adrenaline.com><402DA9F8-41AF-4EBC-9F27-17C48EFFCF4D@btinternet.com> <43E9A976.6060300@adrenaline.com> <00ce01c62ccf$416fb490$af7c8081@lse.ualberta.ca> <43EA567B.2010907@adrenaline.com>

  Scott McLoughlin wrote:

> Good info! My JPEG curve is the popular, updated version of
> the old "White Wedding" curve.
>
> In a digital context, how would one "test" ISO.

Excellent question.

Film ISO is measured as defined by the International Standards  
Organization.
I suggest that the following method would allow one to develop an  
equivalent "ISO" for digital sensors:

The method that I've used to measure my own "ISO" has been to shoot a  
textured grey fabric with a meter at the recommended exposure and  
then each frame at a successive underexposure of one stop.

Using the Zone system, Zone 0 ought be pure textureless black.  
consequently the proper "ISO" for a particular film/dev combo ought  
be when 5 stops of underexposure give a blank textureless negative.  
That is to say the exposed part and the unexposed edge have the same  
density (film base + fog).

In digital one can likewise underexpose until the image becomes  
background noise. 5 stops up from this gives an "ISO" for which a  
Zone 0 is background noise (pure black in the digital world).

By this method one could actually compare the DMR with a Canon sensor  
-- it may be that Leica is just being more conservative than Canon  
regarding the ISO ratings. Underexposing by more stops than is  
supported by the sensor is just like pushing a negative -- although  
you get a picture you loose shadow detail.

Do you fault Kodak or Ilford for selling ASA 3200 films that really  
have a true ISO rating of 800-1000 (given the film base + fog  
measurement technique)? This might be the exact same situation as  
Canon allowing a higher "ISO" for the 20D than the Leica DMR. Has  
anyone measured this?

Jonathan



>
> Tim Atherton wrote:
>
>> Bear in mind that the on camera histogram may be a little  
>> misleading - it is
>> usually the histogram as applied to a to a JPEG which has had the  
>> camera's
>> default conversions applied to it (including a fairly steep S curve)
>>
>> So what you see there has already dumped a lot of the information  
>> that would
>> be in the RAW file - including highlight detail
>>
>> Add in - that rather like the old days you still need to tweak the  
>> iso of a
>> digital camera (remember shooting slide film at 80 or 125 instead  
>> of 100?
>> etc or rating 160 neg film at 125 or 100?) . A digital camera's  
>> 100 iso may
>> actually be closer to 80 or 125 - you need to run your own tests -  
>> just like
>> the old days  :-)
>>
>> tim a


Replies: Reply from ericm at pobox.com (Eric) (Digital ISO, was Re: [Leica] I have given up)
Reply from scott at adrenaline.com (Scott McLoughlin) (Digital ISO, was Re: [Leica] I have given up)
In reply to: Message from feli2 at earthlink.net (feli) ([Leica] I have given up)
Message from scott at adrenaline.com (Scott McLoughlin) ([Leica] I have given up)
Message from Frank.Dernie at btinternet.com (Frank Dernie) ([Leica] I have given up)
Message from scott at adrenaline.com (Scott McLoughlin) ([Leica] I have given up)
Message from tim at kairosphoto.com (Tim Atherton) ([Leica] I have given up)
Message from scott at adrenaline.com (Scott McLoughlin) ([Leica] I have given up)