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[OM] ISO sensitivity, was: Recommendations for compact digital camera

Subject: [OM] ISO sensitivity, was: Recommendations for compact digital camera
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Wed, 19 Nov 2008 07:11:32 -0500
After Mike posted a DXOMark link the other day I checked their ISO 
ratings of the 5D and was a bit surprised to find their ratings show 5D 
ISO performance to be slightly less than the nominal value.
<http://www.dxomark.com/index.php/eng/Image-Quality-Database/Canon/EOS-5D>

That conflicts with what DPReview reported long ago and which I have 
always treated as gospel when using an external meter.  According to 
DPReview the 5D (and other Canons) show 1/3 stop greater sensitivity
<http://www.dpreview.com/reviews/canoneos5d/page21.asp>

This caused me to reflect back on a problem I had a few weeks back.  I 
was taking some shots of my grandchildren in bright, late morning sun 
and using a flash for fill.  I measured the exposure with my Sekonic 
L-358.  Both were wearing white sweaters and I was surprised to see that 
parts of the sweaters had blown highlights.  I did a lot of muttering 
under my breath but it only just occurred to me now that perhaps I 
didn't (as I normally do) set the meter for ISO 125 when the camera was 
set at 100.

Looks like it's time to take a bright white target into the sun and do a 
real world controlled test of meter and camera together both with and 
without flash.

Chuck Norcutt


Moose wrote:
> Wayne Harridge wrote:
>> Just how is ISO determined for a digital camera ?  As I recall the ISO 
>> rating of a film is determined from the film's characteristic curve which is 
>> distinctly different to the curve (straight line !) of a digital sensor.
>>   
> The rules are easy enough to find with a search. I don't recall the 
> details, but I recall deciding that there was no obvious problem with 
> measuring the ISO sensitivity of a digital camera using the same rules 
> as for film. I believe the film spec only uses the linear, central 
> portion of the curve.
> 
>  From a practical standpoint, though, at least with current technology, 
> the correct speed to use is strongly affected by the highlight and 
> contrast situation. Avoiding clipped highlights is so much different 
> with digital than film.
> 
> I try always to put my cameras away in 'ready for anything' settings. 
> One of those is an EV adjustment of -2/3. When I've finished shooting 
> with a different exposure comp. setting, I try to remember to reset to 
> that. For me, it's been true from the original S110 to the 5D, for Canon 
> and Fuji, digicam P&S and DSLRs, that -2/3 gives the best chance of 
> correct histogram in bright, contrasty light.
> 
> So I could be cynical and guess the Panny marketing people fudged the 
> numbers. But it's possible that some engineers said something like "The 
> ISO standards just don't give the average user the best results with our 
> cameras. Let's just use settings that give the best practical results."
> 
> Just a thought ...
> 
> Moose
> 
> 
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