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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