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Re: [OM] ISO may not be accurate

Subject: Re: [OM] ISO may not be accurate
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Fri, 30 Jan 2009 07:18:45 -0500
Interesting.  I'll stick with the gray card.

Chuck Norcutt

Tom Fenwick wrote:
>> 2009/1/30 Chuck Norcutt
>> That's interesting.  DPReview has been reporting actual ISO test results
>> for a long time.  DXO says the Canon 5D's ISO 100 is actually 92.
>> DPreview reported it as 125.  And, of course, Canons says it's 100.
>> Who's right?  I doesn't make any difference until I have to get out the
>> external meter for flash.
> 
> I came across this discussion elsewhere a couple of weeks ago, which may be
> relevant:
> 
> -----------------------------------------
>> I have a hard time trying to reconcile DXO results with real world
>> testing by Phil and many other reviewers.
> 
>  The discrepancy between DxO's results and the manufacturer's ISOs is very
> straightforward to explain, and it's all down to how metering and exposure
> works.
> 
> Basically the conflict is between two ways of measuring ISO. One method is
> to determine the exposure required such that, if you spot meter off an 18%
> grey card and then take a picture of it, the grey is rendered at a luminance
> of 50% in the resultant image file. I'd argue this is the method most
> relevant to how the majority of photographers work, and if you test cameras
> this way they will all give essentially the same answer (always bearing in
> mind that ISO 12232 allows a tolerance of 1/3 stop either way, probably as
> much to cater for manufacturing variations as anything else).
> 
> However DxO use a different method, which is based on the point where
> highlights clip to white. This does not necessarily give the same answer as
> the grey-point method due to tone curve differences between cameras, and the
> difference between the two methods generates the discrepancy in DxO's
> results. In fact it turns out that the difference actually reflects the
> highlight dynamic range of the camera; the lower DxO's "true" ISO, the more
> higlight DR you've got.
> 
> Perhaps the easiest way to consider this is to think in terms of taking two
> images of a high dynamic-range scene, using identical shutter speeds and
> apertures but with and without Highlight Tone Priority turned on. Visually,
> both will be correctly exposed, and by the grey card test, both shot at the
> same ISO. But the highlight clipping point will be shifted by a stop between
> the two, and therefore using that definition they will have been shot at
> ISOs one stop apart, with an ISO 200 HTP shot considered to be ISO 100.
> 
> Now this doesn't actually mean DxO's method is wrong, as they are only
> concerned with RAW data not output image files, indeed it's the only way
> they can compare RAW output on an equal footing. But it's purely about
> technical comparison of RAW data, not about using the camera.
-- 
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