DPR chickens out.

Started Apr 18, 2013 | Discussions thread
Great Bustard
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Re: A few quick questions:
In reply to jrtrent, Apr 20, 2013

jrtrent wrote:

Great Bustard wrote:

I was talking about my particular camera, a Samsung GX-1S, or Pentax *ist DS2 clone.  It's lowest ISO setting is 200, and test reports, as well as my usage, indicate that it gains a bit of highlight dynamic range by using ISO 400.  A more dramatic difference is found in my Olympus E-450, where highlight clipping is far less likely at ISO 200 than at ISO 100.

I'd love to see an example.  Please show me a photo at, for example, f/5.6 1/100 ISO 100 that clips highlights more than a photo of the same scene at f/5.6 1/200 ISO 200 or f/5.6 1/400 ISO 400.

I don't post images, but the explanation for the difference in the Olympus can be found in DPReview's review of the E-620 (my E-450 works the same):

The thing that jumps out about the E-620 is what happens to the dynamic range and response curves at ISO 100 and 200. ISO 100 is showing considerably less highlight dynamic range and a much less subtle roll-off to white (resulting in a harsh transition to blown-out regions beyond the camera's dynamic range). Whereas ISO 200 and above use a more gentle and highlight-preserving tone curve.

It's worth noting that the ISO 100 tone curve is virtually identical to that used by the E-520 - the E-620's ISO 200 mode is effectively that ISO 100 underexposed by a stop, with a different tone-curve applied to pull the data back up to the correct brightness. This is essentially what Canon does with its Highlight Tone Priority mode, or Pentax with its DRange expansion mode, where the sensor is intentionally under-exposed and the data pulled-up in order to retain highlight detail. http://www.dpreview.com/reviews/olympuse620/16

The difference with the Samsung between ISO 200 and ISO 400 is more subtle, with test reports estimating about a 0.3 EV difference as opposed to the 1.2 EV difference in the Olympus.  DPR had noted the same effect with a few cameras from that era using the same 6 megapixel sensor.  Something about its native sensitivity being closer to ISO 270 than ISO 200, with the consequence that a bit of highlight room was lost in providing that lowest ISO setting.  My Sigma SD14 also lost highlight dynamic range going from ISO 100 to ISO 50.

So what you're talking about is different tone curves being applied at different ISOs for OOC jpgs.  That's another matter entirely.

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