Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 17,020
Re: It's about useful comparisons - not which is better

I additionally know, and can prove, that the DPR studio scene samples offer no set of camera ISOs such as to meet your preferred criterion. However, they do offer one set of camera ISOs such as to meet another criterion that I find acceptable, namely one in which the cameras are given the same signal (as I defined the word signal above) although the mean raw values are not the same.

I don't think that they do do that. DPR exposes as I understand for a fix point in the sRGB output - that is that 18% (though they never specify 18%) on their step wedge gives the appropriate value in the JPEG file. You have to relate that back to the effective (meter ignored) ISO value that they discovered (and do not always report) in their ISO 'sensitivity' test.

Furthermore, the viewer of the DPR SCT RAW thumbnails is seeing Adobe ACR's RAW-level post-scaling applied without regard for individual deviations which may exist between cameras at either the same, or differing, ISO settings, as well as the possibility existing that DPR "tweaked the brightness" - which, in the case of the G3 test-shots processed using ACR 6.x, means the ACR 6.x "Brightness" control which is in fact a Gamma control (not a RAW-scaling control), and thus (when adjusted from the default value of 50) alters the apparent SNR of the image. Food for thought ?

See Andy Westlake's statement here: http://forums.dpreview.com/forums/post/40965292

Whether two cameras got the same exposure at the same camera ISO is nothing we have to speculate about DM. We can just check it. See here:


Yes, I am familiar with that bit. My post was about other things that may affect the DPR SCT RAW thumbnails as they are presented, and at what display luminance levels image-noise may appear. I recognize that this is an issue restricted to the practice of casual reliance upon those thumbnails - as one is free (with the downloadable RAWs) to eschew Adobe's potential proprietary idiosynchracies, and use RAW processors that are not silently messing around in numerous (and largely unknown to users) respects with the RAW conversion and processing. Only "Adobe-ites" seem satisfied with that.

OK. I see your point. But as you say, one can always download the RAWs and play with them oneself. What you would want to know in that case is simply whether the exposure was the same and we can check that by means of the methods I have used.

I agree with what you have been saying. Your method of using the ratio of DxOMark Saturation ISOs combined with RawDigger statistics represents an accurate way to determine the actual exposure.

It seemed to me that the appearance of DPR SCT RAW thumbnails (not downloaded RAWs) is (at least a partial) factor in the making of value judgment by (at least some) readers of this thread.

Beyond issues of the exposure used in the test-shots involved, it seems to me that the apparent SNR (when viewing such DPR thumbnails) is an important factor. In that respect, the possibility that use of the ACR 6.x "Brightness" (which alters gamma-correction, as opposed to linearly scaling the output image-data) with respect to the G3 test shots represents a legitimate concern.

The "Brightness" control no longer existing in ACR 7.x, it is unclear what ACR control may now occasionally be "tweaked", and unclear whether the underlying mechanisms are understood by those making the adjustments, or by those viewing and relying upon such thumbnail views.

Please note that my raising of this issue related to the apparent SNR of these particular thumbnails is not intended to impeach your arguments in a major way. Instead, it seems to me to be a valid general concern which may possibly have some impact that it seems should rightly be noted.

Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow