On the contrary. It seems to me that you just took a whole page to
explain a distinction without a difference.
Chuck Norcutt
On 9/9/2011 10:15 AM, Ken Norton wrote:
> Not to disagree with the Moose in public, but I must...
>
> Initially, ETTR was as per what Moose suggested, and that's to protect the
> highlights. Since then, it has evolved to maximize the bit-depth available
> for the converted RAW (Raw, raw) file. This is based on the theory that the
> top stop of dynamic range contains half of the available bits for the
> image's tonal depth. The next stop of dynamic range contains a quarter, the
> next contains an eighth, and so forth. Provided that the sensor isn't
> producing just noise, the bottom stop of dynamic range is represented by a
> single bit and posterization is visible.
>
> So, ETTR, if used to protect the highlights will result in the image being
> underexposed. During conversion, the exposure must be brought back up and
> adequate curves applied to move everything back to the right, but the
> highlights will end up being compressed a bit to keep them from clipping.
> (this is my prefered reason for using ETTR with the E-1).
>
> If ETTR is used to maximize dynamic range AND tonal separation (maximum bit
> availability), the image is overexposed in-camera. Inotherwords, you
> overexpose the scene as much as you can to the point where you start to
> clip, and then back off a teeny tiny bit.
>
> If the entire scene is, say, a foggy morning where the entire dynamic range
> of the scene is three stops, what you want to do is in-camera push the
> exposure so far to the right that those three stops reside up at the right
> hand of the histogram. During conversion, you "normalize" the exposure,
> which means that you'll pull the exposure back down the same amount that you
> pushed it up.
>
> There are two distinct advantages to doing this with a low-dynamic range
> subject. You get more bits available during capture to represent the usable
> image. During conversion, we're going from a 12-14 bit capture medium, to a
> 16-bit per channel editing environment. As such, the problem with available
> bits is not in the editor, but just in the camera. More bits mean smoother
> gradients. The second reason has to do with noise. This has its roots going
> back to the basis of Dolby noise reduction. The thinking is that noise is
> more apparent in the shadows (due to sensor noise AND available bit depth),
> so by amplifiying the incoming signal to the maximum, when we convert we
> lower the signal back down to desired level, but in doing so, the noise is
> also pushed down by the same amount.
>
> As to the noise reduction, this is easily seen in my E-1 Dyamic Range
> article where you can see the noise build-up in the images which were
> boosted up in conversion. Meanwhile, the images which are pushed down in
> conversion are squeeky clean. More or less, of course. The E-1 is a bit
> unique because Kodak-Olympus applied a dithering noise to imply a smoother
> tonal gradient across the entire image and to mask the shadow noise buildup.
> Most Panasonic-sensor image files don't handle being boosted up as much
> because the transitions in the tonal gradients are much more abrupt.
>
> To the last point, CMOS sensors use on-chip noise-reduction. This is ALWAYS
> in place and is used to balance out the noise profile of the sensor itself.
> This is something that occurs directly on the sensor and has no
> user-settings available. It's baked into the chip. Does it work? Yes it does
> and extremely well. It's required as CMOS is normally noisier than CCD, so
> this technology not only evened the playing field, but then advanced it much
> further than CCD was able to go. But what happened is that CCD would add a
> bit of random noise to the image, where the CMOS's noise tends to be more
> fixed. Although this on-chip noise reduction does NOT smear or reduce detail
> in any way because it is acting on each individual pixel, what it does do is
> reduce or eliminate the random noise which gives a false texture. It also
> means that in your bottom two stops of dynamic range, any gradient is
> severely affected and stair-stepping occurs with reckless abandon.
> (hyperbole alert--OK, under some circumstances where you are cranking the
> curves around, you'll see the posterization). By using ETTR and overexposing
> the scene, we are able to have more bits available for the deepest shadows.
> The final presented image will be free from having any visible artifacts or
> stair-stepping (posterization) in the shadows.
>
> AG
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