On 9/9/2011 7:15 AM, Ken Norton wrote:
> Not to disagree with the Moose in public, but I must...
Nice try, but I don't think so. In line with the thread that got us here, I
paid most attention to the problem of wide
DR subjects. Here, you are dealing more with low DR subjects. It's still "make
the snake kiss the post".
> 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 someone has a different rational than someone else. The technique used is
the same.
> 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.
Let's make this clearer. ETTR may move the tonal midpoint up, overexposure, or
down, underexposure OR, in a great many
cases, leave it unchanged. Of course, we may not think of the ordinary light,
ordinary subject shots as ETTR, because we
made no special effort. But if the top of the brightness just kisses the top of
the histogram, it is ETTR.
No, I didn't go into the fact that in conversion or post one may have to move
the tonal midpoint up, high DR ETTR, or
down, low DR ETTR. My interest is simply getting all the DR possible into the
captured file with as much tonal detail as
possible and minimal noise. What I may have to do later in post is of much less
concern. I'll readily trade an extra
five minutes in conversion/post for more and cleaner image data.
I realize that's not the way things are for event shooters, ( I assume portrait
is a controlled situation without these
issues.), those with tighter time limitations and those who are not digital
darkroom jockeys. But hey, those in the old
days who weren't wet darkroom jockeys had to live with more limited
photographic range, too.
> 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.
Nice rational, and may well be right. I certainly don't have the skills to tell
about theory. The practice of ETTR is
still the same, though. Actually, in my experience, many low DR subjects start
to look wrong if stretched too far,
anyway. As a practical matter, it seems to me that just getting the pig in the
python above center is enough.
> 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.
Always better to be pushing down than pulling up. Still, there are lots of
subjects where it's not a choice with today's
cameras. I'd rather be working on noise reduction than throwing the image away
'cause there are crucial highlight
details that are just plain gone.
ETTR Moose
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