Re: With 12/16bit RAW do you still need an ND Grad

Roger N. Clark (change username to rnclark) wrote:
Floyd Davidson wrote:
"Roger N. Clark (change username to rnclark)" <username@xxxxxxxxx> wrote:
Floyd Davidson wrote:
The "lowest usable signal" is determined by the Signal to Noise
Ratio, and with digitally sampled images that is generally
accepted to be a zone that has at least 8 levels. It is indeed
limited by the bit depth, and with JPEG that is 8 fstops. (It
is not directly determined by the 8-bit depth though, because
gamma correction is applied.)
This is not correct. First, this continued reference to "gamma"
correction is a misnomer.

It *is* correct, though you are certainly right that I'm not
using pedantically correct terminology to describe it. But if
we get tied up with terminology, people lose sight of how it
applies to the questions they asked...

No your statement
>>> It is indeed
>>> limited by the bit depth, and with JPEG that is 8 fstops.

is NOT correct. Non linear encoding allows you to record a larger range
than 8 stops. What you sacrifice is precision between stops.
For example, I can encode 26 stops by encoding integer log base 10,
so each bit is a factor of ten, or 10^8 -1. The non-linear encoding
in digital cameras typically encodes 11 to 12 stops.

The function for the "standard" transfer
curve in digital cameras matches closely the characteristic curve of print film.
It is a continuously varying gamma function, meaning if you applied
a gamma function, the exponent would have to be variable with scene
intensity. A gamma function plots as a straight line on log-log
plot. The transfer curve for digital cameras do not plot as straight lines.
e.g. see:
http://www.clarkvision.com/imagedetail/dynamicrange2

All true, but of no particular significance to the question I
had at hand, IMHO. However, your point *is* interesting and
probably of value to some readers, so I do think it is good to
bring up in a followup.

It is exactly to the point. It is the non linear encoding,
not gamma encoding, that allows digital camera images to cover
the range they do.

The
standard definition is max signal / noise floor.

No it isn't. That is the definition for the dynamic range of
an analog electronic sensor. It is interesting, and is a useful
value to know about, but it doesn't necessarily translate
directly to the dynamic range of the digital data produced by a
codec. It certainly defines the upper limit for the digital
data though.

It is also the definition used for digital imaging data. It is the
definition used in the scientific terrestrial and planetary remote sensing
fields. It is the definition used by sensor manufacturers in
defining the dynamic range of their products.

In DSLRs, for example,
that is higher than 9-stops.

You mean that for CCD and CMOS electronic sensors it is higher
than 9 fstops. But that data is currently being digitized and
captured via a 12 bit codec, and images that result from such a
process have a *useful* dynamic range of 9 fstops, maximum.

That is a figure arrived at simply by the empirical observation
that if zone has fewer than 8 values it will appear to the human
eye to be posterized. It is entirely arbitrary and subjective
to choose 8 values...

So we are using your subjective definitions?

Some measured dynamic ranges are shown as
a function of ISO in Figure 5 (~11.5 stops) at:
http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary

A 9-stop definition would mean a lower end signal-to noise ratio
of around 5.5. Note some high ISO film images do not have that high a
ratio. But people still view them as acceptable images.

And that is what causes so much confusion. It might well be
that for *some* particular use a zone with only 2 values will be
considered "useful"; and just as clearly that will not normally
be the case.

Incorrect. See below.

An 8-bit RGB image with gamma adjustment actually has more potential DR
than a 12-bit linear RAW file. You don't see it, because of the noisy
JPEG files are limited to an 8 fstop useful dynamic range. 12
bit linear
data is limited to a 9 fstop useful dynamic range.
The data at
http://www.clarkvision.com/imagedetail/dynamicrange2
show over a 10 stop range for jpegs.
The data at
http://www.clarkvision.com/imagedetail/raw.converter.shadow.detail
show no clipped highlights at +2.0 stops in the sunset image to
shadow detail down to -7.6 stops. That is more than your 8 stops.

You are not addressing the same definition of "useful dynamic
range" that I am. All we have to do is assume that 6 levels is
satisfactory, and JPEG has 9 fstops of dynamic range. That is a
reasonable, and very arbitrary, assumption.

I have made it clear that the commonly acceptable number is 8
values, and that I am not going to argue that value. I will,
obviously, argue the significance of the value chosen though!
:-)

The following page is under construction, but has enough data
to illustrate what we are talking about. It is a test series
showing exposure latitude of a digital camera. I did an exposure series
from -11.4 to +14 stops on both a 1D Mark II and some print film.
The page so far shows the 1D Mark II results.
The metered value = 18% gray card. The macbeth color chart, bottom
raw, forth patch from the left is 20% reflectance, so closest
to 18% gray. The brightest patch is +2.2 stops, the darkest -2.7 stops.
The idea is to produce the best image at each exposure.

You can look at the images and decide for yourself what is acceptable.
Exposures -5 to +2 stops produced excellent images, with
no saturation and excellent shadows. The meter +3 second from left
white panel is not saturated (59% reflectance) but the whitest
panel is, so that is +1.6 stops, so the real dynamic range is
+3+1.6 to -5-2.7 = 4.6 + 7.7 = 12.3 stops of excellent image quality!
The meter -7 is still pretty good, for a total range of 14.3 stops!
Each can decide on the lower limit to useful images. The meter -8
stops is also pretty good (better than some high speed film images)
and there is still information recorded in the -11 stops (label is 11, but
actual is 11.4). (Also note the -10 = 10.4 and -11 = 11.4 and were obtained
in shadow other in full sun, so light was blue sky and reds are lost.
I need to redo the low end).

Anyway, it is an impressive range, and a range greater than 12-bit linear
encoding. There is a scientific explanation too.

Oops: here is the URL
http://www.clarkvision.com/imagedetail/exposure_latitude-1

Roger
.

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