From: Porte Rouge on
On Oct 7, 8:21 pm, "Wilba" <use...(a)CUTTHISimago.com.au> wrote:
> Porte Rouge wrote:
>
> > I set my exposure to slide the histogram to the right, without clipping
> > ( when I have time), to capture the most tonal levels . So, now when I
> > am editing the photos they are over exposed(not clipped). A sunrise is
> > a good example. The deep colors are washed out. The obvious fix(to me
> > anyway) in Lightroom or CS4 is to reduce the exposure. Now my question
> > is, by reducing exposure in post, am I just ending up in the same place
> > (histogram to the left) as if I had just ignored the histogram when I
> > was shooting and set the exposure to properly expose the image using
> > my light meter? I guess in short I am asking if Lightroom or CS4 loses
> > tonal values when you reduce exposure in editing.
>
> Yes, reducing the exposure in Lightroom just puts your histogram back to the
> left, so that's a waste of time. To avoid having the deep colours washed
> out, you need to make use of the full tonal range available in the image. If
> you have done a good job of exposing to the right, your highlights will be
> pretty much where they need to be. So then all you need to do is raise the
> black point (e.g. using the Levels dialog in Photoshop), so that the dark
> tones in the scene end up as dark tones in the image. If you are using the
> Levels dialog in Photoshop, while you drag the black (or white) point
> slider, press the Alt key to see which pixels are clipped (or blown).
>
> (The following relates to my experience with a Canon 450D. YMMV.)

Aha!

>
> There are two parts to exposing to the right - levels in the image and
> saturation of photosites. Whether you are shooting JPEG or raw, the camera's
> histogram display shows you the levels in a JPEG produced from the raw data,
> so the shape of the histogram curve depends on your settings for sharpness,
> contrast, saturation, colour tone, etc.
>
> If you are shooting JPEG, all you can do is avoid piling up the histogram on
> the right, which prevents gross areas of blown highlights in the image.
>
> If you are shooting raw, you can overexpose (beyond the point at which the
> histogram curve touches the right boundary), by about two steps and still
> produce an image from the raw without blown highlights. IOW, the headroom
> between blown highlights in the JPEG and saturation of the photosites is
> about two steps (in my experience, with the JPEG settings I use).
>
> So if you are bracketing for the best possible exposure, go at least two
> steps over what the histogram tells you is "correct". The maximum possible
> dynamic range occurs at the point where photosites in the highlights begin
> to saturate.
>

What do you mean by steps? Do you mean stops, or steps on the aperture
or shutter dials?


> In everyday raw shooting, you can be quite relaxed about having a small
> spike on the right of the histogram - that's actually a good thing.

From: Doug McDonald on
Floyd L. Davidson wrote:


I have performed tests on my Canon 30D camera to see
what happens if I expose less. I exposed 1/3 and 2/3
stops less than before.

At 2/3 stop less than before yes, even at contrast setting zero,
I do see a teensy bit of space between the top of the red histogram
and the top of the display. This means that if I'm willing to
lose 2/3 stop of unclipped data at the bright end, Floyd is "effectively"
right.

But if I want that extra 2/3 stop, I'm still correct.

I should add that my method works --- I use -4 in the camera,
use the histogram, and find my raw images are very nicely exposed,
no clipping (except as desired for specular highlights) and the
data right up to the clipping level.

Doug McDonald
From: Doug McDonald on
Floyd L. Davidson wrote:

> Hence if the right edge is set correctly with high
> contrast or low contrast, either way it is exactly the
> same exposure and neither is more accurate than the
> other.
>
> You continue to say that low contrast makes it easier to
> see where the edge is, but that is only true for special
> cases, and for an equal number of special cases high
> contrast would make it easier!
>

In NO CASE would high contrast make it easier ...
at least on a Canon 30D.

Mr. Davidson: Let me ask again: What is the serial
number of the Canon 30D you have checked this on.

If you have not done so, SHUT UP.

We admit that you are right if somebody using
a Canon 30D is willing to allow 2/3 stop more "slop"
than is really necessary.


Doug McDonald
From: Paul Furman on
Floyd L. Davidson wrote:
> Paul Furman <paul-@-edgehill.net> wrote:
>> Floyd L. Davidson wrote:
>>> That is exactly the point I *am* making. WB changes
>>> the histogram in a useful way. The Luminous-Landscape
>>> article did not talk about WB at all. They discussed
>>> changing the camera's setting for contrast, and that
>>> simply does *nothing* useful for the histogram/exposure
>>> issue. And if you doubt that the contrast setting will not
>>> change the histogram as stated, *try* *it*. I gave an step by
>>> step description of a very easy way to show exactly what
>>> does and does not happen. Why argue from supposition when
>>> you could actually learn something about photography...
>>
>> I am interested in learning. I tried fiddling with the camera contrast
>> settings to compare histograms and the difference was very minor, at
>> least for the conditions I tried just now.

A second try showed a significant difference.


>> I was thinking of
>> post-processing where increasing contrast blows highlights & blocks
>> shadows. Maybe not the very final edge of 100% saturation but contrast
>> adjustments can make a big difference in what appears to be blown or
>> blocked. What you are describing sounds more like a gamma adjustment
>> than contrast where only the middle part shifts.

Scratch that comment, they both have an effect.


> "Gamma is equivalent to contrast.

I dunno, maybe your linked definers are thinking of a different context.
Common sense and experience tell me gamma is adjusting the middle-value,
more like brightness, without changing the the ends. Contrast
adjustments tend to hold the mid-point and use an s curve to raise
highlights & lower shadows or the inverse. A gamma adjustment curve is a
fairly simple bowed shape, not an s shape. Sure a gamma adjustment has
an effect on contrast but that's a side effect, not the goal. Gamma
adjustments boost contrast in either the highlights or shadows, not both.


> This can be observed
> in traditional film curves, which are displayed on
> logarithmic scales (typically, density (log10(absorbed
> light) vs. log10(exposure)). Gamma is the average
> slope of this curve (excluding the "toe" and
> "shoulder" regions near the ends of the curve), i.e.,
> the contrast."
> http://www.imatest.com/docs/glossary.html
> (The above is a web page owned by Norman Koren.)
>
> "Gamma -- A way of representing the contrast of an
> image, shown as the slope of a curve showing tones
> from white to black."
> http://www.hp.com/united-states/consumer/digital_photography/articles/scanner_glossary.html
>
> That might help make things clearer as to why changing
> contrast does not make the histogram any more or less
> accurate for exposure.
>
> When the histogram indicates precisely the correct
> exposure, changing the gamma (contrast) moves the curve
> between black and maximum white values, but it doesn't
> change the value of either. Note too that it might
> spread the histogram out from the center (if the gamma
> curve is moved by picking a point at its center), or it
> might move towards ether the right or the left (if the
> gamma curve is moved from point closer to the ends
> instead of at the center).
>
> In any case, it changes the contrast but if there are
> image values at the maximum white value they will not be
> moved.

The maximum white value may be already off the edge so you are talking
about part of the middle of the curve, which is effected, showing more
detail with low contrast.


>> An interesting related issue I don't understand is how the exposure
>> slider works in Lightroom or ACR. I don't know how to duplicate that
>> effect in photoshop with curves, levels, etc. Those all do like you
>> describe, moving the middle parts of the histogram but there isn't an
>> easy way I can see to shift the whole exposure. Hmm, the middle slider
>> on levels comes close but still doesn't match the effect.
>
> I don't use those programs so I can only provide general
> instructions about how that is typically done. I'll
> leave it to someone who can be more specific.

Understanding this would help in simulating exposure changes in an
editor - to better understand when shooting & interpreting the histogram.

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam
From: Floyd L. Davidson on
"DRS" <drs(a)removethis.ihug.com.au> wrote:
>"Floyd L. Davidson" <floyd(a)apaflo.com> wrote in message
>news:87ab02ql1h.fld(a)apaflo.com
>> "DRS" <drs(a)removethis.ihug.com.au> wrote:
>
>[...]
>
>>> I have always said it changes the indication of exposure. It is you
>>> who made the false claim that I and others were saying it had
>>> anything to do with changing the actual exposure, which we have
>>> repeatedly denied.
>>
>> *All* *five* *show* *the* *exact* *same* *exposure*.
>
>Yes, how many times do I have to tell you that before you get it?
>
>> Not one of them shows any 1 f/stop difference as you
>> claim.
>
>Yes, they do, in terms of *accuracy*.

You simply do not know how to read a histogram, and until you
learn there is little point in talking to you on any topic
that requires it.

>> The only thing that moves 1 f/stop are the tall peaks in
>> the graph that have *nothing* to do with exposure. It
>> is well to the left of the right edge, and it is that
>> right edge that indicates exposure.
>
>The right edge moves.

It doesn't. You are *not* looking at the right edge, you are
looking at a peak that is left of the edge. The peak moves, the
edge does not.

>>> And because of the different contrast settings the 5 histograms show
>>> the exposure differently. The exposure hasn't changed but the
>>> accuracy of its representation has. Which is all that has been
>>> claimed.
>>
>> They *don't* show the *exposure* differently.
>
>Yes, they do. Everybody can see it except you.

Everyone who knows how to read a histogram can see the same
thing that I do.

>>> None of the 5 histograms indicate overexposure. What they do
>>> indicate, to different degrees of accuracy, is the room for exposure
>>> compensation. Only 1 can be the most accurate and it is the -4
>>> histogram.
>>
>> All of them indicate there is no room for any
>> "compensation". Increasing the camera exposure will
>> result in clipping, and each of those histograms shows
>> that.
>
>No, they don't. Try looking at what is there.
>
>[...]
>
>>> In this instance that indicates specular highlights, which as has
>>> been noted by several people, may be blown without spoiling the
>>> image. That is a choice by the photographer.
>>
>> The tall peaks are *not* a "specular highlights", and
>
>I didn't say they were. Do you ever pay attention? You can't even follow
>when I'm addressing your argument!

There are *no* specular hightlights in the images shown.

>> You know, logically if what you said made sense the
>> article would have discussed it in some way, but it does
>> not even hint at it. If changing the contrast displayed
>> a more accurate *exposure* (right edge location), why
>> didn't they point to it?
>
>"What does all this mean? In order to display the full dynamic range of
>your camera in the histogram displayed on the back, you must set the
>contrast to minimum. You now can see the same dynamic range in the
>histogram on the camera as you will see it in your RAW processor on your
>computer."

Nice switcheroo, but the article that shows the histograms says
nothing like that. Here is what it says, in it's entirety:

"Contrast adjustment

Adjusting the tone alters the shape of the 'S curve'
used to map the linear image data captured by the
sensor into the correct gamma. A lower contrast
setting maintains more of the original data's dynamic
range but leads to a flatter looking image. A higher
contrast setting stretches the grayscale (dark to
light) of the image and could lead to clipping of both
shadow detail and highlights. The Neutral Picture
Style was used for the samples below."

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com