Revisiting the "expose to the right" dogma

[devoman]

"Or in other words, the birghtest F-stop used 2048 values (YES) which represents the righmost one-half of the histogram (NO)."  The right half of the histogram is more than one stop - it's one half of all the stops represented.
[a href=\"index.php?act=findpost&pid=53251\"][{POST_SNAPBACK}][/a]

Thanks, but Just to be sure I understand this, you're saying that the X-axis of the histogram is NOT linear; it is logrithmic.  Is that correct?

[boku]

FACT: the X-axis of the histogram is NOT linear; it is logrithmic.

[Gary Brown]

My point is that the scale (x-axis) on the histogram is linear.  Or at least I assume it is.  [{POST_SNAPBACK}][/a]

The values you see in the histogram (if my understanding is correct, anyway) have been converted from the linear raw values (where doubling or halving the value would be a one-stop difference) to the logarithmic RGB "pixel" values, for lack of a better term.

So, although the histogram x-axis is linear, in terms of RGB values, it's logarithmic, in terms of f-stops.

Somewhat related: Take a look at [a href=\"http://www.normankoren.com/digital_tonality.html#Human_vision]http://www.normankoren.com/digital_tonalit...ml#Human_vision[/url] and scroll down to the table showing "RAW levels in each zone" on the left, and "Gamma = 2.2 levels in each zone" on the right.

[Jonathan Wienke]

With all due respect, I don't think you addressed my question.  Yes, I know that the brightest F-stop uses 2048 values (assuming 12 bits).

My point is that the scale (x-axis) on the histogram is linear.  Or at least I assume it is.[a href=\"index.php?act=findpost&pid=53246\"][{POST_SNAPBACK}][/a]

You know what they say about assumptions...

The histogram is based on an in-camera JPEG conversion of the RAW data, which means that a tone response curve has been applied to the histogram data, and it no longer has a linear gamma. Each of the 5 segments of the displayed histogram represents 1-1.5 stops of exposure depending on the camera and camera settings.

[BernardLanguillier]

The ability to display a linear RGB histogram based on demosacied RAW data would be great to have, since the brightest stop would represent half of the histogram width for more accurate evaluation of the highlights situation.

Unfortunately, and for reasons that I have a hard time understanding, no camera manufacturer that I know of is currently proposing this.

Regards,
Bernard

[devoman]

Thanks, all, for the clarifications and elaborations!

[Jonathan Wienke]

The ability to display a linear RGB histogram based on demosacied RAW data would be great to have, since the brightest stop would represent half of the histogram width for more accurate evaluation of the highlights situation.

Unfortunately, and for reasons that I have a hard time understanding, no camera manufacturer that I know of is currently proposing this.

There really isn't even any need to demosaic/interpolate a RAW histogram. I'd be ecstatic if a camera offered a 16-channel bar graph of the uninterpolated RAW data, where the rightmost bar represented clipped pixels and the remaining bars went down the tonal scale in 1/2 or 1/3-stop increments (set to match the camera's exposure adjustment increment) as they progressed to the left. Make the rightmost bar red, the next one to the left yellow, and the rest green to make it even easier to evaluate. It would be trivial to program and far less CPU intensive than a JPEG-based histogram, and far more useful for evaluating exposure.

Simply adjust exposure until you get a bit of yellow bar but no red bar unless there's some specular highlights you don't mind clipping. And knowing that each exposure adjustment increment would move things one bar to the left or right would make dialing in the perfect exposure trivially easy in most circumstances, especially if the test shot was somewhat underexposed.

[BJL]

Jonathan's post suggests a related wish: a histogram which covers the full RAW highlight gamut, with a marking (flashing bars?) for the highlight clipping limit of the current JPEG conversion settings. And a clear description of where the f-stop levels lie, at least in the manual.

[BernardLanguillier]

Agreed, that would already be great.

Cheers,
Bernard

[bob mccarthy]

Simply adjust exposure until you get a bit of yellow bar but no red bar unless there's some specular highlights you don't mind clipping. And knowing that each exposure adjustment increment would move things one bar to the left or right would make dialing in the perfect exposure trivially easy in most circumstances, especially if the test shot was somewhat underexposed.
[a href=\"index.php?act=findpost&pid=53350\"][{POST_SNAPBACK}][/a]

Isn't that basically what the auto exposure system within the camera does . Or perhaps better, what the exposure system of the camera "should" do!

Bob

[Tim Gray]

Isn't that basically what the auto exposure system within the camera does . Or perhaps better, what the exposure system of the camera "should" do!

Bob
[a href=\"index.php?act=findpost&pid=53452\"][{POST_SNAPBACK}][/a]

I think there's a difference between what can be accomplished by today's autoexposure systems, and what can be accomplished via a live histogram, which is what I think you're describing.  Maybe given what Sony has been able to do with the R1, a live histogram type AE isn't that far off...

What I'd like to be able to to is simply dial in my tolerance of what is clipped and the camera sets the exposure based on that.  So nomally I want 0 clipping - but if there are specular highlights, maybe 2% is ok - so that's what I'd dial in and that's what would  drive the exposure to be captured in the RAW file.

[bob mccarthy]

I think there's a difference between what can be accomplished by today's autoexposure systems, and what can be accomplished via a live histogram, which is what I think you're describing.  Maybe given what Sony has been able to do with the R1, a live histogram type AE isn't that far off...

What I'd like to be able to to is simply dial in my tolerance of what is clipped and the camera sets the exposure based on that.  So nomally I want 0 clipping - but if there are specular highlights, maybe 2% is ok - so that's what I'd dial in and that's what would  drive the exposure to be captured in the RAW file.
[a href=\"index.php?act=findpost&pid=53457\"][{POST_SNAPBACK}][/a]

If we ever get wide DR sensors, then we could dial in contrast range (expansion-contraction) and all be digital Zone system shooters. The only difference would be setting the exposure off the highlight rather than the shadows. With tone mapping, maybe even a shoulder/toe effect could be accomplished

Whew, that would be cool

bob

[Jonathan Wienke]

If we ever get wide DR sensors, then we could dial in contrast range (expansion-contraction) and all be digital Zone system shooters. The only difference would be setting the exposure off the highlight rather than the shadows. With tone mapping, maybe even a shoulder/toe effect could be accomplished

If you shoot RAW, you can already do this with the RAW conversion settings and level/curve adjustments. The future is here.

[bob mccarthy]

If you shoot RAW, you can already do this with the RAW conversion settings and level/curve adjustments. The future is here.
[a href=\"index.php?act=findpost&pid=53475\"][{POST_SNAPBACK}][/a]

It's just with film and development (expansion) I can get 2 to 3 more stops of DR without going through the HDR or equivalent process.

It was just a thought.

Bob

[Andrew Larkin]

The histogram on the 5D is marked into five equal sections with vertical lines.  Each line is one stop of exposure apart.

This makes the histogram linear only in terms of exposure - not in terms of raw sensor values.

Andrew

[Tim Gray]

The histogram on the 5D is marked into five equal sections with vertical lines.  Each line is one stop of exposure apart.

This makes the histogram linear only in terms of exposure - not in terms of raw sensor values.

Andrew
[a href=\"index.php?act=findpost&pid=53485\"][{POST_SNAPBACK}][/a]

I thought the 5d would have somewhere in the neighbourhood of 8 - 9 stops of dynamic range, 5 doesn't seem right.

[Jonathan Wienke]

The 1-stop thing is an approximation at best, and varies depending on camera settings, especially contrast. Remember that histograms are derived from camera JPEGs even when shooting RAW.