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Live Histogram

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Offline snapshot

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Live Histogram
« on: 05 / June / 2008, 15:10:34 »
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I've been having fun with CHDK and AllBest 50.  When taking pictures though, I've noticed that the live histogram is totally different from the histogram displayed with the picture on playback mode.  I put the camera on a tripod, noted the live histogram, hit the shutter, then checked the picture in playback mode, totally different.  I changed the live histogram parameters>Histogram layout to different modes -RGB, Y, etc, doesn't matter.  How useful is this if I can't trust the information it's giving me?  Shouldn't they be identical?  Sorry I don't know how to show thumbnail images from my camera display.

Thanks for any help!

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Offline cybercom

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Re: Live Histogram
« Reply #1 on: 05 / June / 2008, 15:31:08 »
While I'm really just guessing here, I think it might have something to do with the jpeg compression.

Before you take the picture, you are looking at unprocessed input, afterwards, you are looking at what's actually on the card, e.g., finished output. Somewhere in-between, the camera made decisions.

If you ever had a cassette recorder, the parallel would be the difference between listening to the input signal - clean, noisefree and very wide dynamic range - and listening to that same signal played back. :o
« Last Edit: 05 / June / 2008, 15:37:04 by cybercom »
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Offline snapshot

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Re: Live Histogram
« Reply #2 on: 05 / June / 2008, 16:02:07 »
That makes sense.  So I guess I could use the live histogram as a guide, knowing that the camera will compress the data.  I remember noticing the dynamic range was always shorter in the playback histogram than the live histogram.  Maybe it's more accurate or relevant when shooting RAW.

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Offline cybercom

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Re: Live Histogram
« Reply #3 on: 05 / June / 2008, 16:21:47 »
Quote from: snapshot on 05 / June / 2008, 16:02:07
I remember noticing the dynamic range was always shorter in the playback histogram than the live histogram. 

That would support the theory....

Quote from: snapshot on 05 / June / 2008, 16:02:07
Maybe it's more accurate or relevant when shooting RAW.


Time for more testing... ::)

However, what's apparent is that the Histogram is still useful because, if you can place your values where you want them before you shoot, you know they'll be within the smaller dynamic range of the jpg.
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Offline jeff666

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Re: Live Histogram
« Reply #4 on: 06 / June / 2008, 09:40:10 »
Quote from: cybercom on 05 / June / 2008, 15:31:08
Before you take the picture, you are looking at unprocessed input

Actually, that's not true. The histogram, as well as the zebra, take the live preview as source of information. This way it has way less data than the processed image. It's still good enough to be useful.

Cheers.
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Offline cybercom

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Re: Live Histogram
« Reply #5 on: 06 / June / 2008, 11:07:27 »
What is the "live preview?"

Where does it fit into the Optical/Electronic image creation chain?

My understanding is that optical components (lens elements, shutter, aperture, ND filter) create the photonic data presented to the electronic components (CCD or CMOS imaging chip, control, scanning, clocking, decoding, compression, etc. circuitry) that create the file structure that is ultimately recorded, as well as outputting data to be decoded as imagery for EFT/display.

Can you give some specifics about "live preview" to help us understand the differences in the histograms?


TIA,
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Offline wontolla

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Re: Live Histogram
« Reply #6 on: 06 / June / 2008, 12:08:30 »
I think he means it takes the info from the display. Just like motion detection.
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Offline jeff666

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Re: Live Histogram
« Reply #7 on: 06 / June / 2008, 14:19:04 »
Quote from: wontolla on 06 / June / 2008, 12:08:30
I think he means it takes the info from the display. Just like motion detection.

You're right.

Cheers.
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Offline cybercom

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Re: Live Histogram
« Reply #8 on: 06 / June / 2008, 15:45:57 »
Quote from: jeff666 on 06 / June / 2008, 14:19:04
Quote from: wontolla on 06 / June / 2008, 12:08:30
I think he means it takes the info from the display. Just like motion detection.

You're right.

Cheers.

And therefore, the histogram is also taking it's info from the display when an image is viewed in Playback mode, so any differences in the histograms would be solely attributable to the jpg compression...

Okay! That's good to know. Gives a better understanding of what the Histogram is showing. And eliminates even more variables. 8)
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Offline DataGhost

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Re: Live Histogram
« Reply #9 on: 06 / June / 2008, 17:05:32 »
Uhh... at least on the S5, in review mode the viewport is 'locked' to the last image on it. I don't know if it's possible to have the CHDK histogram on in review mode (never used that feature) but if it is, all you'll see is the histogram data for the specific thing you were pointing at when going into review mode. It'll probably be the same for all other pictures, right?
As for possible other differences: since the data is taken from the viewport (active display) this also means it's affected by exposure compensation the camera might apply in order to focus better or get more light into the picture. When using auto mode, it's generally WYSIWYG, but if you override stuff you can't really use the histogram unless the particular override is active (and influences the display) before shooting.
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