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Old November 6th, 2008, 04:00 PM   #1
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Histogram, Shmistogram

Can anyone point me in the right direction to learn about the histogram? Or perhaps explain what it should look like?

It's one of those useful-looking tools that I don't know how to use.
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Old November 6th, 2008, 04:11 PM   #2
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They're one of those things that is useful...sometimes.

On average, you want the histogram to have the full range between 0-255. 16-235 if you're going for Broadcast save specs. It should normally look like a bell curve. Again, this is in a well exposed, average scene.

The problem is that a lot of what you encounter and the look your after won't work this way. Sunsets, night shots, moody effects, bright sand or snow all make the previous paragraph moot. Your histograph won't look anything like "normal". And that's fine because your after a different look.

I've used it more over the years with Photoshop than I do for video. I'm able to expand the tonal range of an image greater that how it was exposed by using curves to either bring up the shadow areas or darken over exposed areas.

Again, I just see it as another tool to see how your image is exposed. Useful for some situations but not all.

Hope that helps some!
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Old November 6th, 2008, 04:17 PM   #3
 
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A histogram is, basically, a bar chart. In the exposure histogram, the number of occurrences of a particular exposure level(frequency of each level) are plotted on the y-axis and the particular exposure level is plotted on the x-axis. So, low exposures levels are on the left side and high exposure values are on the right side.

In statistical analysis, the "bell curve" is talked about, which implies a distribution of levels such that a few appear at each end, but, most of the population is in the center. In the ideal case, exposure levels in the scene will demonstrate a bell curve distribution, if the scene is properly exposed. However, scenes are rarely perfectly distributed. For example, a frame that shows nothing but sky will skew the histogram so highest population of exposure values are on the right. The opposite for a scene that contains all shadows.

The most outstanding feature of the histogram is that neither the left end, nor the right end, should extend into the edges of the plot, or it shows data is being lost. In a scene of high dynamic range, i.e. a scene where exposure values run the gamut from full black to full white, a camera sensor probably can't record the whole range, so, one can't help but have one end extend off the scale. The trick then becomes one of how to best capture the image so that the least significant data is not recorded.

The last hooker to this whole story is that exposure histograms come in several varieties. Still camera histograms started out by showing all one black, shades of grey, and white histogram. It wasn't long before someone figured out that it was possible to show all values in range on the B&W histogram, but, still have one color, usually blue, exceed the range of the scale. More developed histograms now show 3 or 4 channels, corresponding to R,G,B and B&W.
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Old November 6th, 2008, 04:48 PM   #4
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Thanks, guys, this really helps. I appreciate the responses.
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Old November 6th, 2008, 05:28 PM   #5
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yes , histograme shows where you data are, not if this is nice or not.
imagine the histogram like a square.
each border of the square (bottom, left , right, top) is a limit.
bottom is no signal, top is too much signal (clipping, burned zone)
left is dark , right is bright.
what you want is always fill the square as much as possible without touching borders.

For example if you got a lot of value (high colum) on the right (bright) side, you better should set your zebra on because you probably have overexposed zone.
If you got all the data filling with high columns the left and the right side of you histogram, with almost nothing in the middle, you got crushed blacks, and over exposed white (typical when filming somebody in front of a sunny window)
here there is not much you can do with the camera. you have to bring light in the dark space of your scene and diffuse the bright light.

now you can choose to move you data in the histogram depending your plan.
if you shoot something dark and plan to make it lighter in post, push all data to the left (not too much, but stay close as possible as the left border since you will not use the empty space left) , stretch the data so they fill 3/4 of the bottom line.

So you would say: How the hell can i move the data in the histogram.
that is the tricky part. Close the iris, make the scene darker, put a ND filter, increase shutter speed, all of this will make the data move to the left.
adding light, opening iris, lowering shutter speed, removing ND filter push the data to the right.
If you have settings for the blacks and whites you can even influence how the data spread in the histogram.
but basically , this will not change the shape of your curve.
So the even trickier part is not moving the histogram horizontally , but changing the shape of the curve , a challenge that you can solve only by playing with light.
Most of the work is done by filtering/attenuate/diffuse hard light (making bright (right)columns going down and toward center of histogram.
The same for the dark (left) columns by increasing ambient light.
When you make a column lower, it means that another column will get higher.
the problem is that a good balanced histogram could give you a totally dull picture (especially if you like hard constrast) so working with the histogram is nice if you know what ou want.
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Old January 15th, 2009, 03:26 PM   #6
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Can you tell me why my histogram doesn't go all the way to the right

I thought this might be the most appropriate thread for this rather than starting a new one.

I have been noticing that with the Cine 4 setting I'm using that the histogram stops short on the low end and the high end if I have gain of -3 set. If I open the aperture till the picture is blown out, the histogram runs into a wall on the high end. I think this has something to do with Cine 4 or those settings compressing the highlights before the maximum. With some other settings (such as off) the histogram does go all the way to top with -3db gain set.

No matter what I do it seems the histogram always stops short.

Seems to me I'm losing some valuable latitude there is the histogram is an indication of the possible range from low luminosity to high. What am I missing?
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Old January 15th, 2009, 03:40 PM   #7
 
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When the gain is set to -3dB, you don't get the full latitude of the sensor as the upper end of the gamma curve cuts off at about 90%.
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Old January 15th, 2009, 03:50 PM   #8
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Thanks Bill, but if I chose a different picture profile, say "OFF" or another like PP2:TC1 (based on your PP set) and still have the gain set to -3, I get a histogram that goes all the way up. So I think there is some interaction between PP's and the histogram, and sometimes it's related to the PP, but not entirely.

No matter what I can't get the histogram to go all the way to the left though, what's up with that? I feel like I'm missing out on some latitude.
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Old January 15th, 2009, 08:03 PM   #9
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Ted if you have a copy of Photoshop, open up a tonally balanced photo and then open up the "Levels" dialogue box, Command-L is the shortcut on a PC. and have a good look at the historgram. You will see several triangle sliders below the histogram. One for the shadow detail, one for the midrange and the other for the highlights. Yank on the the midtone triangle slider going back and forth. You will be moving the midtones from one end of the tonal spectrum to the other when you do this. In doing this it will give you more of a sense of what the histogram represents. Another thing to do is once you have yanked on the histogram to modify the midtones, click OK and then open up the Levels dialogue again and study how the histogram has changed. Again this should be instructive.
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Old January 15th, 2009, 08:40 PM   #10
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Agreed. Your comments on the histogram are good. I'd like to see a different tonal display - a 2D level meter. It's kinda of like zebra stripes, except that you don't have to worry about setting them to 70%, 95% or 100%. The entire dynamic range would be divided up into 10% bands corresponding to the colors of an audio level meter, various greens, yellows, oranges, and reds. Each pixel on the LCD display would instead be a level meter, so it would be easier to see the exposure of the entire picture without being obscured by moving zebra patterns. I'd even be up for a blinking 2D map. In addition, the histogram could be superimposed on the entire image.
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Old January 15th, 2009, 10:25 PM   #11
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I don't know how to use it either, even after this thread. However I do understand it better. One application though that it seems good for, measuring the "Eveness" of light on a green screen. Am I on the right track here? If so, what would you want it to look like on a green screen?
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Old January 16th, 2009, 03:49 AM   #12
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Brian...

For green screen, use the zebras set at 50 IRE.

Adjust the lighting so the zebras just barely show up. It'll give you a quick indication of what's hot and what's not.
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Old January 16th, 2009, 07:11 AM   #13
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Dean, thanks for the tip!

Does anyone have experience lighting infinite white? I'd imagine that without talent, the histogram will look pretty odd - just one edge at 100%. Where would you set the zebras for this?
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Old January 16th, 2009, 07:18 AM   #14
 
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Keith...

A couple of comments....
1-You can play with the aperture to intentionally under-expose. I would expect this to force the histogram to the left, just to see if the brick wall you're complaining about is real.
2-The histogram Sony provides is only 25% of the story. Better histograms will also show the individual RGB channels. It's possible to over/under expose a scene in only one channel, like Blue, and not have it show up in a composite histogram on the camera. The method of importing into Photoshop, described above, would verify whether or not that's what you're seeing.
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Old January 16th, 2009, 08:01 AM   #15
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What I look for in the histogram is that the bars to the extreme right side are not crashing into the right wall (whites blowing out) and that I do get the left side bars to 'almost' touch the wall (perfect deep black). This therefore gives me the most dynamic range.
As far I know, changing exposure will not give me the above. Yes, the bars all shift, and to avoid clipping whites that's what I do. But gain settings and the gamma settings are where I need to make changes to get the left side bars move close the wall, while retaining the right side.
Unfortunately, as a doc film maker, I rarely get the time to fiddle for 10 minutes under the hood, while the action whizzes by me. So that's where the PP's come into play - to extend the left edge of the bars - and the right, to where I want them. I never worry what's happening in the middle too much. This may be too simple, but it is all I have time for in time-critical situations.
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