Histogram question.

jwbryson1

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Can a histogram be used to determine how far your image is underexposed or overexposed?

For example, just from looking at the image of the histogram can you ascertain that your photo is 1 stop underexposed or 2/3 stop overexposed, etc?

Or do you just use the histogram to determine how close you are to both extremes (clipped or blown), and make appropriate adjustments from there?

Thanks!
 

amolitor

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Sure, why not?

Find yourself a white wall. Or a grey card, whatever.

Take a picture at the exposure the camera says, and look at the histogram. Now dial in -1 stop of exposure compensation, or underexpose by one stop. Take another picture and look at the histogram. Repeat for various +/- exposures. The histogram should be pretty much a single spike in each case. This should give you a notion of what +/- a stop does to the histogram.

On my camera the spike doesn't quite march up and down in even increments, it seems to slow down as it gets to the bottom of the range, at least. This is to be expected, there's probably some sort of curve involved. The technical details don't interest me. Anyways, to me, on my camera, it looks like if I want the histogram to move something like 15% of the display left or right, that's about a stop, roughly.

Inspecting the results of this experiment in more or less detail will give you as much or as little intuition into how the histogram relates to stops of exposure as you care to have.
 

unpopular

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It can, but typically the scale isn't there on the histogram. Raw Photo Processor is the only one I am aware of that has comprehensive exposure data:

$Screen Shot 2013-07-08 at 8.05.51 AM copy.jpg

Here, the roman numbers represent Zone scale, the numbers at top represent greyscale in percentage, the green/magenta ticks represent (i think) EV scale. The grey line represents the referenced point in the image, which I had fixed to middle grey and the orange numbers represent stops from that reference to the corresponding zone. The reason why it's biased by +0.3 EV is because the reference was initially 1/3 stop above Zone V.

The main problem is that the greyscale is linear while the EV scale is logarithmic.
 
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480sparky

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Once a pixel is underexposed to the point it reaches pure black (0,0,0) or pure white (255,255,255), then any further change in exposure will not change where it shows up on the histogram.
 

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As you shoot, the histogram can tell you how under or overexposed you are. This is why I like it better than blinkies. I can not only see whether I'm blown out but by how much, or I can see how much room I have before I clip. I think it's better than the blinkies, which tell you only that you've clipped, not by how much. I don't use it for every shot but a quick glance is usually all I need to make a quick adjustment for perfect exposure.
 

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I dunno where this stupid baby is. I am beginning to suspect that my wife just put on a lot of weight and learned some incredible abdominal muscle control somewhere. We're 5 days overdue, and everyone's pretty sick of waiting by now. The 3 year old is going BONKERS because she has presents she gets to open "as soon as baby sister gets here".
 

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Once a pixel is underexposed to the point it reaches pure black (0,0,0) or pure white (255,255,255), then any further change in exposure will not change where it shows up on the histogram.
This. Once it's clipped, it's past the point of no return, and you can't tell how many stops over or under it is by looking at the histogram.

Try this experiment: Take a photo with the lens cover on. Look at the histogram. Now, using ONLY the histogram, determine how many stops underexposed it is. Now change the shutter speed and do it again. Compare the two histograms, and using ONLY them, determine how many stops apart they are from one another.
 

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I use my histogram a lot. My main goal with the histogram is to see it pushed to the very far right without blowing my highlights. (when shooting people) I use the histogram and I use my blinkies, but when I shoot people, or mainly skin, I usually overexpose 1/3-1 stop anyways to ensure proper exposure. When I shoot everything else, that doesn't involve skin, I will keep my histogram more toward the middle and I won't compensate my exposure.
 

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Can a histogram be used to determine how far your image is underexposed or overexposed?

For example, just from looking at the image of the histogram can you ascertain that your photo is 1 stop underexposed or 2/3 stop overexposed, etc?

Or do you just use the histogram to determine how close you are to both extremes (clipped or blown), and make appropriate adjustments from there?

Thanks!

Both.
The way I teach my DSLR class is that we mainly use the histogram for two things. The first is to check for clipping and the second is for verifying exposure. Although, I don't take it to the point of being able to calculate just how far the exposure is off.

I break down the histogram into 6 zones; black, dark dark, middle dark, middle bright, bright & white.
So with a little bit of experience, someone should be able to judge the brightness/reflectivity of something, and thus guess where it should show up in the histogram when properly exposed. For example, if you shoot a high key shot, with lots of bright background and bright clothing...then you should know that a properly exposed photo will produce a histogram that is heavily biased to the right.

Once the students have that concept down...I will often add in the 'Expose To The Right' principle.
 

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