Histogram - what for?

hello,

I've never really understood what purpose the histogram in the upper left corner serves. Could someone give me a good explanation why I need to use this, and how? Thanks.

-Al.

Hi,

Histogram is a graph that shows the distribution of light within your photos and therefore a very useful tool to verify whether your photos are well exposed or not, independently of whether you are using a profiled monitor/screen or not.

The left side of a histogram represents the darker part/tones of a photo. The middle represents the middte tones and the right side the lighter tones.

Generally a well exposed photo would have a histogram, which looks like this. The light is well distributed within the whole range of the histogram.



Underexposed looks like this. The darker/left part is very dominant.



And overexposed - the right/light part is dominant, whereas the dark part is non-existent.



Of course this is only a general rules. There are photos, in which the histogram is very right oriented but are still well exposed, e.g. high key photos, or photos which mostly contains light color and via versa -> low key photos.

And what about the IRGB function in the Histogram window? what additional information can I gain from that compared to the regular Histogram explained above?

cheers,
PhotoWebber

The IRGB-Tab shows you all three channels (red, green and blue) at the same time with different colors.

OK. Thanks. And is there also a rule of thumb how the IRGB-Histogram should look like for a good picture?
Above was explained, that the Histogram of a well exposed photo should look similar to a sinus curve. But what if I shoot a picture with lots of blue sky and a blue sea. There will be probably only few green and red parts in the IRGB-Histogram, i.d. no sinus curve distribution for all three colors but only for the blue part, right?

cheers,
PhotoWebber

Hi, this could be the case. But there is no real general rule for this, as it depends on the color mixture. At least I do not know any

Most colors consist of RBG. So in a dominant blue photo, unless it is pure blue, you will still find more green and red then you might expect. So more probably is that either all 3 channels have a sinus curve, but the amplitude of the blue one is signicantly higher than the other's or they are just differently distributed, e.g. the dark part is dominated by red and green, while the light part by blue...etc...

Best regards

Hoang-Tran

he etymology of the word histogram is uncertain. Sometimes it is said to be derived from the Greek histos 'anything set upright' (as the masts of a ship, the bar of a loom, or the vertical bars of a histogram); and gramma 'drawing, record, writing'. It is also said that Karl Pearson, who introduced the term in 1895, derived the name from "historical diagram"

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Hello

a histogram is a graphical representation, showing a visual impression of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson . A histogram consists of tabular frequencies, shown as adjacent rectangles, erected over discrete intervals (bins), with an area equal to the frequency of the observations in the interval. The height of a rectangle is also equal to the frequency density of the interval, i.e., the frequency divided by the width of the interval. The total area of the histogram is equal to the number of data.


Thanks

The Histogram shows the total tonal distribution in the image. It's a barchart of the count of pixels of every tone of gray that occurs in the image. It helps us analyze, and more importantly, correct the contrast of the image. Yes, photographers know that color prints must first have the exposure and contrast set right, exactly the same as for B&W, and only then do you worry about the color balance.

Technically, the histogram maps Luminance, which is defined from the way the human eye perceives the brightness of different colors. For example, our eyes are most sensitive to green, we see green as being brighter than we see blue. Luminance weighs the effect of this to indicate the actual perceived brightness of the image pixels due to the color components. The world won't end if you simply think of luminance as brightness, that's actually quite fine for our purpose
Every pixel in the Color or Gray image computes to a Luminance value between 0 and 255. The Histogram graphs the pixel count of every possible value of Luminance, or brightness if it helps to think of it that way. Luminance is brightness the same way the human eye sees it, as opposed to absolute brightness. Anyway, the total tonal range of a pixel's 8 bit tone value is 0..255, where 0 is the blackest black at the left end, and 255 is the whitest white at the right end. The height of each vertical bar in the histogram simply shows how many image pixels have luminance value of 0, and how many pixels have luminance value 1, and 2, and 3, etc, all the way to 255 at the right end.

Thanks