Understanding the Histogram.

[Fogey]

How many of us fully understand the histogram? I didn't until I was given this explanation by a zoologist/semi professional photographer.

We've all heard the expression of a 32 or 64 bit computer operating system; bits and bytes, pixels and megapixels; Yes? Well all these are based on the binary numerical system of 1,2,4,8,16 etc., each number doubling going up or halving when coming down. They equally relate to speed of the computer processor, file size and size of digital transfer.

Well the digital camera shares exactly the same principles. The aperture measurement double and halves when going up and down, as does the shutter speed and ISO - and strangely enough the histogram.

It has to be appreciated that the ISO settings on the camera and the resulting histogram of the shots taken are closely related.
So, to explain.

The histogram gives a pictoral graph of the black and white tones of the photograph, with the optimum, (correct exposure), being 18% grey. On the graph, the left hand side deals with under-exposure, the right, over exposure.

So, the first reading would be one shade of grey, (black), the second reading two shades of grey, the third four shades of grey, and so on, with each subsequent reading doubling until we reach white. Now, remember; the shades of grey on the histogram are derided from the actual colour palette, so it's fair to say that any given scale on the histogram, (ISO setting), will contain a certain percentage of the colour palette chosen for the photograph.

Assuming the camera's settings have been set to show the ISO in 1/3 of a stop, the sequence would be:
100, 125, 160
200, 250, 320
400, 500, 640
800, 1000, 1250
and so on.
Now don't take my word for this; try it for yourself. The 2/3 rd settings, (in green), will give the optimum colour rendition, reducing the colour banding in the skies being the most obvious; also look for a more atmospheric feel to your photography, The next most optimum will be the 1/3rd setting and the interger, the least.

 

[Madison]

When I taught, I called it a population chart. GS is 256 shades of gray, color is 3 (in rgb) 256 gray levels for each color channel. The chart is how many pixels correspond to each density level.

You can’t call left or right of 18% gray as over- or under-exposure— they’re the shades of gray in the image. You move the center histogram slider to change that 18% location, and left and right sliders to move pixels nearby the ends to black or white. Abbreviated for conciseness.

 

[Anton Largiader]

18% is middle because the stops are on an exponential scale, with about four to five stops being considered the range of what we can discern. When you fit that curve to a 100% scale, you get something like this, from -> this page <- with the percentage doubling with every stop:

 

[Infrared Guy]

How many of us fully understand the histogram? I didn't until I was given this explanation by a zoologist/semi professional photographer.

We've all heard the expression of a 32 or 64 bit computer operating system; bits and bytes, pixels and megapixels; Yes? Well all these are based on the binary numerical system of 1,2,4,8,16 etc., each number doubling going up or halving when coming down.

Computer hardware is built on the hexadecimal system. Eight bits make a byte. Binary is only 0 or 1's, no 2,4,8.etc.

They equally relate to speed of the computer processor, file size and size of digital transfer.

???

Well the digital camera shares exactly the same principles. The aperture measurement double and halves when going up and down, as does the shutter speed and ISO - and strangely enough the histogram.

It has to be appreciated that the ISO settings on the camera and the resulting histogram of the shots taken are closely related.
So, to explain.

The histogram gives a pictoral graph of the black and white tones of the photograph, with the optimum, (correct exposure), being 18% grey. On the graph, the left hand side deals with under-exposure, the right, over exposure.

I think it would be more accurate to say the left of 18% are shadows or properly exposed dark objects. To the right would be highlights.

So, the first reading would be one shade of grey, (black), the second reading two shades of grey, the third four shades of grey, and so on, with each subsequent reading doubling until we reach white.

I don't understand this. What you seem to be saying is the histogram is a static display regardless of the image (which it is not). Would the fourth be 8 shades?

Now, remember; the shades of grey on the histogram are derided from the actual colour palette,

I'm missing what you 're saying here too. The color palette is a set range of color standards. I can choose sRGB or Adobe RGB on my Nikon. Any picture I take will only use some of the possible colors in that range. Any color photographed outside that palette will be assigned a color closest in the color palette the camera is using.

so it's fair to say that any given scale on the histogram, (ISO setting),

The histogram takes aperture and shutter speed into consideration the same as ISO.

will contain a certain percentage of the colour palette chosen for the photograph.

I'm guessing that each level would contain 100% of what is in the photo for that range.

Assuming the camera's settings have been set to show the ISO in 1/3 of a stop, the sequence would be:
100, 125, 160
200, 250, 320
400, 500, 640
800, 1000, 1250
and so on.
Now don't take my word for this; try it for yourself. The 2/3 rd settings, (in green), will give the optimum colour rendition, reducing the colour banding in the skies being the most obvious; also look for a more atmospheric feel to your photography, The next most optimum will be the 1/3rd setting and the interger, the least.

 

[Infrared Guy]

This guy does a pretty good explanation of the histogram.

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[Madison]

All interesting information. Thanks

 

[Fogey]

@ infrared guy: I also found this concept very difficult to get my mind around. Yes the the fourth would be eight shades, the 5th 16 shades and so on. The confusion arises if you take the 256 shades of white in the histogram as gospel. Certainly, on my Nikon camera the ISO light sensitivity goes as high as 12,800, but on the histogram equates to 256 shades of white as does the Nikon D560 at 25,800, anything above that is regarded as clipping.

What I'm suggesting is to use the histogram proactively; by setting the ISO to suit the general lighting on the day, to the figures posted in green, will result in a better colour rendition of the finished photograph.

I'll repeat - don't take my word for it - try it and see for yourself.

 

[Cap'n Fishy]

There are 10 types of people in the world. Those who understand binary, and those who don't.