How the brain "sees"

Many people wonder why we need to do things like HDR [ high dynamic range] to our pictures to make them look ''right'' .
As a side issue there are many overdone versions of HDR images which gives the technique a bad name but the main idea here is having an image that is easy to look at where the shadows and highlights both look the same way they would to the human eye .
Actually it is more like the way the image looks to the human brain because the brain processes the  information that enters your eyes much like the way the camera's built in computer processes images that fall on the sensor ..... except it can change the iso in different areas of the " brain sensor "- at the same time - in one image .
Why does this image look relaxing ?

The answer is in the question .....
"Relaxing" means not doing any work . The lighting is pretty even so your brain doesn't have to do any work when you look at it ! It just happened to be good lighting as the sun was going down between the clouds and the image needed no extra editing because everything looked right .... you don't often get that type of lighting !
But with some images there are very dark shadows and bright highlights that your brain has to process which is hard work , the opposite of relaxing - more so in real life than when looking at a screen .....
In the late afternoon when you have your back to the sun everything looks relaxing because the light is even and your brain doesn't have to do any processing ... it can just relax while you enjoy the scene .

When you turn and look to the side where there are a lot of shadows
it suddenly has to work harder so you can see detail in the shadow areas and sunlit areas .... not so relaxing ! 

This is even harder [easier?] to understand when you are taking pictures and the results don't look the same as it looked "to your eyes" .

The main thing to remember is that your brain is doing the 'seeing' , your eyes are just transmitting the image to your brain to process . Apparently our eyes can adjust aperture from about 1mm to 9mm and our eyes' 'shutter speed' is around 1/30th second - from what I have read - besides that all I really only know and care about is what I see when comparing images to what a scene looks like ... such as in this example ...

This is what the view looked like to me when I looked out the kitchen window today , this is two images combined .....

 But interestingly when I look away from the scene and blink I see something like this .....

This is because my brain was darkening the centre area and brightening the area around it so that I could see the two different scenes "correctly" .
What two scenes ? 
First there is the outside scene .... I had to set the shutter speed to 1/250th to capture all the detail my eye was seeing .

But it didn't look like that to me on the inside ? I had been sitting at the computer and the inside looked much brighter to me [ to my brain actually ] .
To get the inside looking as it did to my brain I had to drop the shutter speed to 1/5th ...... 50 X slower than 1/250th !

So what has happened to the outdoor scene now ? Well actually that is exactly how it should look in comparison to the inside of the room because it is 50X brighter than the light indoors .... but my brain won't accept that , it has decided that I can see both scenes as it has determined they should look and has adjusted two different areas of my "brain sensor" to accommodate this . Since the eye/brain is limited to about 1/30th second and it can only select one aperture for any one scene it has only one setting left that it can change ... iso .
That's why when I blinked I saw an image of the window , my brains sensor had modified the centre of the scene to darken it .... it had set the centre of the frame to 'iso 100' and the outer part of it to 'iso 5000' ,  which is why blinking showed an image of the bright window , my brain sensor had two different iso settings at once ! 
[ These are made up iso's by the way ;) ]

Because it was such a drastic difference it would take a while before the entire area of my 'brain sensor' was reset back to an even pattern .
This is why people do 'HDR' images , an advanced process of the quick-and-nasty combined image I did in the beginning to show how it looked to my 'brain' ....

On a side note if you are not into shooting multiple scenes with a tripod and editing them later you can fix the problem in one shot with a flash . I shot the same scene again a little later in the day but this time I exposed for the outside scene and used bounce flash off the ceiling to light the inside .... apart from the tacky light fall-off due to the angle I was at this is the way to expose for indoor scenes when you specifically want the outside to look 'right' as well .

After taking these shots I went for a walk in the hills on the farm , halfway up I took this shot  ..... this is how the shadow area looked to my eyes but the background is blown out

And this is how the background looked to my eyes ...notice we are back at that 1/250th that I ended up with inside the kitchen !

 
 So in this scene the sunlit area is 4X brighter than the shadow area according to how my brain sees it . If I had exposed the shadow area properly it may have been a lot more but my brain told me that since it was shadow area it should look a bit darker than the background [ this needs to be remembered when processing an HDR image for it to look natural ].
So what was lighting the shadow area so that it was brighter than the 1/5th of a second exposure inside my house ?
Well , when I turn around and take a picture I get this light source ....

It is getting its light from the white clouds and blue sky behind me .
This should also make it easier to understand why you need a 'shade' wb setting , a lot of the light in shade areas is blue light coming from the sky which needs to be canceled out by adding a yellow cast that cancels out the blue of the sky ... this is another aspect of how the brains sees ... it can make adjustments for colour casts that your camera actually shows accurately but that is a whole different topic :) .