What do automatic contrast and levels do?

[lucmv]

I use these two commands — automatic contrast and automatic levels — to improve photos. I usually like the results. Sometimes, not often, they seem to have no effect at all.

Would someone be so kind to explain what exactly those two commands do? No, their names are not self explanatory, at least not to me. I would love to get a deeper insight on what they do.

Anyone?

TIA

[xnview]

'Auto levels' try to expand the histogram of intensity
'Auto contrast' apply brightness/contrast following histogram

[mobi]

Where do I find this auto level/contrast option?

[xnview]

Where do I find this auto level/contrast option?

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