In Photoshop's bag of tricks, there are a pair of functions. They are called ‘Dodge’ and ‘Burn’
These two functions have the effect of lightening of darkening the elected part of the image respectively.
But where did these functions get their start?
These two functions have the effect of lightening of darkening the elected part of the image respectively. They are great tools if used gently, and can dramatically improve a photo.
To ‘Burn’ a photo, first, one would take a photo normally, and then cover parts of it and re-expose the unobscured area to darken only that part. The longer the exposure, the darker the exposed portion will become.
‘Dodging’ is a similar process. A card or other object is held between the enlarger lens(a tool for developing film negatives into photograph prints) and the photographic paper. This creates a lighter effect in parts of the photo but can lead to yellowing if overdone.
Both of these techniques were used with stencils to add lettering or other designs to film during production.
If applied more gingerly, they can add light to a too-dark photograph, or darken down a part that is too bright. The ability to add in contrast during post-production was and is a book to the photographers' art.
In Photoshop, these effects are far easier to achieve with the click of a button. It is important to remember that they had their start in film.