If you look at a Daguerreotype, you can realize the plate was present in the camera, and the light reflected off of the subject onto the plate.
The photograph was literally there.
In 1839, photography began to take over the task that painting had had until then, representing reality.
Later, the invention of the tintype process made the representation of reality affordable to all.
Photography allowed painting to go off and do other things, such as impressionism, abstract expressionism, and so forth.
Picasso said, to Brassai:
When you see what you can express through photography, you realize all the things that can no longer be the objective of painting.
Why should the artist persist in treating subjects that can be established so clearly with the lens of a camera?
It would be absurd, wouldn't it?
Photography has arrived at a point where it is capable of liberating painting from all literature, from the anecdote, and even from the subject.
In any case, a certain aspect of the subject now belongs to the domain of photography.
So shouldn't painters profit from their newly acquired liberty, and make use of it to do other things?1
Even before Photoshop, photographs were manipulated.
Manipulation can carry an insidious charge.
However, it can be done to make up for technical deficiencies.
Gustave Le Gray, in the 1850s, in photographs such as Mediterranean at Sète (1856-59), combined two negatives to make his seascape photographs.
He used a negative of the sky, and one of the sea, because a single negative could not properly expose both areas of the scene at the same time.2
Manipulation can also be done to improve a photograph, without changing its meaning.
Manipulation can be insidious, of course.
There's a photograph of Hitler with several other Nazi officials.
A few months later, the same photograph was published, with one of the officials expunged.
This was done mechanically, with some effort.
Even a photograph that has not been manipulated may be untruthful.
The absent truth may be in the context outside the frame that's not depicted, or in a context that cannot be depicted visually.
The untruth may be in what accompanies a photograph, such as a caption, text, and other photographs.
Today, even though we know that photographs can be manipulated, we still have a feeling of authenticity when looking at a photograph.
Diane Arbus wrote:
A photograph is a secret about a secret.
The more it tells you the less you know.
1 Brassai. (1966). Picasso and company (F. Price, Trans.). New York: Doubleday & Co. Reprinted in Goldberg, V. (1981). Photography in print: Essays from 1816 to present. New York: Simon and Schuster.
2 Marien, M. W. (2002). Photography: A cultural history. New York: Harry M. Abrams.