Most of the students who sign up for a painting class with me are surprised to learn that I believe it's easier to paint from life than it is to paint from a photograph. I need to qualify that statement a bit. It's easier if you are a fast painter. Of course you don't become a fast painter until you put in the hours necessary to make quick decisions while you're painting. That is where practice comes in. A photograph is never going to duplicate all the colors our eyes can see. It's a flat rendition of one view of a subject. The human eye, on the other hand, is capable of discerning subtle value changes and luminosity that is quite foreign to the camera lens. A photograph captures the line and the value, while the eye sees the shape and the forms. I enjoy painting from life very much and I take every opportunity to do so. All mys still life subjects are painted form life. It would be very boring for me to take a picture of some "things" and then copy the picture,. The joy arises when I am in the moment and making the decisions about where I want the light and shadow to merge, how much I want to push the color etc. It's hard to explain to artists who have never tried it.
In the warmer months I paint landscapes outdoors: plein air painting. The subject needs to be captured in two hours or less if you're not planning to return another day. This kind of painting forces the artist to make snap decisions about line, form and color. It effectively increases your speed and makes you a better painter. It's great training for painting the human form from life, another very enjoyable but not always easy activity.
Painting from life will train your eyes to look for and find variations in color that artists who depend on photos will never see. After you've put in hundreds of hours directly observing and painting what you see in nature you can use a photograph for reference and safely invent the color, shadows, and highlights you know is missing from it.
Here are a two recent paintings done from life. The woman was a five hour pose and the boy posed for two hours.