Another expression that is used in the art community is alla prima. Alla prima means "in one sitting". Just as many plein air works are created alla prima as their are paintings completed by going back to the same spot under the same working conditions until the painting is complete. The light changes considerably every two hours, so unless you can complete a painting in that length of time, you probably will need to finish the painting another day, or take some reference pictures along the way and finish the work in the studio. Most plein air painters believe that an authentic plein-air painting is done live, on location in front of the subject. It is begun on site and substantially completed while experiencing the landscape firsthand. More than one session may be required to complete it, and each of these should also be on location. Some minor adjustments can be done back in the studio, but not so much as to radically alter the work done on location.

The perception of the plein-air subject is dependent on the conditions encountered while on site, and the artist's response to these is usually evident in the finished painting. Plein-air paintings that follow this standard tend to have a quality that reflects the immediacy of the artist's interaction with the subject.

Artists have long painted outdoors, but in the mid-1800s working in natural light became particularly important to the Barbizon school and Impressionism. The popularity of painting en plein air increased with introduction in the 1870s of paints in tubes (resembling modern toothpaste tubes). Previously, each painter made their own paints by grinding and mixing dry pigment powders with linseed oil. The Newlyn School in England is considered another major location of such painting in the latter 19th century.

It was during this period that "Box Easel" was invented - typically known as the French Box Easel. No one knows for sure who developed it first but these highly portable easels with telescoping legs, built in paint box, and pallette made treks into the forest and up the hillsides easier. Still made today, they remain a popular choice even for home use since they fold up into something the size of a brief case and thus store away quite easily. I paint on location using a pochade box which I attach to a camera tripod. The pochade box is like the French Easel, but smaller and can hold canvas panels like a drying box so the work can be safely transported while wet and kept in the box if necessary until dry and ready to frame.

French impressionist painters such as Claude Monet, Camille Pissarro, and Pierre-Auguste Renoir advocated en plein air painting, and much of their work was done outdoors. American Impressionists, too, such as of the Old Lyme school, were avid painters en plein air. In the second half of 19th century and beginning of the 20th century in Russia, painters such as Vasily Polenov, Isaac Levitan, Valentin Serov, K.A. Korovin and I.E. Grabar were known for painting en plein air. The popularity of outdoor painting has remained constant throughout the 20th century and into the 21st century. Many organizations activily paint together. A great resouce to find these organizations can be found at the National Academy of Plein Air Painters.