Studying from a original work rather a print or photo has several advantages. Specially if one practices traditional mediums, even though i believe it helps digital painting as well.
First of all, the fact is the original. How many times have i been surprised, looking at the original and think, how much better it is than any reproductions. Some cases i thought of the photo or print to be a improvement on the original in the sense the values being stronger and simpler. Also, the scale of the work being much bigger or much too small than i thought to be(like in this case), affecting perception of detail, color, modeling etc...
Photo and prints cannot(yet) capture color and values the same way the eye can. The eye is far more capable of perceiving values and more subtle color variations. Studying color from works of Georges Seurat would be far more effective when done from life. Or Ivan Shishkin's landscape paintings where one can see texture left from initial brush marks, and its effectiveness of each brush mark, the scale of his work forces one to step back just enough were all of it works together.
One more is laying and texture. From original works, it is possible to see(to some extent) how the artist worked. Which brush marks went on first and which ones on top of that, and break down the sequence in which the artist proceeded. The direction of brush marks and how they blend. Textures like Francis Bacon work can only be 'felt' when seen in life.The brilliancy of the paints and layers of glazing under the light should also be mentioned here, on works of masters such as Van Dyke and Rembrandt.
So, when work from original art works over reproductions? If you have the means, i would say, always! But much depends on what's your purpose for the study.
Bellow are composition studies that i did from reproductions of Edward Seago's work. Looking only at basic values and make it readable in a small scale. And some works done from life were i try to apply the lessons from such studies.
|Value composition studies after Edward Seago|
Cityscape, from life
sketch of man painting, from life