Is there a formula for locating a center of interest or focal point?

Answer 1

The Focal Point is what drew your attention to the image in the first place... to make you want to paint it. There is no formula you can use to find it, nor is there one to help you find a place to put it. The "rule of thirds" is a general suggestion, but like all rules, is frequently broken.

In an inspired piece of art, most of the painting is a complimentary backdrop leading your eye toward this point. It's generally NOT dead in the centre of your canvas, but in a location your eye is lead toward by use of technique... either value, pattern, line, colour, texture... etc. When a painting is properly executed, your eye will travel through the picture but keep returning to this point.

Things that distract from the focal point are: a similar colour/value elsewhere in a non-important part of the image, a bold line leading the eye out of the frame, a warm (red/orange/yellow) area in a place where the image should recede (blue/green/violet).

Answer 2

It isn't a good idea to rely on formulas in most cases, but if you use one as a starting point, it may be useful as you discover ways to change it. Take a rectangular piece of paper and draw diagonal lines from one corner to the other so the two lines intersect in the center of the paper. Now draw a line from each corner to one of the intersecting lines to form a right angle with the line. This spot has the ideal qualities for a focal point: It is a different distance from every side of the paper and not too close to the center. Don't lock every center of interest into these exact spots, though. Vary them in as many ways as you can think of. And remember that there are some subjects that work well with the focal point in the center, so don't be afraid to break that rule. Just don't do it by accident!

Another method is to use the photographer's "rule of thirds". Divide the paper in thirds horizontally and vertically and use an intersection of lines as your focal point..