To make an image look right in the frame, it needs space. It also needs to be balanced. However, like everything else visual, there's a catch. The human eye won't see a mathematically correct center as correct. Instead, it sees the image as low in the frame or out of balance.
Consequently, the image needs to be optically centered for display. Optical center is a position slightly higher in the frame (or a designed page) than the true center line. However, it must remain an equal distance from the left and right (vertically true).
To calculate optical center, measure the total top-to-bottom length of the frame. Divide this length in half. Divide half by 10. The optical center will be 1/10th of the distance of half the page above true center.
A typical frame may be 16 inches wide by 20 inches high. The optical center would be:
20 / 2 = 10   Frame length divided in half.When measuring this distance for image placement, the image size must be subtracted from the total length and width of the frame. Again, the width is mathematically centered. Therefore, divide the remainder by two and this is the distance to mark the outside edges of the image from the sides of the frame.
10 / 10 = 1   Half the length divided by 10.
10 + 1 = 11   Optical center from the bottom (or 9 from the top).
For our example, the image area is 8x10 inches. The width measurements are:
16 - 8 = 8   Frame width minus image width.To make the image height optically correct, divide the remainder by two. Then add the 1/10th increment to the bottom measurement (or subtract from the top). This will optically center the image within the frame.
8 / 2 = 4   Remainder divided by two equals the distance from frame sides.
20 - 10 = 10   Frame length minus image length.As a result, the sides of the image area are 4 inches from the frame sides while the top of the image is 4 inches from the top of the frame and the bottom of the image is 6 inches from the bottom of the frame. The image is now optically centered in the frame.
10 / 2 = 5   Remainder divided by two equals distance to true center placement.
5 + 1 = 6   True center plus optical center adjustment equals the height to place the image from the bottom (or 4 inches from the top).
To be fair, I chose the image and frame sizes for this example because the math is easiest. As long as the frame is larger than the image, it can be optically centered and look correct. Most folks choose a frame one size larger than the image (11x16 for an 8x10 image) for personal use.
However, the more space an image is given to itself, the more impressive it appears to the viewer (or client). The clean area around the image keeps viewers' attention longer because there is no competing visual information outside their primary field of view (on the optic nerve).
Since many PJs typically aren't great at math, simply paste the following formula (include commas) into a spreadsheet, insert the dimentions and it should spit out the answers. Depending on the program, PJs may need to remove the quote marks from the front of the formula to make it work.
,Mat board width
,Mat board heighth
The results with be decimals (base 10). Since inches are measured in base 8, this chart could be of help as well.
1/8 = .13I'm avoiding a mat board discussion at the moment, but understand PJs typically place their work on black mat boards (with black cores) rather than white or museum mat boards.
2/8 = .25
3/8 = .375
4/8 = .5
5/8 = .625
6/8 = .75
7/8 = .875
Enough for now,