How to set up grid lines for a Canon camera.
Grid lines are helpful when aligning subjects vertically or horizontally. With a Canon Rebel T3i, there are three grid options: None, 3 x 3, and 5 x 5. This feature is called Grid Display in the Menu options.
Step 1: Press the Menu button.
Step 2: Navigate right, to the fourth tab.
Step 3: Select "Grid display" and then your preferred options: None, Grid 1 (3x3) or Grid2 (5x5).
Step 4: Make sure you're in Live View to be able to view the Grid overlay.
How to turn on grid lines on a Nikon, like the D300.
The question now becomes which of the four sweet spots should you use when deciding where to place your subject. When it comes to the left or right side, you should go with the right side if you live in a country that reads from left to right. Our eyes are used to reading an image, in the same way, we read words. If you place the subject on the left side, our eyes come into the image on the left and then stop on the subject and we don't really look at the rest of the image. Whereas, if you place the image on the right side our eyes come into the image from the left, go through most of the image and then run into the subject. Don't worry if you accidently shot the subject on the right side, there is an easy was to fix it in Photoshop. Here is an example: