For those of you who don’t know, the aspect ratio of an image is the ratio of the length to the width. For example, most digital cameras, including all 35mm formats, have an aspect ratio of 2:3, very rectangular. Medium format cameras have a ratio of 3:4, so more square. 1:1 would be perfectly square, and so on.
Usually, images work great in one ratio, but when cropped to another loose part of their strength. All this is due to how the image was originally composed.
Most of the time when I shoot, if a ratio is not specified, I will compose the image to work best at 3:4, the native aspect ratio of my camera. For example, this image of a kitchen (right) works great at 3:4. Now you could crop it, but parts of the design would start to be lost, such as the anchor of the chandelier or the molding on the left, and the image becomes less impactful.
Sometimes though, clients have a preferred aspect ratio they like to work with. Such is the case of a client of mine in New York who only ever wants square images. This family room image (left) was composed with that mind, and does not really work in any other crop.
The end use of an image can also dictate what ratio to use. For instance, this Absolut image (below right) was composed knowing it would be used as a full spread. The dimensions of this spread were 13.5 x 21.5 inches, which is the crop I set when composing it (with an extra quarter inch added on for the bleed of course).
So, if you are hiring a photographer and you know how the images are being used (or most used), giving he/she the aspect ratio can help greatly in making sure the images are most impactful.