The shape of an image, or its aspect ratio, is frequently something that photographers decide when they’re sitting at their computer editing images. Often they’re trying to work out how to make the shot look a little better and in a flash of inspiration they reach for the cropping tool and make it square or a letterbox shape.
In many cases that will do the job of improving the image, but if they had assessed the scene at the shooting stage and considered the best aspect ratio at the time they would’ve produced an even better picture.
By default digital cameras shoot in the aspect ratio of the sensor, either 3:2 or 4:3. However, most cameras have an aspect ratio control that lets you shoot in other aspect ratios and crucially for helping with composition, you see the impact either at the shooting stage or immediately after.
While mirrorless cameras will show the cropped version of the image in the viewfinder or on the screen when you’re composing the image but this is harder with a DSLR. In some cases, a DSLR may display guidelines in the viewfinder that indicate the cropping that is applied to images with the selected aspect ratio. If your camera doesn’t do that you can either shoot in Live View mode and compose the image on-screen (this will allow you to preview the crop), or you can shoot and review on-screen to assess the impact.
The camera crops the image to create the alternative aspect ratio, so just as cropping with image editing software, you wind up with a slightly smaller image. However, it gives you the opportunity to contemplate the composition while you still have the option to change it.
That might seem like a very minor point, but if you think about how the rule of thirds works you’ll realise that the location of the lines that divide up the frame will change with the aspect ratio. That could mean an extra step or two to one side or the other could have a significant impact upon the balance to the image.
What’s more, rather than cropping an image to 16:9 post-capture to eliminate an empty foreground, if you think about the aspect ratio at the shooting stage, you’re far more likely to fill that void or make better use of the width of the frame.
Using your camera’s aspect ratio controls will help you get better at deciding which scenes suit a particular format and with time you’ll find that you instinctively know whether an image is going to be square, 3:2, 4:3 or 16:9 or any other aspect ratio.
It’s worth noting at this point that if you shoot raw and jpeg files simultaneously, the jpeg is cropped but the raw file is usually saved with the data from the whole sensor and you can change the cropping post-capture if you need to.