Depth of Field Simplified

If you try to look at your lens, you will see some writings indicating its product model and a few numbers that show its focal length and maximum aperture. This Nikon 18-55mm VR lens for example, has a maximum aperture of 3.5 on its wide end (18mm), and 5.6 on its long end (55mm). Since this is a variable aperture lens, its maximum aperture changes when you zoom in or out.

The aperture of the lens determines the depth of field in images – the smaller the value, the more you can blur the background of your subject. When the value goes up, the background becomes clearer in the image too. If you look inside the lens through its front element, you’ll see the aperture blades - these blades adjust the size of the hole inside the lens. Think of it this way – big hole = smaller aperture number, small hole = higher aperture number. Now you know how simple it is, let’s talk about how it affects the image.

When the hole inside the lens is big (meaning, small aperture number), it allows more light to pass through it. If you are outdoors, you might overexpose your image using a small aperture number, slow shutter speed and high ISO setting. For an outdoor shoot and you want to get a blurred background, it’s best to keep your ISO low, around 100-200, and faster shutter speeds to avoid overexposing your image (unless you are doing it on purpose).

Shallow depth of field blur backgrounds

Higher F-stop will get backgrounds in focus

If you are shooting a group photo, using a small aperture number will have other people in the picture out of focus. Since you want all of your subjects in focus, you need to shoot in a higher aperture number (I usually shoot groups in F8). Since the hole is smaller this time, you might get an underexposed image – there isn’t a lot of light passing through the lens. If the place isn’t that bright, it’s best to turn down the shutter speed, or bump up the ISO a little for better exposure. Landscape photographers usually take photos using higher F-stops (aperture number) since they want everything in the image focused. If you’re interested in landscape photography, the tripod is your best friend - It allows you to take super sharp images at slower shutter speeds.