Understanding Depth of Field in Photography
In photography, Depth of field (DOF), also called focus range or effective focus range, is the amount of distance between the nearest and farthest objects that appear in acceptably sharp focus in a photograph.
This focus range will vary from photo to photo. Some images may have very small range of focus which is called shallow depth of field. Others may have a very large range of focus which is called deep depth of field.
With shallow depth of field, the background quickly become blurry, this is great for shooting portraits, for example, where you want to concentrate attention on your subject. However, in landscape photography, the whole scene is your subject, and you want as much depth of field as possible, to make everything in the picture sharp, from the flowers and stones at your feet to a distant treeline on the horizon.
There are four main factors that will affect how you control the depth of field of your images are: aperture (f-stop), distance from the subject to the camera, focal length of the lens, and the sensor size on your camera.
The aperture is the opening at the rear of the lens that determines how much light travels through the lens and falls on the image sensor. The size of the aperture is measured in f-stops. The f-stops work as inverse values, such that a small f/number (say f/2.8) corresponds to a larger or wider aperture size, conversely a large f/number (say f/16) results in a smaller or narrower aperture size,
Using the aperture (f-stop) of your lens is the simplest way to control your depth of field as you set up your shot.
It may be easier to remember this simple concept: The lower your f-number, the smaller your depth of field.
Choose small f-numbers if you want a shallow depth of field where only the main subject looks sharp, and to choose big f-numbers if you want a broader depth of field where more in front and behind the subject will look sharp.
For these reasons, it’s good to use wide apertures when photographing subjects you want to stand out from their surrounding, such as people and wildlife, because a wide aperture will blur backgrounds. And that’s why it’s good practice to use narrow apertures when photographing landscapes – you want to ensure your scenes are in focus from foreground through to the horizon.
Focal Length refers to the capability of a lens to magnify the image of a distant subject. The focal length of the lens also has an impact on the depth of field - the longer you set your focal length the shallower the depth of field which means the more background blur you will have. For example, a photo with an aperture of f/4 and a focal length of 18mm will not have much background blur. However a photo with the same aperture of f/4 and a focal length of 200mm will have a substantial amount of background blur.
Basically, the wider-angle your focal length (e.g. 18mm on your kit lens) the more depth of field you’ll obtain, even at wider or middling apertures. This is why wide-angle lenses are great for shooting landscapes and cityscapes when you want the whole scene in focus, and why you can shoot with a wide lens at around f/8-f/11 for cityscapes and architecture while still ensuring the scene is acceptably sharp throughout. Longer and shallower Conversely, the longer and more telephoto your focal length (e.g. above 70mm), the less depth of field you’ll obtain – which is why telephoto lenses are brilliant for portraits and nature photos. They help to further blur foregrounds and backgrounds so your subjects stand out in your photos.
Your angle of view also changes depending on your focal length. Wide-angle lenses will capture a big angle of view; whereas there’s a dramatically reduced angle of view the longer your telephoto focal length. So be careful when shooting full-face portraits: wide lens = distorted faces, but telephoto lenses = slimmer – and much more flattering!
The distance to the subject also plays its part on the depth of field: the closer you are to the subject, the less that will look sharp.
So if you're after a particularly shallow or broad depth of field, you can use the lens focal length and subject distance to help you. Choose wide angle lenses with larger f-numbers for a large depth of field, and go for telephoto lenses with the smallest f-number possible for a shallow depth of field with a blurred background. To further accentuate the shallow depth of field effect, move closer to your subject and ideally place the background as far away as possible. An extreme example would be macro photography where you may only be a few centimeters from the subject - at these distances it's easy to achieve a shallow depth of field even if the minimum f-number isn't that small. A quick word of warning though, if you're shooting at big f-numbers, remember they're letting in less light and will need longer exposures or higher ISO sensitivities to compensate, the former increasing the risk of camera shake or motion blur and the latter decreasing image quality. Always keep an eye on yuor shutter speed when you're adjusting the aperture.
The distance between the subject and the background. If you’re taking a photo of a newborn baby, you might as well give up on trying to use depth of field. Newborn babies are always lying down, so nothing you do can make the blanket behind the baby’s head blurry. It is so close to the focus point on the baby’s face that it is not possible to make it blurry.
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