4 aspects of photography that blurrs the background of a photo
In case anyone is not aware of the aspects that determines the intensity of the sharpness of parts of a photo (usually the backdrop), here are 4 things we need to take note of.
The size of the aperture
Aperture opening size determines the depth of field of a photo. It is a common saying and most beginners even know that a fast lens with the biggest aperture you can get is desired for portrait work and throws the background out of focus so that attention is locked and given to the subject.
The distance between the subject and that of the backdrop
Another way to achieve the shallow depth of field and a blurry backdrop is to angle your shot in such a way that the backdrop is cleared of obstructions and you have a greater void of empty space between the subject and the closest solid object in the background. Simple to say, if you have a clutter-free background and an empty space behind your subject, you are good to go.
The focal length of the lenses
If you have a super zoom lens and you zoom it out to the longer range, it is going to isolate your subject more from the backdrop. The concept is the same for a telephoto prime lens. The longer the focal length, the more isolation there is between the subject and the backdrop resulting in a more out of focus backdrop. However the same principle applies in the case of backdrop distance to the subject. If you have a wall directly behind the subject, zooming out the focal length is not going to help much in blurring the background.
Focusing distance of the lens to the subject
If you place your lens very near to the subject/s, it will cause a decrease in the depth of field and the backdrop is going to be thrown out of focus. This is very common in macro / micro photography and photographers usually find that they have to step down the aperture (to f/16 or maybe more) in order to have sufficient DOF on the area of interest or centre of focus.
Combining the above factors, you are going to have an amazingly smooth and shallow DOF photo.
The above photo was taken at 90mm focal length, 1:1 macro with 50mm extension tubes, f/29 aperture and a short distance between area of interest and backdrop. Note that the aperture has to be closed down to a high f/29 and even though the backdrop is close to the subject, the focus is only placed on the head and eyes of the tiny spider. This shows how the focusing distance affects the DOF of a photo. Macro shooters will understand what i mean.
Photo credits: Unteroffizier.