|Wide-open apertures cause out-of-focus backgrounds with long lenses.|
It is relatively easy to make the background blurry with a sharp foreground. It is well worth your while to understand the various factors that create depth of field. Manipulating the camera's controls to obtain the precise shot you want is an important part of creating successful photographs. It turns out there are five different ways you can alter depth of field.
1. Aperture changes depth of field by varying the size of the circles of confusion which make up the image. Wide-open apertures (small f/numbers) give less depth of field than small apertures (large f/numbers). Shooting in aperture priority mode is a common practice, partly because you pick the aperture you want to use. In the above photo, I used the lowest aperture value I could muster.
2. Focal length alters depth of field; long lenses (high mm values) have less depth of field than wide-angle lenses (low mm values), given everything else being the same. Telephoto lenses are more likely to produce an out-of-focus background than normal or wide-angle lenses. I used a focal length of 180 mm (relative to a full-frame camera) in the above shot; it helped the background be blurred. I had to be fairly far away though for the whole thing to work.
3. Point of focus is where the lens focuses. As the focal distance increases, there is more depth of field. The closer your subject is to the camera the less depth of field there is. This is part of the reason why macro shots have such little depth to them. I could have been closer to the duck, but then to get the bridge in I would have used a lower focal length. Also, due to perspective differences, the bridge would have been smaller relative to the duck than it is, and I did not want that as my composition.
4. Size of sensor plays a role as well. Large sensors must have longer focal lengths than smaller sensor cameras in order to obtain the same field of view. I shot the above photo with a 1-inch sensor bridge camera, which has a crop factor of 2.7. The actual focal length was around 65 mm, whereas I would have used 180 mm to get the same field of view on a full-frame camera. Smaller sensor cameras have to use wider angle lenses to achieve the same shot, and they get more depth of field because of this.
5. Amount of enlargement relates to how big you make the image and how far away you view it from. As mentioned earlier, a thumbnail image may all look in focus, but enlarge it on you can clearly see the difference. An 8x10 print will look different than a 2x3 print for this reason.
When shooting I think about these factors and try to play with them accordingly. Usually, I am concerned with the first three as they are the ones I have immediate control over. Play with these factors and see what happens. It will make you a better photographer.
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