One of the most important things that affects the look and feel of a photograph is the depth of field. The depth of field, to my mind, is the amount of elements in the photograph that are in focus. When something is in focus, it is rendered as sharp in the photograph.
What’s in focus depends on what we were focussing on in the picture: if we focus on a person’s eye, the eye will be in focus. Depending on the depth of field, the rest of the face may be in focus too, or the ears may begin to get blurry. If we have a background in the portrait, some of that may be out of focus.
If we want to change our depth of field, we can do three things:
1. Change the aperture.
This is what photographers usually think of first when wanting to change the depth of field. Wide apertures (lower f numbers ) like 1.4 and 2.8 will give narrower depths of field than tight apertures (greater f numbers) like 11 and 16. What we have to remember when changing our aperture is that it changes the exposure triangle, so we will have to change the shutter speed or ISO accordingly to get good exposure. Aperture is usually changed using a dial on your camera.
The image below was shot using a 35mm prime lens at f/1.8. The focus in on the left eye (her right) and the mole on her left cheek is slightly blurry.
2. Change the focal length.
Say we are shooting using the kit lens at 35mm. If we want to get more in focus or, in other words, increase our depth of field, we can zoom out to 18mm. At 18mm, more elements of the photograph will be in focus than at 35mm. Do you want fewer things in focus? Zoom in to 50mm and we reduce our depth of field, so fewer elements will be in focus.
3. Change the distance between you and your subject.
Want to get more in focus just by moving your feet? Then stand further away. As the distance between you and your subject increases, the depth of field gets bigger too, so more elements are in focus. Want to get very narrow depth of field? Then get close, then get even closer. Only what’s in the frame will be in focus.
That’s it. These are the three things that you can change to affect depth of field. All you need to do is go out and decide for each picture you take, how much you want in focus. Typically a landscape is shot at wide angles and with high f numbers like 11 so the depth of field is huge. Portraits are often shot with telephoto lenses like 85mm with low f numbers like 2.8 (or lower) so that only the face is in focus.
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