Drowning in the Depth of Shallowness or… Why does Depth of Field Change?

Here’s the scenario: You take a photograph of something that’s quite close to you and doesn’t move around too much (a heron for example – very obliging animals!) and you choose a wide aperture for a shallow depth of field. The image is as you envisioned with the animal in nice crisp focus with a soft out of focus background (and foreground). Nice. The heron now moves away from it’s original position and settles in, say, open green reeds. The bird is at least three times further away from you now but is ‘posing’ perfectly. You think that a shot of the animal in its current location would make an even better image than the first because the green foliage/reeds would give an even better out of focus back and foreground in contrast to the bird’s grey/white plumage. The aperture you chose for the first shot seems to be ideal… offering a nice shallow depth of field whilst maintaining an out of focus foreground and background. You take the shot…Oops! The depth of field has changed! More of the foreground and background is in focus…in fact, quite a lot more! So what’s happened?

There are four main things that play an important part in depth of field.

  1. Aperture: one of the easiest ways to control your depth of field. Wide open lenses (small F-stop) give the shallowest DOF (depth of field) and a nice out of focus background.
  2. Distance from the camera to the subject: The closer you are to your subject the shallower the DOF. Move further away and the DOF will increase even though you haven’t changed your aperture.
  3. The focal length of your lens: the general rule of thumb is that the longer the focal length the shallower the DOF (portrait photographers will often use a 200mm lens – try it yourself). Landscape photographers will often use a wide-angle lens with a short focal length to get the whole scene in focus (big DOF).
  4. Sensor size: Simply put, the bigger the sensor the shallower the DOF. If you want to know the technical reasons then there’s plenty floating about in the ether.

So there you have it: Aperture, Distance from subject to camera, Focal length of your lens and Size of your sensor. All these play an important part in achieving the DOF you want. And, like the exposure triangle, it’s about getting the balance right. Go out and try it. Experiment with all four ingredients. And next time that heron moves, you’ll be prepared and get the shot you want. And remember the five P’s…

Proper Preparation Prevents Poor Performance

Actually there are six P’s but we’ll skip that!😉

Happy snappin’ folks

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