The basics of depth of field. Or, how to get those blurry backgrounds!

November 14, 2016  •  Leave a Comment

 

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The above photo shows shallow depth of field.  Notice how the background is blurred and the only real sharp part of the image are the tree trunk and leaves.  This creates a sense of distance and helps to minimise the background.  400mm lens at f5.6 Although both use f5.6, see the difference between the long lens (above) and the very short lens (beach picture, below)

 

What is depth of field?

This is how much of a subject is in focus in a photograph.

For instance, there might be foreground to background depth of field, also known as wide depth of field.

Or just one flower in a field full of flowers.

This can have wonderful creative effect. For example, it is often better to use shallow depth of field when shooting portraits- of people or animals! This is because the progressive blurring away from what you've focussed on isolates the subject and makes you concentrate more on the subject when looking at the photo. It effectively minimises the distractions of backgrounds etc.

On the other hand, wide depth of field is sometimes necessary in order to avoid unsightly blurring.

So, how does this happen and why?

Several things cause shallow (or wide) depth of field:

  1.  Lens length or focal length.  The longer the lens, the shallower the depth of field.
  2. Focal distance- how close you are to the subject. The closer you are, the shallower the depth of field.
  3. For SLR or more sophisticated cameras, aperture (f-stops) are crucial for influencing and controlling depth of field.

On the back LCD panel or on a ring on the lens of an SLR (digital or film) camera, and many bridge or compacts, you'll see numbers like 2.8, 5.6, 11 etc.

Ever wonder what those settings are for? The lens's aperture is like the pupil of your eye. When it's dark, your pupil gets bigger to let more light in. When it's bright, your pupil gets smaller.

But with a camera lens, the depth of field becomes greater (wider) when the pupil gets smaller- and the same happens with our eyes, to an extent.

This is why we squint when we can't quite see something: squinting has the effect of seeing out of a smaller hole and the area in focus becomes a little larger, although this is somewhat negated by the pupil getting larger. Try looking at something through a tiny gap in your fingers, much the same happens.

So, with apertures (f-stops), the smaller the number, the larger the lens's “pupil” and the less is in focus at any given lens length or distance. Make the aperture a bigger number (e.g f11) and you'll have more in focus.

This is why SLRs and other more sophisticated cameras have aperture settings.

Another factor is sensor size. The sensor is the part of the camera- any digital camera- that captures and records the photograph. Some higher end SLRs have “full frame” sensors, roughly the same size as a single 35mm film negative. Most other SLRs have “cropped” sensors, which are usually around ¾ the size of a full frame. Most compact cameras and mobile phone cameras have even smaller sensors.

The smaller the sensor, the less pronounced the shallowness of depth of field is.

So, if you have a phone and it doesn't have control over apertures (f-stops) how do you achieve depth of field?

It's not as easy as with an SLR, but bear these tricks in mind:

  1. Get as close as you can to the subject.

  2. If you can, lower the camera's ISO to force it to “open up” any aperture (if it has one) and hence make the depth of field shallower.

  3. If you camera really does zoom, use the longest lens setting (“T” rather than “W” for most) and get as close as you can.

  4. If all else fails, there are lots of Apps for “faking” depth of field. Examples are AfterFocus and Big Lens.

  5. Some mobile phones have attachments which go over your phone camera's lens, making it longer or able to work closer up.

 

This photograph shows very wide depth of field.  Notice how sharp it is from foreground to all the way in the distance. 14mm lens at f.5.6  Compare this to the photo at the top, taken with a long lens.  Wide lenses like this give huge amounts of depth of field.

 

 

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Fly Agaric mushroom. Yes, they really are red with white spots! This was taken with a Macro lens, which allows you to get within cm of the subject. But the close you get, the shallower the depth of field, as shown here.  50mm Macro, f.8.  WARNING: Fly Agaric are poisonous, so don't go eating them!

I hope this is of use to photographers of all types.  This is a basic introduction to Depth of Field and how best to achieve it on different cameras. More will follow for those with SLRs and other more sophisticated cameras.

 


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