Photography Mistakes – Minimum Depth Of Field

If I had to put one ‘error’ at the top of my list of photographic mistakes, it is the constant striving for as shallow a depth of field as possible.

We may want the subject to stand out from the background, but the ideal is not going to be achieved if the subject itself is out of focus.

Let’s say we are shooting with a full-frame camera – in other words, a film camera or a digital camera with a sensor the size of which is the same area as 35mm film.

Let’s say I am using a 50mm lens and I am two metres (about six feet) from the subject.

And let’s say I have the aperture set nice and wide at f2.8

The distance from front to back that will be acceptably sharp is about 30cm (about twelve inches).

What I mean by’from front to back’ is this. Imagine a person facing me and I am taking their picture. Imagine that I focus on their left eye.

With the set-up I just described, there will be a depth of about 12 inches – say 4 inches in front of that eye and eight inches behind the eye – that will be in focus.

As long as the person doesn’t move more than eight inches back or four inches forward, then their eye will be in focus.

That means there is plenty of latitude in case my focusing is off or they move.

Now try it with a more typical portrait lens – let’s say a 135mm lens.

The distance that will be sharp from front to back is about 4cm (about an inch and a half).

In other words, if you focus on the eye of the subject, and the camera’s focusing mechanism is a bit off – and it often is – or the person moves just a couple of inches – then chances are that the eye will not be in focus.

With an APC-sized sensor that one finds in a typical consumer dSLR, the front to back sharp region is smaller – just 3cm or just over one inch.

And with a micro 4/3 camera, the region that is sharp is less than 2cm (about three-quarters of an inch).

With a typical point-and-shoot camera it is less than 1cm (about 1/3 of an inch) – and there is practically no chance of being sure that the eyes are in focus.

So don’t get hooked into ‘open apertures at all costs’, because sometimes the cost is too high.

Here is a link to Bob Atkins’ Depth Of Field Calculator.

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7 thoughts on “Photography Mistakes – Minimum Depth Of Field

  1. You are a real photographer – I’m an iphone and whatever it seems to grab for me phototaker. Most end up in the computer for whiling away wasted moments. It is lovely to see what can be done, we all have our own ways to appreciate, express and remember.
    Elspeth

    • It’s nice of you to say, and thank you. One my photography portfolio site that I recently reconstructed, I put a reminder to myself –

      “The biggest obstacle to becoming the photographer I want to be, is the age-old one of not getting on with it…

      As the Buddha said, “I am not sure whether there are any sins, but if there is one it is laziness.”

  2. I read the whole thing :) You lost me somewhere around ‘the distance from front to back’… I just like it when the background is blurry …the blurrier the better. That was one of the reasons I wanted a DSLR camera [the other one being the silky-looking water].

    • Thank you for commenting. I added to the wording to make it clearer (I hope).

      I too like the background blurry.

      What I was describing was the problem that photographers make for themselves by inadvertently making the photo blurry or unsharp where it is supposed to be sharp.

  3. David, I own a fancy mobile phone and a Nikon p/s so I don’t know jack about apertures, etc… truly don’t even know all the features on either ‘camera’ I use, yet I always learn such useful tips from you. I find your examples and tips to be straight forward and adaptable that now I get much better shots even with my junior gear.

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