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Capturing Dragons by Andy Astbury - part 4 of 1 2 3 4 5 6

by Andy Astbury Published 01/02/2009

Right, that's the equipment covered for the most part, so what I'd like to do now is get down to some 'nitty gritty' image talk and DoF in particular.

To me DoF is a sort of two-headed monster, where too much is just as bad as not enough. Firstly we need to define exactly what it is that we are talking about here; yes, some of you will think it unnecessary but we'll do it anyway.

DoF, depth of field can be simply defined as the portion of a twodimensional image of a three-dimensional scene that APPEARS to be in focus.

Now, even though the lens is focused on a particular, finite plane within the subject, areas both behind and in front of the subject appear to be as sharply focused as those in the plane of focus itself, so what we have in effect is a near-focus and a far-focus limit, with the true plane of focus lying in the middle. It is in the middle at macro distances but moves to about a third through the focus zone at 'normal' working distances.

As we vary the size of lens aperture then these two limits move either closer together or further apart, whilst the true plane of focus stays exactly where we put it when we initially focused on the subject. The wider the aperture we use, then, the closer these limits move towards each other and the true plane of focus, and conversely, the smaller the aperture we use the further these near and far focus limits move apart.

So in effect, what we have is a 'box' that varies in depth along the axis of the lens, relative to the aperture we use, and anything inside the 'box' will appear sharp and in focus.

If you've followed me so far then you are doing well - but wait, there's more to consider. Just for clarity let's agree that our three planes of focus, near, true and far, all lie perpendicular to our lens axis; that the linear distance between the near and true planes is the same as the linear distance between the true and far planes; and that the linear distance between the near and far planes defines our total depth of field.

However, as I said before, there is more to consider - for a start, reality is more like this, where it's plain to see that parts of the subject fall outside the near and far limits of our 'DoF box'.

Secondly, the depth of our box varies not just with aperture, but also with subject distance; the further from the subject the lens is then the greater the DoF for any given f-number, which is all well and good but somewhat irrelevant; and conversely, the closer the lens is to the subject the less DoF we will have at the same aperture - which is HIGHLY relevant!

Thirdly, while all the DoF 'malarkey' is going on around the subject, we can well be losing sight of the equally critical image background - overlook this at your peril!

A nice smooth 'studio-style' background, as already hinted at, helps to concentrate the viewer's attention on the subject and all its glorious detail. If the background is too close to the subject then maximising DoF at that point or plane will lead to increased background detail. It could still be hugely out of focus, but its many tonal and brightness changes will become more and more apparent as we stop the aperture down - something to avoid at all costs if you wish your subject to be isolated and indeed for the image to look as it does in the viewfinder. Don't forget, you are usually viewing your subject at maximum aperture, or 'wide open' as it's commonly referred to.

So, how do you achieve that good wide open background once you are shooting?

Well I have a simple rule, whatever the f-number you are using then that number multiplied by your subject distance will give you an appropriate subject to background distance. Using this little formula you'll at least be on the way to achieving those studio-style backdrops to your images.

You do, however, still have to choose your backgrounds with care, overall colour and tonality matter greatly, but at least this method of calculation will help you pick your subjects and camera positions with a bit more accuracy

• Capturing Dragons by Andy Astbury page 1
  
• Capturing Dragons by Andy Astbury page 2
  
• Capturing Dragons by Andy Astbury page 3
  
• Capturing Dragons by Andy Astbury page 4
  
• Capturing Dragons by Andy Astbury page 5
  
• Capturing Dragons by Andy Astbury page 6
  

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