Manipulating the depth of field when you take an image is a useful skill to learn whether you want to achieve front to back sharpness for a landscape or whether you are after an artistic effect or want to direct attention to one particular part of an image with distracting background or foreground elements artistically blurred. When you are trying to lean the relationships between factors that affect depth of field, there’s nothing more guaranteed to confuse you than an in depth tutorial. Does the thought of juggling formulae like

H=(f x f/Nc)+f

and

Dn=s(H-f)/(H+s-2f)

and

Df=s(H-f)/(H-s)

put you off completely? Well read on to learn a much simpler approach.

Maybe not but the reality is that whilst these are great fun to play with in the comfort of your own home to get results accurate to a millimetre and where you have access to a calculator or spreadsheet and lots of coffee, it’s hardly a practical way to estimate depth of field when you are out and about with your camera.

Of course if you want to play with the above formulae then feel free and if you want to, you can read more here but what I am providing here is much more valuable than a couple of equations and its simple enough to keep in your head. It’s simply this!

What factors affect depth of field and how?

Since a lot of people are now using digital cameras that have the ability to zoom in on the image you’ve just taken, it is possible to get a pretty good estimate of the depth of field or zone of acceptable sharpness in your image. The first step is to take your picture and look at it on your camera to see how it looks. Once you have established that, if you want more or less depth of filed you have some choice to increase or decrease it. So here they are:

Aperture or f number.
The aperture is actually the size of the hole in the lens that determines how much light reaches the sensor. At a later date, I will do a tutorial to explain why the size of the aperture increases or decreases depth of field but for now just remember the relationship.

1. The smaller the aperture, the greater the depth of field you get in your image.

And remember that because of the way the f number is calculated that a small f number is a large aperture and vice versa. So all else being equal, an aperture of f/2.8 gives less depth of field then f/8 which in turn gives less depth of field than f/16 and so on.

OK, so far so good.

Focus Distance
If you have elements in your scene that includes objects close to your lens, then to keep the close objects in focus, you need to set a short focus distance. This has the effect that.

2. The shorter the focus distance, the smaller the depth of field.

Focal length.
Similarly, the focal length of the lens has an effect on depth of field. One of the reasons that landscape photographers like to use lenses with short focal lengths like 17 mm or even less is that it is relatively easy to ensure a depth of field that extends from very close to the camera to infinity. So the rule here is

3. The longer the focal length of the lens, the smaller the depth of field.

So just 3 easy rules to remember then. There are other factors that make a difference but not that you can realistically control. These are the sensor size (unless you have a selection of cameras that you carry with you) and the circle of confusion which is somewhat related to the sensor size. In any case, it can be successfully argued that sensor size only gives an apparent change in depth of field so in reality you can ignore this since all you really want to do is increase or decrease depth of field relative to the original image you took.

In fact it doesn’t take very long to learn these relationships especially if you experiment a little with them. And before long you will find that you will be making the changes to aperture, focal length and focus distance almost subconsciously with a pretty good idea of how much depth of field you are going to get before you take the picture. If you simply want maximum depth of field for any given combination of focal length and aperture then check out my Hyperfocal Distance Made Easy tutorial and download the table for your camera.

So in reality, it doesn’t matter whether your depth of field is 3 metres or 3.1 metres, 10mm or 20mm. What matters is that the depth of field suits your image and now you know how to increase it or decrease it to suit your needs.

Happy Snapping.