Depth Of Field (DOF)

This is a bit of a technical subject but in my opinion and experience absolutely key to getting accurate focusing and getting the DOF correct, which results in sharp pictures where you want it to be sharp.
Ok so DOF is described as: the area of acceptable sharpness within a photo that appears to be in focus, every image you take has a certain amount of the image acceptably sharp in front of and behind the actual point of focus.
What controls depth of field:

1. Aperture
2. Focal Length
3. Distance to subject

Ok so far so good this is what makes little sense to me, a Large aperture means a Small f stop which equals a shallow or narrow DOF
Whilst a Small aperture means a bigger f stop which equals a deeper or wider DOF. 

 The above picture taken at 100mm f2.8 1/6th ISO100

Notice how only the 4th and 5th legs are in focus all the rest of the pins are not in sharp focus the DOF in this case is no more than a few mm

 

The above picture taken at 100mm f8 1/1th ISO100

Notice how we now have much more of the IC in focus simply by adjusting the aperture from f2.8 to f8 we have achieved a much greater DOF.

The simplest way to remember this is the lower your f stop number the narrower your DOF whilst the opposite will happen using a higher f stop number will result in a wider or deeper DOF, as we can see from the two examples above f stop of f2.8 gives us a narrow DOF, and a f stop of f8 gives us a much wider DOF.

OK so now we understand what our f stop does and how to adjust it to achieve the result we want right, well not so as there are two other factors to bring into play that directly affect the resulting DOF.

Focal Length.

So we have a  focal length of 100mm, distance to focal point 1m, f stop of f2.8 this results in a total DOF of 1.53cm, that is 0.76cm in front of the focal point and 0.77cm behind the focal point.

If we keep the same parameters and adjust our f stop  to f8 we get a total DOF of 4.32cm that is 2.11cm in front of the focal point and 2.21cm behind the focal point.

Distance to Subject.

Using the same focal length 100mm and f stops as above this time we will change the distance to the focal point at f2.8  if we double the distance to the focal point to 2m we get a total DOF of 6.45cm, that is 3.17cm in front of the focal point and 3.28cm behind the focal point, if we move the focal point out to 3m we achieve a total DOF of 14.8cm, that is 7.2cm in front and 7.6cm behind the focal point.

Bringing these parameters into a birding situation using a Canon 500mm f4 L IS with a distance to the bird of say 10m at f4 we will have a total DOF of 9.12cm that is 4.54cm in front and 4.58cm behind the focal point, which is not a great deal when you are pointing at for example a kingfisher  which is sitting at an angle you will struggle to get the entire bird sharp you may find the tip of its beak or the tail are not as sharp as the rest of the bird.

Taken using Canon 5dmk3 700mm f7.1 1\250th ISO640

The above image resulted in a DOF of 7.66cm which was just about enough to get this bird sharp end to end.
Using the above as an example it throws up all sorts of issues for me, I know for sure that the sharpest aperture on my 500 is f7.1, so lets say I want to use apertures from f6 to f8 to keep my image as sharp as possible, I also know that the average kingfisher should be about 9cm from the tip of its beak to the end of its tail if its sitting at an angle, I also know the distance to the perch favoured by the kingfisher is 10m from the hide.
Using the above parameters to calculate the DOF to cover the whole bird, f4 gives me total DOF of 9.12cm, through to f8 which gives a total DOF of 18.2cm.
If we now add a 1.4x tele converter into the equation and push the focal length out to 700mm then even at f8 we only have 9.1cm total DOF to play with, really to be safe we will need to go to f9-f11to give a reasonable DOF.
Using such shallow DOF's also means it is important where you focus on the bird, if the bird is facing toward you at a 45degree angle and you focus on the eye as a lot of people do, you waste a lot of the DOF in front of the bird and end up with the tail being soft or out of focus, for me I try to focus part way down the body, as advised by my good friend Dave, this seems to work very well, but it depends on the situation these are all rule of thumb guides.

What this also highlights is how important it is to ensure that your camera and lens combination are focusing where they should be i.e. at the focal point, and not either front or back focusing, although for birding it can be advantageous to have a slight back focus dialled in.

See my AFMA post on how to fine tune your lens and camera combinations.