It’s something that Instagram fakes rather well, and other mobile phones have copied. It’s something that can be added in afterwards… and then there is the right way of doing it. We are of course talking about Depth of Field (DOF), and when done correctly, can produce some really stunning images.
We have all seen images that make use of DOF, and works best when subjects are close to medium-length away from the photographer.
What is Depth of Field?
In Layman’s terms, DOF is the act to placing an object in focus but having the foreground and background out of focus – creating a striking image that highlights a specific area on a subject in the images. The best example could be that of a cat with an outstretched paw – the cat’s head would be in focus, but its paw and the surrounding background would appear to be blurry.
There are actually a number of technical formulas to work out the exact DOF needed for an image, but you won’t have to worry about it – unless you are a Portrait photographer, do studio filming or take stock images for a living.
How is it done?
Luckily with today’s technologically advance cameras, there is no need for amateur or professional photographers to know the exact formulas to work out DOF as the lens handles most of that. To explain how to get the affect in simple terms, the best method would be to set your camera on Manual focus first. This way you ensure that you get the desired affect that you want, and not what the camera thinks will be the best option. Aim the camera at the subject you want to capture and slowly rotate the focus ring until the desired part is in focus. The just snap away. Most cameras will actually indicate through the viewfinder if the portion is in focus, and always make sure that Manual Focus is set on the interchangeable lens and the body.
Do all lenses work on the same principle?
Well, yes and no. While the principle in theory remains the same, cameras to tend to behave differently depending on the type and length on lens that is affixed to it. Longer lenses will afford you the ability to play around with the DOF a lot more if the subject is some distance away, while shorter lenses have a limited capacity of DOF. The same goes for Macro photos – you would need an incredibly short lens (like a Pancake lens) to get the same affect.
The formula for DOF?
There is a number of factors that come into play when making use of DOF, such as subject matter, movement, camera-to-subject distance, lens focal length, selected lens f-number, format size, and Circle of Confusion (CoC) criteria. Before starting on the formula, you need to know the CoC of your camera – Canon’s APS-C sized sensors are 0.018, Nikon’s APS-C are 0.019, for full frame sensors and 35mm film it is 0.029. So the formula would be: CoC (mm) = viewing distance (cm) / desired final-image resolution (lp/mm) for a 25 cm viewing distance / enlargement / 25.
Once you have the CoC and the Hyperfocal point is precalculated, the entire formula would look like this: TotalDoF[mm] = ((HyperFocal * distance) / (HyperFocal – (distance – focal))) -(HyperFocal * distance) / (HyperFocal + (distance – focal)).
[Image – CC by 2.0/Tim Ellis]