How to play with Bokeh and depth of field

The term bokeh is the effect associated to a blurred background and a perfect sharp subject. Our vision is like a fast prime lens. We focus on subjects all around us, and we never mind about the background, except when we see a panoramic view.

For example do this simple experiment:

With one hand close one eye and put your finger just in front of you but don’t cover the background, just move it a little bit to the right or left side, depending on which eye you had closed.
Now focus on your finger, just look at it and, don’t move your view from it! Try to see the background without moving your view from the eye. You can see that it’s blurred and your finger focused.
Now try the opposite, focus on the background and see at your finger without moving your view from the background. It should be blurred.
You have two fast prime lenses in your head! :)

We are used to see the world through our eyes, which behave approximately like a fast 50mm lens. That means that when we see a picture with a good bokeh our attention will point immediately to the subject and then we have the opportunity to admire the soft and nice texture of a blurred background.
Think about it, we are not able to see that background with our own eyes. Well, actually we are, but we cannot see it directly, we can only perceive it from the external borders or our vision.

To understand the technical details of the Bokeh, you need to understand the depth of field. For a technical explanation check the wiki. Anyway you don’t need to understand to go so deep into technical explanations to be able to use it. Let’s discuss here the practical (and funny) part of it.

The depth of field is directly related with the aperture. That is the amount of light that your lens let pass through it. The wider the aperture, and the bigger the blur. The narrower the aperture, the smaller the blur. Don’t get confused with the f/stop number. Check out the aperture controls in your camera.

aperture aperture

Left: Wide aperture Right: Narrowed aperture

The number used to indicate the aperture is called the f-number or f/stop. There is not a concrete definition of the term stop in this context. Think about it as an attempt to say “something that stops light to enter!”. If we narrow the aperture, in some way we are stopping light to enter. The lack of a concrete context for the term “stops” could lead some confusion. Let’s focus on the practical matter: the f-number has a direct relationship with the diameter of the circle that we saw in the previous pics. Remember a big f-number indicates a narrowed aperture and a small f-number indicates a wide aperture. So don’t get confused. If somebody tells you “increase the aperture!!” what do you understand by that? Increase the f-number, “stops” light to enter or increase the diameter of the diaphragm?

Remember:

The smaller the number, the wider the aperture and the bigger the amount of light that will enter through the lens.

To obtain a good bokeh we need a wide aperture, that means a small f-number.
The best lenses to obtain this are the fast prime lens, the most common ones are the 50mm and 85mm at 1.2, 1.4 or 1.8F. Anyway, any lens that could be set to an aperture less than 2 or 2.5 is enough.

With this kind of photography we have to be specially careful of two things:

It’s really hard to focus with such a small depth of field. We cannot rely 100% on the autofocus. Sometimes it’s necessary to deal directly with manual focus.

In well illuminated environments we have the camera to shot at a huge speed. Depending on the equipment, sometimes the camera cannot afford the required speed. In such cases we can use an ND filter

nd filter
ND FILTER (Neutral Density)

Check these three pictures that will show the meaning of the depth of field

depth of field

depth of field

depth of field

If we narrow the aperture, the depth of field also changes and the sharped zone increases as well.

depth of field

depth of field

Also, the depth of field could be used to bring to focus the subject that we are interested to show. For example, in these two pictures, the second one shows the doll in the back giving a sense of depth. If we compare the second pic with the first one, we can see how we increase that sense of depth focusing subjects that are not in the first plane.

In this video, you have the depth of field in action.


Video took with a Pentax K-X and Super Takumar lens

Direct link here

The depth of field is a powerful concept that we can use for many different creative purposes. From a basic understanding, it’s used to put in evidence a particular subject and to catch our attention to the sharp point. From a generic point of view, depth of field is just an art, so let’s talk about it a later post.

Check some bokeh in Flickr