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A Primer on Depth-of-Field
Of all the topics I cover in my workshops, I find that depth-of-field is the one that tends to confuse people the most. Using it properly is one of the fundamental skills a good photographer must master.
What follows is a slightly simplified explanation of depth-of-field and focus. I have not taken camera movements into account as most people reading this won’t be using such techniques. It will suffice to give a good working knowledge of the concepts to someone who is just being introduced to them.
What is depth-of-field (DOF, for short)? The definition is as follows:
“The portion of the scene — both in front of and behind the plane of focus — that is considered to be acceptably sharp.”
The Basics of Focus
Clearly, in order to understand DOF, we must first undestand focus. When we focus on an object with our camera, we are aligning the lens so that that particular distance from the camera is in focus. Therefore, anything at that distance from the camera will be in focus, whether it’s above, below or to either side of the object we focused on initially.
“When an object is in perfect focus, light rays from a point on that object — or any other at the same distance — will converge to a point on the camera’s sensor.”
The above statement tells us that in any photograph, only objects at that specific distance are truly in focus. This is referred to as the focal plane. Any objects in front of or behind the focal plane will be out of focus. The illustration below shows this.
You can see here that the red rays coming from the candle converge back to a point on the sensor, resulting in a sharp image of the candle. However, the blue rays, coming from the glass, converge behind the sensor, resulting in an out-of-focus image of the glass. This is because the glass is in front of the candle and therefore at a different distance.
So if we can’t have objects that are not in focus be, well, in focus – how do we make photographs where everything is sharp from the photographer’s feet to the distant horizon? The answer is in exploiting the fact that our eyes have limited resolving power – they’re not perfect.
Circles of Confusion
In the illustration above, the blue rays don’t converge to a point on the sensor – they converge behind it (the dotted lines show their path if they were able to continue beyond the sensor). This means that instead of describing a point on the sensor, they describe a disc. These discs are known as circles of confusion. Any out of focus object will be projected as a mass of these overlapping discs – that’s why an object that’s not in focus looks blurry.
It is in managing the size of the circles of confusion in out-of-focus areas that we’re able to create this illusion known as depth-of-field. The way we do this is by varying the size of the aperture in the lens.
The Aperture
All camera lenses have a variable aperture. This is simply an opening in the lens through which the light is permitted to pass. We can make it bigger or smaller to suit our needs, and through that we can manage the DOF in an image.
In the illustrations below, light rays from an out-of-focus object are being directed through the lens and onto the sensor. In the first, a large aperture is used, and in the second, a smaller one. Nothing else has changed – the object is equally out of focus in both examples.
In the first example (above), the gray spot on the sensor is the circle of confusion created by this out of focus point. You can see that it’s quite large, which means it will be obviously unsharp to the viewer.
In the second example, the gray spot is far smaller, and to the viewer may appear sharp (depending on how large the photograph was printed). It’s smaller because the aperture was smaller, and therefore forced the light rays from our point to enter at a shallower angle. They still converged on the same point in front of the sensor, and then diverged again. However, unlike the first example, they weren’t able to diverge very far before striking the sensor and making the image.
It’s precisely this behaviour that creates the illusion of depth-of-field.
In a Print
Again, the whole idea of depth-of-field is to trick the viewer into seeing something that’s out of focus as being sharp. We do this by ensuring the circles of confusion of out of focus areas are so small that they are indistinguishable from a point.
This works very well in small prints. However, when enlarging your photographs, the larger the final size, the more likely it is that the viewer will start to see the out of focus areas as being unsharp. Therefore, it’s essential to take care when making the photograph to ensure that you use a sufficiently small aperture to have everything appear sharp, even in a large print.
In the Field
How do you put this into practice? Well, where you want to isolate a subject and make it stand out from the background, such as with a portrait, you would use a large aperture. Because the circles of confusion are larger, out of focus areas will very quickly become unsharp, as below.
Where you want to have sharpness from front to back, such as with most landscape photography, you want to use a much smaller aperture. Because the circles of confusion are smaller, they appear sharp to the eye. If done properly, this will work even in a large print, as below.
Proximity and Focal Length
There are two other factors which affect DOF. Your proximity to the subject is one – the closer you are, the less DOF you have. This is why in the photograph below of the skua chick, the foreground and background are so extremely blurred.
The second factor is focal length – the longer it is (i.e., the more telephoto the lens), the less DOF you have. Again, in the image of the skua chick, it’s a combination of my close proximity to the subject and the long focal length (400mm) used that creates such a shallow DOF.
These two things work against you when you’re looking for DOF and are both close and zoomed in on your subject. However, in most landscape photography, you aren’t that close to the nearest subject (being at least a few feet away), and you are generally using a wide-angle (read: short focal length) lens. This works in your favour, making it relatively easy to get sharpness from front to back.
The second example below illustrates this, as I was using a very wide-angle lens (12mm).
Examples
Here are two examples. The first, an image of a great skua chick, was made with a telephoto lens and a large aperture of f/5.6.
You can see that the chick itself is perfectly sharp, but the grass in front and behind it is increasingly unsharp.
In the example below, a tiny aperture of f/22 was used to ensure sharpness from front to back.
Note that both the rocks behind the waterfall and the rocks and foliage in the foreground are sharp.
In Conclusion
Understanding how to control depth-of-field is critical to your growth as a photographer. It requires a little practise and some knowledge of theory to get it consistently right, but it’s well worth the investment in time and effort.
I hope that you have found this tutorial useful. If you’d like to improve your camera or post-processing techniques, I run regular workshops at my studio in West Cork and in Dublin.
On top of that, I also run ten-day touring workshops. Contact me for details.
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