Circle of Confusion explained and what it means to be in focus.Posted by Leigh Diprose
Have you ever heard photographers saying 'that photo's so sharp' or 'it's tack sharp'?
Do you actually know what they mean by this? I mean to say what defines sharp and can sharpness even be measured? How should you go about obtaining the sharpest image? These are some important questions you should ask yourself before going out to take photos professionally.
The sharpness of a lens can directly be related to the Circle of Confusion - umm...the Circle of what?
Let me explain...the Circle of Confusion refers to the measurement of a point of light that falls on the focal plane (camera sensor) that's often referred to as the focal point. It's this point where the light converges the Circle of Confusion can then be measured.
It sounds like a mouthful and it can be, so here's a diagram to better describe it.
To help explain it further, let me take you back to your childhood. Imagine you were holding a magnifying glass and the sun was your light source. If you hold the magnifying glass above the ground you can see the light falling on the ground in a circle. The magnifying glass would act as your camera's lens and the ground would be the camera's sensor or focal plane. As you move the magnifying glass closer or further away from the ground the circle of light (focal point) will become larger or smaller. This movement of the magnifying glass is exactly like focusing on a camera's lens. As you focus to your subject or away from your subject the focal point hitting the focal plane becomes larger or smaller.
It's at this point you can sum it up as if you have the smallest point of light hitting the focal plane then you will have the sharpest image. The Circle of Confusion refers to the measurement of this small point of light on the focal plane.
The diameter of the Circle of Confusion is actually measured in fractions of a millimeter on the surface of the focal plane. Different lenses will vary in focal lengths, resulting in different Circles of Confusion. Now that we can understand this, Depth of Field tables can be calculated based on the value of a lens's Circle Of Confusion and aperture.
Depth of Field tables are used to determine a lenses Hyperfocal Distance. Again that's a mouthful, so let me explain. Hyper Focal Distance is very important for photographers to understand. What it essentially refers to is making sure that everything is in focus, in other words you have the sharpest possible image from the foreground, right through to the background of your image.
To do this out in the field, a photographer will refer back to their Depth of Field Table. The table will tell them how far away from the camera they should focus. Focusing at the correct distance will mean they will have the background of the scene in focus as well as the foreground. Let me give you a few examples...
Have you ever captured a landscape photo only to find everything on the horizon is completely sharp, while everything in the foreground is out of focus? It's a photo many landscape photographers dread, as they want to obtain everything in focus, so how do they go about capturing that perfectly sharp image?
It's simple all they do is use their Depth of Field chart, which is specific to two things, their lens and what aperture they are using. The numbers on the Depth of Field chart will tell them how far away they must focus from their camera in order to have the foreground and background in focus, so rather than focusing on the horizon they might only focus around 20 meters in front of them. Many lenses (and electronic viewfinders) support these calculations as they have depth of field scales inscribed on the focus ring of the lens. The scale is represented in meters. Simply turn the focus ring to the corresponding known Hyperfocal Distance and you'll obtain sharp focus.
Fortunately there are many Depth Of Field tables available online which have already calculated the Hyperfocal Distance for the majority of lenses out there. So you don't have to be brilliant mathematician to work out what measurements you'll need to obtain a perfectly sharp landscape photo. For those who are interested though, here's the math behind how Hyperfocal Distance is calculated: