Thursday, March 4, 2010

Digital Pinhole Follow Up

After reading the post on digital pinhole posted a few weeks back, one of our technicians here at KEH Camera thought that in addition to diffraction itself being the factor in the softness of the digital pinhole images, that there was much more to it. He decided to delve into deeper research to support his theories. For the technical people out there, this post is for you. Kris has shared his findings and sources as to why this happens.

In addition to light diffraction, a couple of other factors contribute to digital pinhole images being fuzzy and out of focus ( for essentially, they are out of focus.) These factors are focal plane & depth of focus.

Pinhole photography differs from traditional photography in that it is lensless. The photography concepts that we are taught, and so familiar with in traditional photography are completely absent and do not apply to pinhole photography. Namely, focusing mechanism and focal plane.
As you may know, we need a focusing mechanism and a flat plane (along with a proper shutter speed and a stable platform for supporting the camera) to obtain a sharp image. A pinhole camera has no focusing mechanism and, since it is lens-less, depth of field does not exist.

The focal plane in a film camera is different from that of a digital SLR sensor. Film is one flat plane once it is properly loaded inside a camera. A digital SLR's sensor, although it appears flat, is made up of several different layers of light gathering and protective materials. The nature of a pinhole's poor image resolution is also amplified by the sensor's micro-mirrors and photo-sites.

A simulation of a Circle of Confusion resting on/at a film plane, by Kris Phimsoutham
A simulation of a Circle of Confusion resting on/at a sensor, by Kris Phimsoutham
 The CCD and/or CMOS sensors have several layers of protective and light-gathering materials built on top of the actual material that does the capturing of an image. When a circle of confusion reaches the camera's sensor, the focal plane is resting several layers above the image capturing layer. Since it is a pinhole-fitted camera, the circle of confusion cannot be focused to fall on the plane of the image capturing layer. The sensor is simply capturing what it sees resting on the top layers, which captures and appears as fuzzy and unsharp.

To further explain these concepts, the following section is being used with permission by the author of www.cambridgeincolour.com.

Circle of Confusion

"Another implication of the circle of confusion is the concept of depth of focus (also called the "focus spread"). It differs from depth of field in that it describes the distance over which light is focused at the camera's sensor, as opposed to how much of the subject is in focus. This is important because it sets tolerances on how flat/level the camera's film or digital sensor have to be in order to capture proper focus in all regions of the image."

Depth of Focus & Aperture Visualization

"The above diagram depicts depth of focus versus camera aperture. The purple lines represent the extreme angles at which light could potentially enter the aperture. The purple shaded in portion represents all other possible angles. The diagram can also be used to illustrate depth of field, but in that case it's the lens elements that move instead of the sensor.

The key concept is this: when an object is in focus, light rays originating from that point converge at a point on the camera's sensor. If the light rays hit the sensor at slightly different locations (arriving at a disc instead of a point), then this object will be rendered as out of focus -- and increasingly so depending on how far apart the light rays are."

Digital SLR sensors were invented to capture images through optical lenses. Until sensors can be made to act like film, conventional pinhole photography, as it is, isn't viable without post-capture processing. This doesn't mean it is impossible to create beautiful, artistic and expressive images from your digital pinhole camera. Artists such as Sam Wang, Nancy Spencer, Eric Renner and numerous others have been exploring and defining digital pinhole imagery with great success and profound impact.

~Kris Phimsoutham + Noted Sources