After writing two articles on the Nature of Light and its relevance to digital photography, I found that the subject of noise still fascinated me and decided that I had to make some measurements. Looking at the wiggly waveforms of my previous article might indicate that camera A is noisier than camera B but can we measure the noise in a rigorous way? This present article explains how to do that using free software. As well as presenting graphs of the measurements I have attempted to explain the results from physical principles – and evidently the noise is predominantly photon noise (aka shot noise).
In the second of two articles I look at another natural phenomenon, photon noise (also known as Shot noise). As with diffraction blur, the problem becomes more serious as the physical size of the sensor is reduced.
Whilst this is not the only source of noise, it is now the dominant one in the darker areas of an image where only a relatively small number of photons are incident on the sensor. It is the counting of photons, which is subject to Poisson statistics, which produces the noise.
Reducing the physical size of a camera, even if the total number of pixels is maintained, inevitably reduces the quality of the images because of two fundamental properties of light itself. This technical article looks at diffraction, usually explained by considering light as waves. A future article will look at photon noise, explained by considering light as particles.
A simple rule-of-thumb is established for determining the “diffraction limited f-number” by relating this to pixel pitch on the sensor.
The earlier, pre-digital, criterion for diffraction limited aperture (based on required print sharpness) is revisited and considered to be still valid – perhaps with a little sharpening.
When I last updated my page on this subject (2012) I decided that it was best to offer the test cards as .png files and leave it to the user to assign a profile (presumably sRGB) and to view the result in an application such as Photoshop. For convenience, I have now added a version of each test card with sRGB profiles assigned and saved as jpegs. These should be viewed in an application that is colour aware (i.e. one that recognises and uses the embedded profile).
Comparison of Demosaicing Methods available in Free, Open Source Raw Processors
My previous article included a table listing the various demosaicing algorithms offered by the four raw processors considered and I wondered why we (as users) needed such a wide choice. The table is reproduced below.I decided to investigate those offered by RawTherapee by looking closely at the detail in an image of tree branches against the sky – the same part of the same raw file processed by each of the algorithms.
Comparison of four free raw file processors: RawTherapee, Darktable, Lightzone and Photivo
With the exception of Darktable, which is not yet available for Windows, all of the applications are available for Windows, Mac and Linux. All are free and open source downloads.
I am looking for a raw file processor that will allow me to develop raw images to produce files ready for projection (at 1400 x 1050 pixels) and files at full resolution for further development, as necessary, to make high quality prints. I don’t expect to print directly from the raw processing application though this might be an advantage.
I have used both RawTherapee and Darktable for over a year and have recently tried Lightzone and Photivo so I will restrict my comments to these four.
As part of a recent event in Saddleworth, we were treated to a flypast of a DC47 Dakota. Perhaps a few tips on how to photograph this sort of subject would be useful to others facing a similar challenge.
Allowing the camera to make the decisions on speed and aperture is not a good idea in this case, so let’s get back to basics. When I first took up photography I was told that the only S A F E way to take a picture was to consider (shutter) Speed, Aperture, Focus, then Expose. Read the full article