This is a comment on the article 'Focusing in the Digital Era - Part I' recently published on the LL site.
I am not going to argue here about the details and findings in the article. What I want to comment on is the author's basic premise that in the case of digital capture, degree of enlargement, defined as the percentage of increase of the sensor dimensions to produce a specific sized print, is of relevance. Assuming this premise results, for example, in the reasoning that an image captured by a (theoretically perfect) sensor smaller but of equal MP number than a larger (theoretically perfect) sensor can 'support' less 'enlargement', just because of its smaller geometric dimensions.
I find it hard to agree with this assertion, although it is a very common one in digital photography fora. A digital image, I believe, is a 'virtual' entity. It does not have dimensions per se, the way a piece of film has. By the time it is captured it exists in storage and it is independent of the geometric dimensions of the capture sensor. I can see no way of attributing absolute geometric dimensions to this image which are then somehow 'enlarged' to produce a print.
'Enlargement' and 'enlargeability' in the digital era depends on MP content, information amount so to speak. The degree of enlargeability is directly related to the amount of information contained and not on any non-existent geometric dimensions of the captured image.
So, can't we draw a parallel between digital and analogue capture with regards to 'enlargeability'? I believe we can but we have to go one level up in the film case to draw parallels. Examine why is that a larger piece of film can support more enlargement than a smaller one.
Information content as a measure of enlargeability is true for the film case as well. But in that case, assuming the use of the same film emulsion with a set ability for information capture per unit of area, a larger piece of film directly means more information storage capability.
Surely there are quantities directly related to the geometric dimensions and MP number of a capture sensor that will affect the DoF (Circle of Confusion), pixel pitch for one, but the author seems to ignore these. In fact, I have yet to see a conscise and convincing essay on the net dealing with the definition of the, largely subjective, CoC using terms applicable to the digital era.
I suppose this is a difference in semantics but a very important one I believe.
I would welcome constructive arguments for or against my premise.