The Recent F/stop Controversy

Started 1 month ago | Discussions thread
Bill Ferris
Bill Ferris Veteran Member • Posts: 4,085
Re: The Recent F/stop Controversy

Michael Fryd wrote:

it's not so much a technical question as a philosophical one. Is the frame of reference designed for the limitations of film the best frame of reference to use for digital?

Perhaps, I am misreading your comment and question. It appears you consider f-stop to be more relevant to film than to digital. Or at the very least, you see f-stop as a concept or setting that may be more relevant to film than to digital. Is that accurate?

As you and probably everybody participating in this discussion know, the focal ratio of a lens (aka f-stop, f/ratio) describes the ratio of the focal length of the lens to the diameter of the aperture of the lens. Both dimensions are typically measured in millimeters. For example, a 100mm lens with an aperture diameter of 25mm, is described as, f/4. That's the lens's f/ratio. It's the f-stop chosen by the photographer.

The use of f-stop to describe the physical characteristics of a lens applies regardless of the light-sensitive medium used. Whether that medium is a chemical emulsion or a collection of pixels, f-stop has the same meaning. Also regardless of the light-sensitive medium used, f-stop along with scene brightness and shutter speed define exposure: the average brightness of the scene at the image plane. This is another fact of which you and most others in this thread are well-aware.

Within this context, I would say it is more accurate to describe, f-stop, as a concept adopted as a tool of photography than as a concept linked more closely to film or any other mode of containing the light-sensitive medium. The term has no different meaning to a photographer using a chemical emulsion (e.g. glass plate, tin type, film) than to a photographer using pixels (e.g. CCD or CMOS) to capture light. It has no different influence on exposure.

The profound physical and performance differences between chemical emulsions and pixels along with the relative performance characteristics of the modes (glass plates, tin types, film stock, sensor formats) used to precisely positioning these light-sensitive media at the image plane introduce a host of issues relevant to the photographic process. Equivalence, is but one. Frankly, there are too many to adequately address in a collection of books, let alone a single online forum discussion thread.

However, f-stop, is a term having a clear definition. Any lack of clarity on that point in this post, is my failure; not a failure of the long-established and accepted definition of the term. Its meaning and role in determining exposure are the same, regardless of the medium or mode used in the photographic process.

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