The Recent F/stop Controversy

Started 1 month ago | Discussions thread
fferreres Senior Member • Posts: 1,499
Re: The Recent F/stop Controversy

Michael Fryd wrote:

Bill Ferris wrote:

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?

I would say that for digital photographers the actual aperture diameter is more important than the ratio of focal length to diameter.

You stated it so many times. We get your point of view and belief. I don't think anybody trying to follow you has missed you make this point over and over.

Yes, if you know the focal length you can convert from one to the other. But I would say one is better off using the value that's relevant, rather than a related value.

If you state that one would say one is better using the value that's relevant, you are stating that the use of f-stop is irrelevant for everyone. You are generalizing to everyone, not just to your case and workflow.

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.


But don't forget other implementation details that perhaps are more important that light per unit area. For instance number of photons hitting each pixel.

We tend not to talk about exposure in terms of photons per pixel because that's an implementation detail, and tells us very little about the final image unless we know how many pixels there are.

Exactly. We tend to not talk about exposure as photons per pixel, or mm apertures, because those are implementation details. We care about exposure, regardless of medium.

My point is that with digital, light per unit area (traditionally called "exposure") is an implementation detail, and tells us very little about the final image unless we know the area of the sensor.


On the other hand total light gathered tells us useful information about the final image without knowing the specifics of either the pixel or sensor size.

It doesn't "tells us"...maybe it "tells you". I want to make sure I am at ISO 100, or whatever is the optimum exposure amount, not less or more. I don't want everyone to care, but if they want the least amount of noise, and not having clipped outputs, I'd advise anyone to care a bit more. However, they all know this already.

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 tradition of describing apertures as the ratio is intended to make it easy to hit a particular target exposure (light per unit area) on the film.

Apertures have been described this way even in most all applications that do NOT involve film at all. Actually, many of these application that do not involve any kind of light recording mechanism. And many of these, do not even involve exposure, light intensity or anything of sorts. They are describing a lens property, regardless of the scale at which the lens is built.

Obviously, it tells us the light per unit area when shooting digital, or other light sensitive mediums.

I think it describes a lens property. You may be having a projector, where there is no light sensitive medium. Using a specific metric to designate the aperture, will tell you something only in relation to the scale of such lens. If I shrink the lens proportionally, the f-stop doesn't change But the max aperture diameter will.

My point is that with digital this implementation detail is not that important in terms of what we will see in the final image. Therefore we are better off with a system that backs out unneeded implementation details.

So you are proposing Auto-ISO, and that this detail be hidden as an implementation detail that the user should care about. This follows immediately from what you are saying, and have repeated many times already. Why? In you statement, because now digital can operate in other ISOs where as film was limited to a certain ISO unless you wanted to change the film.

However, "we are not better off". Maybe you are better off. I don't want the camera you are describing.

When freed from the need to be a slave to traditional S-shaped film response curve, we have the opportunity to elevate the priority of depth of field and the field of view.

You have the opportunity. In my system, I don't need to elevate DOF any further. It's very very high and having mm designated apertures would make it very difficult for me to know if the aperture is staying the same in relation to the focal length when using a zoom, to name 1 out of 250 examples. I don't want the opportunity that my constant aperture 24-105L f/4 now is designated with a variable mm.

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.

The meaning of f/stop is clear. What changes with digital is the priority of hitting a particular light per unit area.

No. Maybe it changes for you and some others. I still want to know the intensity of light, the ISO, the exposure and be able to control it by doubling it and halving it.

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