image stabilization - Where does the ¹/shutter speed = focal length rule for hand shake come from?

The generally accepted rule of thumb is that the shutter speed must be the same or larger than the inverse of the focal length.

As is, it seems that it makes no sense as is:

1. 
   

   On a 24 Mpixels full-frame camera, at 100%, the blur from camera movement will be more visible than on a 10 Mpixels full-frame camera.

   
   


2. 
   

   A photo intended to be printed small can have slight blur at 100%: nobody will see it when scaled down for printing. When doing a high-quality large print, even a small blur will be noticeable.

   
   
   


3. 
   

   Image stabilization (vibration reduction) affects the blur when shooting handheld.

   
   


4. 
   

   The blur will not be the same on a cropped vs. full-frame sensor.

   
   

I imagine that the rule of thumb appeared first when there was no DSLRs yet, and photographers were talking about SLRs with 35mm film. Is this that fact that makes the three of four points irrelevant? If yes, what about the second point? If not, what is the origin of this rule?

Answer

I did some quick Google Books searches, and while I can't pinpoint the origin, there are a number of references to it as a rule of thumb or general guideline in the early 1970s, and none that I can find before that. There are plenty of earlier references to the idea that a longer focal length requires a faster shutter but they're all general advice.

The first reference I find is from Popular Photography in 1972:

A rule that will help you determine the slowest hand-held shutter speed to use is: place the number one over the focal length of the lens (in millimeters). For example, with a 100-mm lens, one over 100 is ¹⁄₁₀₀ (¹⁄₁₂₅ would be the closest speed to set); with a 250-mm lens, the rule gives ¹⁄₂₅₀ sec. Use this rule as a guide. You may be able to hold for somewhat slower speeds if you're steady and your camera holding technique is good. If you're shaky, you may have to shoot at a faster speed than the rule indicates. Experience will tell this. If in doubt, use a tripod or other firm support and a cable release, when possible.

A year or so later, I found this

You can minimize or completely eliminate camera movement if you remember this rule: For hand-held shooting, don't use a shutter speed any slower than the focal length of the lens. The normal lens on a 35mm camera is is 50 to 55mm. When using this lens, set the shutter at ¹⁄₆₀th second. ... — Walter Chandoha, How to Photograph Cats, Dogs, and Other Animals, Crown Publishers, 1973

I doubt that either of these is the first occurance, though. There's a whole bunch of examples from around the same time, like this:

A rule of thumb is to use a shutter speed at least as high as the focal length of the lens: a 60th for the 50mm, 125th for the 105mm, 250th for the 200mm, and so on. But experience may show you are steadier or shakier than this rule assumes. — Robert Foothorap and Vickie Golden, Independent Photography: a biased guide to 35mm technique and equipment for the beginner, the student, and the artist, Simon and Schuster, 1975

So, I don't know exactly where it came from, but it's definitely an idea for 35mm film, and it's clear that in its early form, it was seen as a general guide, not a law.