Thanks Bruce...also, gosh, I need to cut down my replies, by a bunch...leaving this up just for chuckles I guess. Apologize for the rough tone, Morris Sullivan, but it seems that everybody is quick to tie shoelaces together over some perceived differences on the 'net.
All that you can make equivalent (at least reasonably) from cropping a wide-angle lens is the FOV / AOV. There will still be many problems. You've certainly guessed that you will be dealing with fewer details because the lens isn't meant as a telephoto, but the depth of field will also not behave the same as if you'd taken the photo with a longer lens to begin with.
This is incorrect. When you use a longer focal length you are essentially cropping with your camera. It is the same action that would be taking place in a traditional film enlarger. The image gets bigger and whats outside the sensor gets cropped off. No different than cropping off the edges and enlarging it on your computer, aside from the resolution loss.
I don't see where you have acknowledged, let alone rebutted, any of my specific claims, which you will find to be true: 1.) Cropping an image from a wide-angle lens is not ideal, but possible, but will only change the angle of view; 2.) the actual merits of using this approach generally are less than that of shooting a longer lens; and 3.) modifying depth of field (for example) after the fact by cropping is impossible (unless you consider extensive photo retouching acceptable).
What you say about "essentially cropping with your camera" seems needlessly confusing, because 1.) the sensor is not being cropped, taking in less of the light focused at the sensor (as with a 1.6 crop sensor, which is very different from using an enlarger or a reversed lens), 2.) the image is not being cropped either, since the image is only comprised of that light focused by the lens, and 3.) in fact, NOTHING is being cropped "when you use a longer focal length," since the job of a telephoto lens (and any other) is to focus rays of light into a specific flat area! Unless, I suppose, you're talking about cropping in the sense of going out into the real world and boxing out the incoming light, which makes no sense in terms of simplifying how lenses (or croppoing) works to be easily understood (while still accurate).
The main point which should not be lost is that, whenever possible, one should use a lens suited to the task, because simply cropping away an image will not be the same. The ideas of "pixels on taret" (known to wildlife photographers) and of controlling depth of field are
practical reasons why one should avoid this.
The way you appear to be using (I cannot be sure what you mean, actually) the term "cropping" here is of no use to the OP. Cropping usually refers to a couple specific, and related, ideas, and saying something like "well it's almost the same as using a telephoto lens!" is completely missing the point, like saying that "using chocolate on your hamburgers is a bit like using ketchup, because it's also food!" It completely misses the point of why you might want to do one and not the other, and is confusing besides.
The next myth that page discusses, "Larger sensor systems are bulky and heavy," will also be of use to you.
Nobody has mentioned "Larger sensor systems" untill you did.
That's mighty presumptuous of you, thinking you know the content of a page from my having copied down the section heading for the OP's reference.
On the blocked Joseph James Photography website, that section links to the concepts being discussed (I hope) here. For whatever reason the powers that be have blocked the URL. Great Bustard linked this same website to make a point that is still somewhat relevant for this thread, here:
http://forums.dpreview.com/forums/read.asp?forum=1029&message=37923258
So while no two photos from two different systems will ever be equal, Equivalent photos from different systems will be as similar as photos from different systems will get. Clearly, however, the point of choosing one system over another is not simple to get photos as close as possible to other systems (equivalent photos), but to get photos that look "better" (in our eyes) to what other systems can deliver (non-equivalent photos), or for the differences in operation (AF speed/accuracy, size, weight, frame rate, build, price, etc.).
In other words, completely apart from the issue of whether one thing is equivalent to something else is the question of - does it make
sense to do it this way (unless you absolutely have to)?
Total takeaway from this exchange: Words still have meanings, and talking about "equivalency" is too confusing, most of the time. (Including when one says "essentially" in place of "is equivalent to," as in "essentially cropping with your camera.")
