Rob, the standard definition of "normal" focal length for a format is the format's diagonal. It is pretty universally accepted for formats larger than 24 x 36. Cine-camera formats and derivatives -- 24 x 36 is double frame 18 x 24, the classic "35 mm" cine camera format -- use arbitrary definitions that bear no relation to the format. By convention, 24 x 36's normal focal length is 50 mm even though the format's diagonal is 43 mm. Except when the SLR manufacturer can't make a fast 50 mm lens that will clear the mirror, in which case "normal" is redefined as 58 mm. Whence the 58/1.4s "normal lenses" for 35 mm SLRs of the late '50s through mid-60s.
And then there's Humpty Dumpty. " “When I use a word,” Humpty Dumpty said, in a rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” That's you, I think. Ignorant or willfully idiosyncratic.
To get back on topic, 4x5's diagonal is approximately 150 mm.
Dan, my name is not Rob, first of all. Why do you think that it is? Nor is it Humpty Dumpty. Nor was my tone in my post "scornful." Nor was my post off topic.
If you read what I wrote more carefully, you should see that what I did was to equate a 4x5 lens on 4x5 to a 50mm lens on small format specifically, not to the standard definition of a "normal" lens (the diagonal), which would be 43mm on small format (and 163 mm on 4x5). I did not invent or try to push a new definition for the technical term "normal lens." In fact, in my first few sentences of the second paragraph of my post, I say, "210 is your standard lens on 4x5. They are just a hair long." Not only do I specifically
not use the term "normal," but I specifically say that the lens is "long," not normal.
Also, if you call a 50mm, 55mm, or 58mm lens normal on small format, then calling a 180 or 210 on 4x5 "normal" after explaining your point should not lead to confusion...but did I even do that, or was I very careful with my wording?
Nor did I tell the OP that he ought to strive for a "normal" lens. Quite the opposite, in fact. My point was to not be drawn in to what lens to choose by picking the one that meets the technical definition of "normal." A 50mm lens is not "normal" for small format; it is simply "standard," and that doesn't seem to bother anyone. If one gets a 150mm lens for 4x5 expecting it to "feel" like a 50mm lens on small format, then he or she will be disappointed. Based on the diagonal, a 150mm 4x5 lens is closest to a 40mm lens on small format, which is closer to "normal," but only 80 percent of the "standard" 50mm.
Additionally, to be clear, I even defined the common use of the term "normal" a bit later, by stating that it is based on the diagonal, but made it clear early on that I was talking about comparing horizontal AOVs. I was talking about horizontal AOVs not to redefine the term "normal," but to try to give a 4x5 equivalent of the width one gets with a standard small format lens. This was to prevent the OP from being disappointed if he were to select a FL based on comparing diagonals between formats with different aspect ratios.
I was making a point against the common "150mm on LF is equivalent to a 50mm on SF" statement that the OP is sure to encounter along the road, by saying that a 150mm actually seems quite more wide on 4x5 than a 50 seems on small format. Not only is a 150mm actually 8 percent wider than the technically "normal" lens for 4x5, but 50mm is actually 6 percent longer than the technically "normal" lens for 35mm format. (In other words, a 150mm lens is farther from "normal" on 4x5 than a 50mm lens is from "normal" on small format, though in the other direction.) Look at this difference (one 8 percent under normal and the other 6 percent over normal), then consider the difference in aspect ratio, and you have a notable difference between how a 50mm lens "feels" on small format and how a 150mm lens "feels" on large format. I said that if you look at horizontal AOV, a 180 mm lens gives you the most similar match to the horizontal AOV of a 50 mm on small format; then I made the point that a 210 is probably read by our brains as being the most similar to a 50mm lens on small format, due to the fatter frame of the 4:5 ratio.
I was also making a point against the common argument that a 210 sounds too long to be a standard lens for 4x5, by saying that it really is not that different than some standard lenses used for small format (which, again, are called "normal," even though they are not). And since you mention "convention," I should also say here that 210 is pretty much the most common standard 4x5 lens, even though it does not meet the technical definition of a normal lens, just like 50mm is pretty much the most common standard small format lens, even though it does not meet the technical definition of a normal lens.
So, again, my point was that if you like your 50mm lens on your small format camera, and that is what you want for 4x5, do not use the "normal" length lens on 4x5, but use the lens closest to a 50mm on small format, which would be either a 180 or a 210. I am making a point against the argument that a 150 lens should be ones standard lens on a 4x5 camera
if one wants a lens equivalent to 50mm lens on small format.
So, please reread and tell me where I have turned the world on end or scorned anybody by attempting to redefine the technical term "normal lens."