Madweasel wrote in post #8072454
Well, you don't say what body you're using, but either way the difference between the two focal lengths is significant. In the days when primes were the only sensible choice, it was normal to go for steps of 30-50% increase in focal length. These two are that sort of interval apart.
Actually, there generally were two ranges of lenses, roughly based on the 50 mm FL (considered to be the FL of the "standard" lens for FF), and on the ~43 mm FL (considered to be the "true" standard lens FL for FF). Basically, the lenses were steps of ~1.4X away from each other, or ~0.7X going downwards.
Of course, this depended on whether a lens manufacturer actually could design and produce such a lens. Often you'll find slight discrepancies in the ranges between manufacturers, plus, they didn't always produce all the lenses at all steps.
Also note that the size of these steps is not a pure coincidence. The factor indicates a magnification factor, one very common in photography, namely the square root of 2 and/or the inverse of the square root of 2.
Furthermore, you'll also find that the FL indicated on lenses is not always very exact. A difference of up to 8 % or thereabouts (generally 5 %) is considered acceptable, so a 43 mm may be marked as a 40 mm or a 45 mm, although you'll often find that FLs of telelenses are quite often slightly exaggerated (seemingly having more range), f.e., a 380 marked as 400 mm, and WA lenses often rounded down, to make them more WA than they really are (21 to 20 f.e., or 17 to 16). BTW, talking rectilinear lenses here; fisheyes are excluded as there is no need to hide their true FLs as per above .
Anyway, here they are, with in red italics the lens determining the range, and in bold probably the most common lenses on each step.
14/15 - 19/20/21 - 28/29/30 - 40/43/45 - 55/56/57/58/60 - 80/85/86/90 - 110/120 - 170/180 - 240/250 - 350 - 480/500 - 680/700 - 1000
12 - 16/17/18 - 24/25 - 35 - 50/52 - 70/75 - 100/105 - 140/135 - 200 - 280/300 - 400 - 560/600 - 800 - 1100/1200
To come back to the factor indicated above, this actually is a linear magnification factor. If you look at it from a FoV POV, you'll find that the FoV increases by a factor 2 each time you step down in the list (1.4 X 1.4), and decreases by 50 % (factor 0.5, 0.7 X 0.7) going up. This is a difference a human can perceive quite well, which is why it fits so nicely. Less than a factor 1.4 (or 2 in FoV mode), is too small for most people.
The interesting thing is that if one alternates steps from one series to the other, skipping the nearest next lens in the other range, you get an approximate factor of 1.7 X going up, IOW, an increase in FoV by about 3 going down, and decrease by about 2/3 going up (FoV factor of 1/3).
Just skipping a lens every time in any row, gives one a factor of 2, or a 1/4 of the FoV going up, and 4X the FoV going down.
This allows one to really pick a nice, comfortable line-up, and of course, one doesn't have to follow the rules of the lens designers: one can leave out one or many, or odd steps, in a lens line-up .
BTW, these ranges also explain zoom ranges. These steps are the reason why one doesn't normally see odd zoom ranges; these also use the guidelines provided by the steps in the above ranges.
Finally, with the advent of APS-C, many manufacturers have come up with lenses specifically designed for APS-C rather than FF, but even here you'll find that these follow a similar set of rules. Just multiply by 1.6 or 1.5 going up, to see where they fit .
Kind regards, Wim