One fly in the ointment, so to speak, of a linear progression of magnifications:
Many "planetary" observers like their high magnifications to be much closer together because they are always pushing the limits of seeing
and smaller jumps at the high end allow them to maximize magnification while pushing against the ceiling allowed by seeing conditions.
So, though a 40-50% jump may be appropriate for low powers, much smaller jumps may be appropriate at high power, like 15-20%.
I've seen sets of 6mm, 5mm, 4mm, 3.5mm, 3mm, 2.5mm, 2mm, as an example.
Additionally, I find that close jumps at low power usually aren't necessary. If I start out with a really good low power that frames objects well,
if I feel I need a higher power to improve resolution or view smaller details, anywhere from a 60% to 100% jump is just fine for the next step up.
Having a bunch of low powers usually results in my just using one eyepiece to the exclusion of all the others.
Then there is the rational philosophy of having magnifications be closer together near the "sweet spot" of magnification for the type of object viewed.
So if I like a 10mm eyepiece for most of my observing of, say, galaxies, then I might want a 25% increase or decrease from that for other sizes
of object or details, while as I move farther from the sweet spot I can tolerate larger jumps like 40% or 50% or even 60%.
Among the eyepieces I own, I tend to use them in subsets, dependent on seeing conditions or objects viewed.
I either use a set where the increases from low power are: 61%, 63%, 70% or 71%, 66%, 62%. I always have the in-between eyepieces there if I need them, but I tend to use a subset of 4 most of the time: 21..13..8..4.7mm or 17..10..6..3.7mm
But we all will use a much larger skip for particular objects. I'll use a low power to find a planetary nebula, but then immediately jump to a much higher power to view it.
I have often used a 87x to 304x jump because long experience shows me I'll use the 304X to view the object anyway. Why waste my time with in-between magnifications I won't stay with?
So there are magnification protocols that may require a completely different set of eyepieces than Barry's progression. Even in an SCT, I'd find the 20mm to 40mm jump just fine, and I'd
see no reason for the 28 or 56mm eyepieces.
Edited by Starman1, 14 February 2018 - 10:42 AM.