I have no idea where the 1.4x rule or any of the other rules I have encountered originated. I have found that for my lower power observing more than 1.4x seems to be noticeable and useful. However, as magnification creeps up, such as when observing planets, I prefer smaller increases.

I have also noticed that when using my long focal length Maks, small differences in eyepiece focal length give very noticeable magnification increases. In my short focal length refractors, larger differences in eyepiece focal length don't seem to give quite as much of a noticeable difference.

Ed D

 

I think the "rule", as it were, is just a simple function of how we humans perceive things. For example, decibels are a logarithmic scale used to measure sound. Every 10 decibels is double the sound energy as 10 decibels less. In other words, 50 decibels is twice as loud as 40; 100 is twice as loud as 90.

The same is true of brightness, which is why we measure magnitude the same way - logarithmically. 1st magnitude is approximately 2.5x brighter than 2nd, etc., such that as between 1st and 6th magnitude, there is a difference in brightness of ONE HUNDRED times.

The human brain compresses these huge differences, both in sound and in brightness, so that they are not quite as noticeable as you would otherwise think them to be. This is why when they say something like that a 6" scope collects 44% more light than a 5" one (or something like that), it's just not that big a deal. That's not even a third of a magnitude. And I think the human brain compresses differences in magnification, too.

Me, I never heard of this 1.4x magnification "rule" before until I read this post. But I would put it at more like 1.6x. I have both a 32mm and a 25mm Plossl. I use the 32mm all the time, as it gets me just about the widest field of view possible in my Mak at 48x. But I never use the 25mm in my Mak to go to 62x. That's just short of a 1.4x jump. Why don't I? Because 1.4x doesn't do it for me. I'm just not seeing that much more when I go from one to the other.

But a 1.6x jump - to 77x - well, now you're talking. That's significantly larger, more detail. It's quite noticeable, where the 1.4x jump isn't. In my Mak, that would be a 20mm eyepiece.

I recently got a 24mm ES 68, which I'm thinking about using in place of my 32mm Plossl, because the 24 gives me a smidge wider field of view than the 32, having to do with ES pushing out the field stop to as wide as it can go in a 1.25" EP - just like your 35mm Eudiascopic. Going from the 24mm at 64x to my 15mm Paradigm at 103x is a nice, noticeable jump - and is almost exactly 1.6x difference.

Another example is at the higher end. I have a nice 12.5 mm ortho at 123x that I use to start out with some high-powered lunar/planetary observing. From there, I go to an 8mm TeleVue at 193x. That's just short of a 1.6x jump. And that jump makes a big, very noticeable difference in the detail I'm seeing.

When the seeing is good enough, I'll take the TeleVue out (193x), and Barlow the ortho for 246x. Even though it seems like a large jump in terms of the magnification numbers, the magnification, visually, it's just not that big of a difference, really - because it's just short of a 1.3x jump. And that difference isn't enough for the brain to go "Wow, that's much bigger than what I was just looking at!"

 

I have noticed that a lot of eyepiece series are in 1.4, 1.5 or 1.6x jumps.
When working out a set for a new scope I will use 1.5 and 1.6x normally.
You usually end up with a set that overlaps enough that you do not need a barlow and using the other gives you enough gaps to then include a 2x barlow.

 

I agree that the jump is noticeable. My planetary set goes from 3.8-8mm in roughly 0.5mm steps which give about 30x mag difference. From 8-28 they are in steps giving me the same mag difference.

 

With my planetary eyepiece set (0.965" & 1.25" Orthoscopics)
As you can see there is a 20x jump just between the 3.8mm and 4mm eyepiece so small increments can make very large differences.
After 8mm using full focal lengths give the same jump until 12mm. After that it is about 20x which is more subtle and depending on conditions you can start jumping 2-3 focal lengths at a time.
3.8mm - 0.47mm/ 316x
4.0mm - 0.5mm/ 300x
4.4mm - 0.55mm/ 273x
5.0mm - 0.63mm/ 240x
5.4mm - 0.68mm/ 222x
6.0mm - 0.75mm/ 200x
6.4mm - 0.8mm/ 188x
7.0mm - 0.88mm/ 171x
7.4mm - 0.93mm/162x
8.0mm - 1mm/ 150x
The 0.4 f/l jumps are for maximizing what the atmosphere is allowing you to see.
I like going after as much detail as possible on targets

 

Thanks, it has taken a few years and a shameful amount of $ to put the set together though the full set goes from 3.8- 28mm.
I forgot to mention that those exit pupil and magnifications are with my 150mm f/8

 

I like the way you think!
Very true, I love a good Orthoscopic. It is sad that the only really good ones being made right now are the Takahashi Abbe and the Fujiyama HD.

 

The Baader "Genuine" ortho was made by the same company, the "Classic" is made elsewhere. I cannot remember offhand.
The BCO according to Baader uses Schott glass and the Fujiyama uses the equivalent Hoya glass.
Do you find the BCO a touch soft at the edge with the wider AFOV?

 

I remember reading a post by BillP that he had said the field stop had been moved in the BCO to give it the 50* FOV but it made it quite soft at the edge. I have not tried the BCO but they seem to have a good following.
The majority of my orthos are between 30-45* so 50* is extravagant!
The .4mm orthos I have are the Siebert Optics StarSplitter 3 series and are quite nice and 60* AFOV.
I believe they are a reversed Kellner like the Brandons.

 

I know a few people that have the 18 and 32mm and they like them. The 10mm seems to be the favorite with people.
I suppose I should borrow a set when I get home and give them a try.
I prefer regular eyecups to the winged ones or to not use them at all depending where I am viewing from.