If the scope is very large, the secondary will be fairly small, percentage-wise, and f/4 can give astounding views in, say, a 32" if the seeing is excellent, the scope is well-collimated, and the mirror is cooled.
I am curious on this topic as I sometimes see that claim by scope makers. I am especially curious as to honest opinions regarding the <f/4 large dobs. We all know you get a brighter image and can see more, but does anyone feel that you get as crisp an image as an APO?
My 12.5" always beats the 5" and 6" apos in the field, but that's not really a fair comparison. Alas, I have no 10-12" apo refractors handy to compare to it. But, for the price of my high-end 12.5" f/5 with Zambuto mirror, I could have bought a nice 4" apo on a cheap mount.
Nah, 12.5" will ALWAYS show more.
The point is, resolution is aperture-related.
But large apertures are all seeing-limited, too.
In my experience, the seeing always varies enough for the larger apertures to show off more in the way of lunar and planetary details than the smaller scopes. And, they're usable at much higher powers. 200X is really pushing a 4", but a 32" isn't really pushing the limits at 1000X.
Even in mediocre seeing, a 32" will handle 300X OK, while the smaller scope may be limited to <150X or even 100X.
Lots of people argue that they prefer the aesthetics of a view that, though it has lower absolute resolution, is much more stable and consistent. They use that to justify why they prefer their 4" and 5" apos to a big scope.
The thing is, the average resolution of their 4" and 5" apos will be what you see though a good 20" reflector when the seeing is bad, and during moments of truly steady seeing it will be as if the wax paper was ripped away and you are seeing the object in more detail than typical photographs.
I had occasion, recently, to spend some time on the Moon at first quarter with a TeleVue NP101, and the image of the Moon was as photographic as you can get--simply stunning. Contrast was superb.
Because we were having a period of really steady seeing, I took my 12.5" out to look at the Moon. Bear in mind that this scope very rarely sees anything that bright. I use it mostly for DSOs. The image of the Moon almost made me cry. At 304X, the image was dead-steady and the surface of the Moon looked littered with debris and not smooth at all.
Small craterlets were visible inside all the large craters and buried craters were everywhere. And each of the craters near the terminator had its walls casting saw-toothed shadows that looked so sharp and stark it was as if I was in orbit over the Moon. The images of the Moon I saw were simply so far beyond what the NP101 could do that the images were burned into my brain as examples of what can and should be seen when looking at the Moon.
Now, when I look through my excellent 5" Mak or the TV101, I am just disappointed at the softness and blurriness of the image, i.e. the low resolution. And both of those scopes are superb examples of their breeds.
So was it a fair comparison? No. And would an equivalent 12.5" apo refractor beat the reflector? Probably. But I could carry the 12.5" reflector in a Lamborghini Murcielago for the price the 12.5" apo and mount would cost (not counting the dome to house it in).
So, when you are talking high-resolution per aperture, the refractor wins up to about 5 or 6". But that's a pretty small reflector.
And the reflectors start revealing lots of details simply not visible in the refractors once you get to 12" or more. And by the time you get to a scope like a 24" or 28", no one you know or ever will know will see details in a refractor that can be commonly revealed in the reflector.
And that's only talking Moon and planets. Then there's the hundreds of thousands of other objects the big scopes can see the small scopes can only see through long exposures with an expensive camera.
Crispness of image is seeing related, and the small refractor will probably not be seeing-limited most of the time. The large reflector will be seeing-limited most of the time. But not ALL of the time, and therein lies the reason why a big reflector is also the best instrument for high-resolution lunar and planetary use. It also goes without saying it's best on DSOs.