R5 lunar photography with stacked EF extenders

Bigger wrote:

Larry Rexley wrote:

I should clarify --- I wasn't referring to a loss of 50% light loss by adding teleconverters, I was referring to a 50% loss in the whole optical path (some lens reviews use T-stop vs f-stop). This is useful when comparing with telescopes dedicated for lunar or planetary use.

DXOmark measured the EF 600mm F/4L IS II USM at T/4.5, so that's (4/4.5)^2=79% transmission. I can't find measurements for the TC. If we use the guideline in your link below, the EF 1.4x III has 3 groups, and the EF 2x III has 5 groups, for a total of 8 groups, and 0.985^(8*2)=79%. So total light loss is 62%. But for lunar photography, this is really a non-issue because there is plenty of light.

https://www.dpreview.com/forums/thread/4608842

Thanks for the link. Lots of info there.

If the point of this discussion is to see what Moon photos can be gotten with telephotos and equipment already owned --- that's a very fun thing to do and I've done it too --- even bought cheap vintage lenses for $20 and $30 in the hopes I could get good Moon shots, made my own teleconverters, tried a wide variety of teleconverters to see which performed the best, etc. If I happen to be out shooting in the evening and see cool Moon or a conjunction, I'll still shoot it with my telephoto and teleconverter.

Well, yes, that's the point of a thread on shooting the moon with stacked TC.

I don't mean to spoil the party, but if the end goal is high quality lunar or planetary photography, I eventually realized that stacking teleconverters is not an economical way to do this and get the best results, for the reasons I mentioned above. One very high quality teleconverter does generally produce very good results with a high quality telephotos, but by the second I think you are at the point of diminishing returns already. Even if you still get some resolution benefit from another stack, you're still lagging further behind a good telescope's performance of similar aperture.

What do you see as the tradeoffs between a lens and telescope? Obviously, the extra TC increases CA, but that's easily removed in post now. You mentioned light transmission, but I don't think that really makes any practical difference since you can shoot a big stack in several seconds anyway, so even if it takes twice as many seconds, NBD.

My earlier post in this discussion above with a very portable Meade ETX-90 ($100 used or ~ $500 new) on a night with average 'seeing' did about as well as some $2000 - $3000 combinations posted here. For that kind of spend, $1000 - $2000 will get you a reflector or refractor of 6" or more aperture which will put any Moon photo posted here to shame.

Sure, aperture diameter is everything. But what about a 6" scope, other than price, would make it better than a 6" lens, e.g., 600mm f/4 for lunar photography?

Telescopes have diffraction limited resolution (and are usually used at f/20 - f/40 for high resolution solar, lunar and planetrary images). For infinity focus only.

Telephoto lenses do not have diffraction limited optics (residual abberatins are larger than the airy disk). But they are well corrected for a wide focus range, from near to far.

So different kinds of optics for different jobs. Both are optimized for the job at hand.