Petegh wrote: DavidBM wrote: Petegh wrote:
'Same here. I find that I need to step down the 50/2 APO to f/4 for great off-axis performance. (Still not as good as CV 65/2 APO wide open)
I prefer using an extension tube with the CV 50/2 APO.'
Which extension tube are you using Fred?
An extension tube is just a tube, but some of the cheapest ones have mechanisms which can jam potentially damage contacts. So it’s probably worth getting the kenko ones, which are also flocked which is handy, and saves you flocking them yourself.
Thanks for your reply David - I will check out the Kenko ones.
My question to Fred was more along the lines of: which length of tube he uses? has he tried multiple lengths and found one to be the 'best' overall compromise in performance? and why he thinks an extension tube is better than an achromat close-up lens in the case of the Voigt 50mm f2?
I can tell you why I agree with him (which may or maynot be the same as the reasons Fred has!)
First I doubt there is an overall best compromise. What magnification do you want or need? You use what you have to use for the image you want to make. They normally come in a pair of 10 and 16mm tubes and your can use either or both.
The more extension, the more the magnification, the worse the quality, in a pretty linear fashion.
Second in general achromats work better as a lens gets longer, and less well as the lens gets shorter. In two different responds: the first respect is that magnification is higher for a given achromat (or indeed non achromatic diopter) the longer the lens. Pretty simple reason: the diopter fixes the focussing distance. So if it's fixed to, say, 50cm that's quite a bit of magnification on a long lens, but pretty much the usual MFD on a fifty. But they also seem to work better in terms of peripheral IQ on longer lenses.
Third, what works best depends in part on how good the lens already is at MFD; with a lens that is struggling at MFD (like the excellent CV 1.2/40) adding extension will only make it worse still. Adding a diopter, despite the optical compromises that involves, works better, because then the lens can be set to a focussing range in which it works better. But if the lens is already excellent at MFD like the 2/50 CV, then the slight degradation by making it focus closer with tubes is probably less degradation than adding tubes.
That's the theory anyway. I think @Fred Miranda has actual images that show the theory matters in practice...
