Bower 500mm f/8 Mirror lens

On 17/07/2015 10:10 a.m., RichA wrote:
On Thursday, 16 July 2015 16:54:56 UTC-4, Me wrote:
On 16/07/2015 1:17 p.m., RichA wrote:
On Tuesday, 14 July 2015 04:11:48 UTC-4, Eric Stevens wrote:
Does anyone have knowledge or experience of these? How good are they?
(For a particular value of 'good')
--

Regards,

Eric Stevens

Even with focusing aids, 500mm with an APS or m4/3rds camera is a pain to focus absolutely accurately. Also, mirror lenses were never really meant for daytime use. They started out as telescopes. Reason is that light can (even with a baffle tube coming from the primary mirror centre hole) fall in the camera sensor from outside the light cone, reducing contrast. This can be dealt with pretty well in post-processing. Also, mirror lenses of that size are susceptible to heat waves, much more so than smaller aperture camera lenses. This means close-in (100ft or less) objects are generally ok to shoot, but anything beyond that that isn't high above the ground tends to blur. Ironically, the lenses are optimized to focus at infinity which means you get a small amount of spherical aberration when focusing on things close-in, but it's better than heat waves.

Why would atmospheric refraction/shimmer affect a mirror lens more than
a normal telephoto?

Not unless the normal telephoto had a larger front lens diameter. It's all predicated on that. Over 4" diameter, heat waves are very bad, but even less than that, if you are looking through a significant atmospheric blanket, it's bad.

Still doesn't make sense to me, as at the distances where shimmer is a
common problem (more than 50m or so - but of course it depends) then the
difference in angle between subject and the perimeter of the front
element (or mirror) is so insignificant, I doubt you'd see it.

Here's an example of some shimmer at only 300mm or so (100% crop), taken
on cool clear calm winter morning before sunrise:
http://i.imgur.com/NZBRL2w.jpg
The small island is about 40km, already degraded by atmospherics, the
low hills about 80km, the mountains about 150km.