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I am looking to buy a mirror lens for the upcoming solar eclipse. I am using a Canon with an EF mount.

Most of the lens options I see require a T-Mount adapter to fit on my canon. Are there any image quality/focus limitation/other drawbacks I should consider before purchasing one of these lenses?

With other adapter types, I know there is the inherent problem of loosing inifinity focus. Does this same limitation apply to other adapters, including the T-Mount adapter?

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Theoretically speaking, using a T-mount lens to shoot an eclipse should create no such issues related to infinity focus/image quality/etc. Most DSLRs that shoot through a telescope are attached via T-mount adapters and, depending on the quality of the telescope's optics and the technique of the photographer, images taken using such an arrangement can be quite spectacular.

Infinity focus is not an issue because the T-mount specification has a flange focal distance of 55mm. Most DSLRs and even older 35mm film camera systems have FFDs in the 40-48mm range.The adapter that screws onto the 42x0.75mm T-threads needs to be anywhere from 7mm to 15mm thick to fill in the distance between the camera and the lens. Thus no additional optics are needed to achieve infinity focus.

Of course there is no communication between the lens and the camera so you need to be sure to use a camera that can shoot in Manual exposure mode without a lens detected. There are a few entry level cameras from various manufacturers that do not have such a capability.

You'll also need to focus manually, but for astrophotography that's not much of an issue since 1) Most astrophotography is done at infinity focus and 2) Most camera's can't AF on objects in the the night sky other than the moon. Depending on what type of solar filter you use to image the sun, even if you had an AF capable lens it might or might not AF correctly through the filter. In any case careful manual focus, typically using magnified Live View, will usually get better results with astrophotography.

In the real world, though, things are a bit different. Most mirror lenses designed to be primarily a camera lens that have a T-thread at the rear of the lens for use with cameras via a T-mount adapter aren't that good optically. This is particularly the case with the cheapest ones. You're also limited to a single aperture with almost all such lenses, which may reduce your options regarding ISO and shutter time. Note that cheap refractive lenses intended primarily as photographic lenses that have T-mount connectors are even worse optically than their mirrored counterparts.

Higher quality mirror lenses tend to be made with specific lens mount connectors permanently attached to the rear of the lens, rather than T-threads for a T-mount adapter.

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No problems with infinity focus with T mount lenses. The T mount was designed specifically to let you use the same T mount lens with different camera brands by using the appropriate T Ring for the camera - the depth of the T ring compensates for the different camera flange to film/sensor distances.

Big drawback with the T mount is that there's no mechanical or electronic aperture coupling available - which means that a lot of entry level DSLRs won't meter with the lens at all (higher end Nikons that didn't need the electronic coupling worked OK; not sure what the situation is for Canon bodies). Also, native Canon lenses can have a slightly bigger clear aperture than the T mount diameter - so the native lenses can go to slightly faster max aperture values than a T mount equivalent, but that's not usually an issue for the sort of lenses that come in T mount fittings these days.

I've used five different mirror lenses over the years:

-An old Meade 1000mm f11 camera lens/spotting scope (basically, this was a camera lens focusing style mak scope that worked either as a camera lens (with an extension tube to a T ring) or a telescope (with a star diagonal).

-An old Tamron 500mm / f8 mirror lens with an adaptall mount (similar idea to the T mount, but extended so that (with other lenses) you had basic mechanical aperture coupling. The mirror lens was fixed at f8, though

  • A Minolta 300mm AF mirror lens for their APS SLR

  • Old and newer versions of the Nikon 500/f8 mirror lens.

Of those, the AF on the Minolta lens worked fine - I'm surprised there aren't more AF mirrors around.

The Tamron and the newer Nikon also made nice butterfly lenses - they both focused close enough that you could get a butterfly to a decent size in the frame from far enough away that you didn't frighten it off. The older Nikon had a much longer minimum focus distance, which is why I eventually switched to the newer version. Quality on the Nikons was higher than the Tamron, but the Tamron was pretty good - and packed down smaller than the Nikon (The lens hood reversed around the lens for transport, which gave you a shorter body).

The 1000mm Meade lens worked pretty well, on a tripod for wildlife shots or handheld in bright lighting for things like motor racing shots. But it benefitted from improvising a lens hood, since the front element was basically at the front of the tube. Usable without, but contrast improved with the hood.

These days, I usually use my 80-400mm Nikon image stabilised zoom - it's not quite as long as the 500mm mirrors were, but the AF and VR capability make up for that, and the close focus at 400mm also makes for a nice butterfly/ large insect lens. Or if it's worth the hassle of carting a heavy manual focus setup along, then I can use my 106mm astrograph refractor at 530mm f5 or 840mm f8, which gives extremely high quality results. But by the time you add a suitable tripod and mount, it's bulky and heavy enough that it's really restricted to specially planned trips - not something you'd normally walk around with.

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  • Of the various mirror lenses you mention, it appears only the first (1000mm Meade) is a T-mount lens. Is this correct? – Michael C Jul 9 '17 at 18:16
  • Yes. The Nikons were Nikon mount, and the Tamron 500mm used their adaptall mount - a similar idea to their original T mount (That's where the "T" comes from ) but extended to allow mechanical aperture control and meter coupling. Since the T mount has no mechanical or electronic linkages for the aperture, it's restricted to fixed aperture lenses (like mirror ones) or preset aperture ones (where there's one ring to set the aperture, and second to open/close it so you can fully open the aperture to focus, or close it down to the preset value to take the picture or check depth of field. – JerryTheC Jul 9 '17 at 22:26
  • T mount adapters and T rings (and sometimes so-called "wide T" versions that use a larger diameter (some are 48mm vs the normal T mount 42mm diameter for more clear aperture) are also commonly used to connect cameras to astronomical telescopes - either by having a T thread on the scope focuser (or via a threaded adapter to the T thread), with a T ring attached, or via a 2" eyepiece barrel to T thread adapter with a T ring (Though you can also get one piece 2" barrel to camera mount adapters, which may allow more clear aperture than the 42mm diameter T threaded tube). – JerryTheC Jul 9 '17 at 22:39
  • Yes. But those telescopes with 48mm wide T-mounts, or even the higher quality instruments with 42mm T-mounts, are primarily intended for use as astronomical telescopes, not as all purpose camera lenses. Most cheap mirror lenses with T-mount adapters are the later, not the former. – Michael C Jul 9 '17 at 22:51

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