Why so cheap?

[mickyj]

Ok, to sort this out for myself, and to help others with all the questions, learn about the lens from my eyes. I bought a cheapie.

Check out post called "eBay - onlinedigitalshop (ods) - Anyone delt with them ?"
http://www.ausphotography.net.au/forum/showthread.php?t=63294"

[ameerat42]

Hmmm. This thread got interesting. I now doubt whether these lenses are the same as the ones I mentioned above. Mine were metal housings. The consensus on these is that they're plastic.

But about the "mirror lenses".
I read the article about "mirror lenses" in the link posted by Joele
http://www.photozone.de/mirror-lenses
and found it to be disappointingly amateurish, relying on vagaries and generalisations. Just look at the first paragraph, "...strange...", "...there're..."
And in para 5 "...these lenses are mainly based on mirrors rather than glass elements...",
which means - anybody's guess!
Also in para 5 "...The lack of glass elements (apart from the correction element) is a significant design advantage compared to classic (refractive) lenses..."
So, what about the back glass element. In other words they are as much refractors as reflectors, hence correctly, "mirror-lens", or catadioptric.
And para 6 is supposed to be informative(!?!)
"...Unfortunately the design results also in some significant disadvantages like

* no aperture so it's not possible to control the depth-of-field.
* the mirror reflections eat contrast
* the overall sharpness is usually quite mediocre
* the secondary mirror produces an odd effect donut-like effect on the out-of-focus high-lights. The following picture illustrates this problem."

It is informative guff that the system went from fixed aperture to "no aperture".

And so on..., even to a purported "Conclusion".

After a whole page it doesn't prove anything. Like anything else, get a good mirror-lens and you'll get good results. I know of two instances, both f=500mm, f/8, catadiopric, and both of which I've had.

The first was a Sigma. It could not resolve anything at any distance. I think it suffered from dispersion (and perhaps circular aberration). When other people posted photos (in photo magazines) of shots with this lens you could see it in them too.

The second (still) is a Tamron SP (BBAR MC). It is pin sharp within its extremely narrow depth of focus, across the whole 5 degree field. It has very minor vignetting in corners of a frame.

Canon and Nikon also brand a range of mirror-lenses. I don't know what they're like, but they have the name.

The main disadvantages of these lenses, that I could say - and they are only comparative disadvantages - are:
1. fixed aperture
2. quite shallow depth of focus up to about 100+ m.

But that what they are, and that's about it. Am.