What is a 'Mirror Lens?'

i_worship_the_King

What is a mirror lens, and how can they get 1000mm equivalents out of something only a few inches long?

I was searching lenses on ebay and came across this...
http://cgi.ebay.com/NIKON-NIKKOR-Reflex-C-1-8-f-500mm-mirror-lens-NR_W0QQitemZ280205134487QQcmdZViewItem?hash=item280205134487

..or..

Whats the differences in these 2 packages??
http://cgi.ebay.com/500mm-1000mm-Mirror-Lens-for-Nikon-D80-D70-D50-D300-D40_W0QQitemZ300204184374QQcmdZViewItem?hash=item300204184374
and
http://cgi.ebay.com/500mm-1000mm-Lens-for-Nikon-F6-FM10-F100-N85-N75-N65_W0QQitemZ360029720430QQcmdZViewItem?hash=item360029720430


Thanks!

LiquidAir

A mirror lens is a telephoto built like a Schmidt-Cassegrain telecope with two mirrors which fold the optic path back on itself. They tend lighter weight, cheaper, slower and relatively low contrast compared to conventional refracting lenses. The other noticably oddity about mirror lenes is that out of focus highlights take on a donut shape due to the hole cut in the primary by the secondary mirror.

jogle

forget about them if you care about image quality or want to take images of anything that moves.

they ARE cheap though, but remember that you almost always get what you pay for with glass. Mirror or reflector lenses are what tend to prove the rule

SloYerRoll

The bokeh is the most noticeable defect w/ these lens's.

Take a look at the shot here. Perfect example of SC lens's:

http://lh3.google.com/_1eaTHuFsXsY/RwVsUskQeDI/AAAAAAAACcI/PLkZqGoWPZc/s800/IMGP0505.jpg

ziggy53

LiquidAir wrote:

A mirror lens is a telephoto built like a Schmidt-Cassegrain telecope with two mirrors which fold the optic path back on itself. They tend lighter weight, cheaper, slower and relatively low contrast compared to conventional refracting lenses. The other noticably oddity about mirror lenes is that out of focus highlights take on a donut shape due to the hole cut in the primary by the secondary mirror.

Absolutely correct! I have a 500mm, f8 "cat" (short for "catadioptric") that is really an f11, so worthless in poor light. The best light for that lens is extremely contrasty, and then you must use RAW to capture because you must later expand the tones to make the image look natural.

The "donut" bokeh used to be considered acceptable, but these days you won't find many takers.

The lens is also fairly fragile because of how the mirror is mounted, so you must not treat it poorly.

The only time the lens has any merit is when you want to travel extremely light, like backpacking, and you need the telephoto properties but cannot afford the weight of a high-speed refractor. In perfect light, with very careful post-processing and the right subject matter you can obtain acceptable results for small image sizes.

i_worship_the_King

I was shopping for a Sigma 400-500mm in the $1000 range for a birthday present and came across these in the searches. Makes me wonder if they would be any use for something like star/moon photography with one of the 2x multipliers for a 1000 or 1200mm equivalent.

Thanks for the information!

gtc

reflex telephoto
I would be suspect of a TC / mirror combo for moon shots-would be very difficult to focus manually and there is no way to stop down further.

I have a Sigma 600mm /f8.0 "macro" (close focusses)

As Ziggy points out there are some limited use- I keep one in my back pack for snakes and spiders etc that I don't want to get too close too.

Its also sometimes useful for architectural detail shots.For acceptable contrast though you have to add lots of extra shading.

I would prefer a 600mm/f4.0, but for the money it does not take up much space in the pack,its light and reasonably sharp with tripod ,MLU and cable release.

So if you have some specific uses ,then one may be of value.

BBiggs

i_worship_the_King wrote:

I was shopping for a Sigma 400-500mm in the $1000 range for a birthday present and came across these in the searches. Makes me wonder if they would be any use for something like star/moon photography with one of the 2x multipliers for a 1000 or 1200mm equivalent.

Thanks for the information!

Ever think about a canon 400mm 5.6? They are only $150 more then what you stated and would definently work really well

i_worship_the_King

Only if it comes with a Nikon converter

Thank you, though.

Robinivich

There have been a few hints already at the best, well, one of the least limited uses for these lenses, pointing them at the sky.

Bokeh isn't an issue with stars/the moon, everything up there is at infinity, and while everything does move, it doesn't move fast enough for f/8(t/11) to be hugely limiting. Manual focus is not an issue, see above "everything is at infinity", lack of an aperture is also not much of an issue (you're only really shooting wide open with an f/8 lens anyhow).

Think of it this way, mirror (reflex) lenses are essentially little astronomical telescopes adapted for cameras. Astronomers don't worry about bokeh or contrast a whole lot, so the weight and price saving, along with the long focal lengths, work out well. That said, I'm sure the optical quality of reflex lenses are already abysmal by astro standards (these guys don't really care about most of the optical quality we terrestrial photographers need, just resolution)

And when it boils down to it, you can get a decent reflector telescope and a T-mount and adapter to use it for not a lot more than your average "mirror lens", and you won't really lose anything in terms of features.

So why not get something that can get results as good as or better than a purpose built lens, and still have a nice telescope when you're done?

:Edit: Hehe, the only answer to the above question is portability, which has been mentioned

pyry

Robinivich wrote:

Think of it this way, mirror (reflex) lenses are essentially little astronomical telescopes adapted for cameras. Astronomers don't worry about bokeh or contrast a whole lot, so the weight and price saving, along with the long focal lengths, work out well. That said, I'm sure the optical quality of reflex lenses are already abysmal by astro standards (these guys don't really care about most of the optical quality we terrestrial photographers need, just resolution)

Those guys tend to use apertures which are imposible or astronomically expensive to produce in any other configuration than a cassegrain, we're talking more than 8 inches here. So question becomes low(ish) contrast or none at all.

I've tried using my little maksutov-cassegrain as a bird lens, but although it has quite a reach (1500 mm), it's too dim to focus accurately manually (f/12). And because focusing is slow, anything that moves is a tad difficult to shoot.

It's good for it was made for though, a telescope for looking at the night sky

Robinivich

pyry wrote:

Those guys tend to use apertures which are imposible or astronomically expensive to produce in any other configuration than a cassegrain, we're talking more than 8 inches here. So question becomes low(ish) contrast or none at all.

Quite true, though I didn't really mean professional, dedicated observatories, more serious amateur astrophotographers. It's certainly true of the apertures though, there comes a point where glass just can't cut it anymore, and so we go to mirrors. In terms of F/Ratios, most astro scopes are pretty conservative in camera terms, though of course there are some that are actually fairly large aperture, like dobsonian newtonians (that sounds so goofy) Think 1200mm f5.6, 800 f4, that kind of thing. But then, those aren't much use for photography...

pyry

Robinivich wrote:

Quite true, though I didn't really mean professional, dedicated observatories, more serious amateur astrophotographers. It's certainly true of the apertures though, there comes a point where glass just can't cut it anymore, and so we go to mirrors. In terms of F/Ratios, most astro scopes are pretty conservative in camera terms, though of course there are some that are actually fairly large aperture, like dobsonian newtonians (that sounds so goofy) Think 1200mm f5.6, 800 f4, that kind of thing. But then, those aren't much use for photography...

From what I've seen, serious amateur astrophotographers use both the smaller refractors (including general photography lenses) and the larger reflecting telescopes out to 16" and, rarely, even larger. These are in the f/10 general direction because of the configuration, but you can readily get what's called a focal reducer (a teleconverter with a multiplier of less than 1), that push the f-ratios to 6.7, or even 3.3.

Celestron even made a lens assembly for some of it's S-Cs that converted it to Schmidt camera, where the camera is mounted where the secondary mirror should be. Those could then make for something of a 500/2 for wide-fields. Granted, these are properly rare and discontinued ages ago.

And of course, there's simply no way of lugging a huge reflector around in the woods looking for birds. So again they stay as telescopes

In the other end of the scale I also know of a professional sky survey that used a cluster of 6 or 8 Canon 200/1.8 L lenses, because they were 'readily available and cheap'.

Fast newtons are also huge clunkers. When it comes to actually shooting with something fast and portable, there's nothing in the telescope market that can come close to the 500/4 and 400/2.8 kind of figures photographers use every day.

Cheap manual focus mirror lenses aren't good for either use

davemj98

500mm f/8
Handheld on a gloomy windy day, from several blocks away in Indianapolis Indiana, standing in front of Roberts Camera. I didnt even know thay had a observation room at the top ot the Monument; here you can see the man inside it! Sony lens.