Getting started with Catadioptric stereos (Mirror stereos)

written for the stereosite by Gordon Au, USA

If you already take stereopho­tos using the sequen­tial (‘cha-cha’) method with your phone or cam­era, you’re undoubt­ed­ly aware of its lim­i­ta­tions: your main sub­ject has to remain per­fect­ly still; peo­ple and cars mov­ing in the back­ground cre­ate dis­crep­an­cies; chang­ing light between shots is a prob­lem; windy days can be impos­si­ble. How­ev­er, you may or may not know that adding a small hand­held mir­ror to your stereo arse­nal can help you over­come many of these prob­lems. With the mir­ror reflect­ing half of the camera’s view, you can cap­ture an entire stere­opair in a sin­gle shot, elim­i­nat­ing tim­ing-relat­ed issues. Though it depends on the mir­ror and camera(lens), this is usu­al­ly best for close-up shots:

Zoomed-in cata­diop­tric stereo of a female Bi-Col­ored Aga­pos­te­mon sweat bee, A. virescens

This fig­ure shows the basic prin­ci­ple: the mir­ror reflects one half of the camera’s view (in blue) over to the oppo­site side, where it over­laps with the oth­er half view (in yel­low). For every­thing in the green over­lap area, you get 2 dif­fer­ent views—stereopair poten­tial! Tech­ni­cal­ly, a sys­tem that com­bines a mir­ror and lens like this is called cata­diop­tric.

The trick­i­est part of tak­ing cata­diop­tric stere­os is posi­tion­ing and hold­ing the mir­ror. The basic steps are:

  1. Bring the ver­ti­cal mir­ror right up to the cam­era lens (but not touch­ing it!).
  2. Move the mir­ror to the side, until the near edge is no longer vis­i­ble to the cam­era.
  3. Tilt the far edge of the mir­ror back towards the cen­ter, until approx­i­mate­ly half the cam­era view is reflect­ed.

You can do this on either side — whichev­er is more com­fort­able.

The sin­gle-mir­ror cata­diop­tric method

The set­up should look some­thing like this:

Mir­ror and cam­era posi­tion­ing

And your raw pic­ture will look like: 

An unprocessed cata­diop­tric shot

When you start out, you will need to fine-tune the mir­ror posi­tion and tilt. I rec­om­mend tak­ing a series of test shots, using the same sub­ject (at ~1–2 mir­ror lengths away), and with dif­fer­ent com­bi­na­tions of mir­ror posi­tions and tilts:

  • Shot #1: mir­ror 0mm from lens edge / 40% of view reflect­ed
  • Shot #2: mir­ror 0mm from lens edge / 50% of view reflect­ed
  • Shot #3: mir­ror 0mm from lens edge / 60% of view reflect­ed
  • Shot #4: mir­ror 3mm from lens edge / 40% of view reflect­ed
  • Shot #5: mir­ror 3mm from lens edge / 50% of view reflect­ed
  • Shot #6: mir­ror 3mm from lens edge / 60% of view reflect­ed
  • Shot #7: mir­ror 6mm from lens edge / 40% of view reflect­ed
  • (And so on…)

To process the shots, sim­ply:

  1. Straight­en if nec­es­sary, so the mir­ror edge is per­fect­ly ver­ti­cal.
  2. Sep­a­rate the two images.
  3. Un-reflect (hor­i­zon­tal­ly flip) the mir­ror image.
  4. Edit the two images as you would for any stere­opair.
    I rec­om­mend using StereoPho­to Maker’s auto-align func­tion

After pro­cess­ing, go back to your notes, and see which com­bi­na­tion of mir­ror posi­tion and tilt worked the best. (What amount of depth do you pre­fer? Which stere­opairs turn out the clean­est?) You might want to fol­low up with more test shots of sub­jects at dif­fer­ent dis­tances.

Beyond ‘get­ting start­ed,’ there are fur­ther details worth dis­cussing about mir­ror stere­os, includ­ing the ide­al mir­ror (size; shape; type: a front sur­face mir­ror (first sur­face mir­ror), which you can find on Ebay), pre­cise pro­cess­ing (crop­ping, match­ing size and focus, cor­rect­ing for key­stone dis­tor­tion), his­to­ry and the­o­ry, and more. Learn about some of these in “DIY Cata­diop­tric Stere­os” a video tuto­r­i­al I made for the Nation­al Stereo­scop­ic Association’s 3D-Con 2020. Also, check out the cor­re­spond­ing gallery and see #cata­dioptric­stereo on Insta­gram for exam­ple shots.

Feel free to send ques­tions through my web­site WorldOfDepth.com, or via Insta­gram. Have fun exper­i­ment­ing, and I look for­ward to see­ing your mir­ror stere­os!

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Gordon Au (New York, USA)

I am an avid stere­o­g­ra­ph­er and gen­er­al 3D exper­i­menter, mak­ing stereopho­tos, stere­ovideos, anaglyphs, extrac­tions, and con­ver­sions, draw­ing upon a vari­ety of sources, includ­ing TV and film, art­work, NASA data, prod­uct reviews, and more. I exper­i­ment with cata­diop­tric stere­os, asym­met­ric fram­ing, video pan­ning / zoom­ing of 3D stills, x‑rays and fog as depth map sources, and more. I am a proud mem­ber of and have pre­sent­ed and/or writ­ten for the New York Stereo­scop­ic Asso­ci­a­tion, the Nation­al Stereo­scop­ic Asso­ci­a­tion, and the Inter­na­tion­al Stereo­scop­ic Union.

Web­site: WorldOfDepth.com
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