Dave's Stereo (3D) Photography

Genesis

Over the summer of 2009, I decided to put together my own stereo camera rig.  I had grown frustrated by the limitations of the two previous stereo cameras I had owned - one was a cheap, plastic Loreo film camera, the other was a dual Rollei camera rig that was made by wiring together the shutter controls of two digital point-and-shoot cameras.  I needed a solution that would provide professional-caliber image quality and full manual control.  There being no such product on the market, I opted to build my own using 2 DSLRs.

The Cameras

I already owned a Canon Rebel XSi, so it was an easy decision to just purchase another.  They were and still are quite affordable - and as a stereo photographer you must never overlook this fact.  Remember, you have to purchase two of almost everything!  It adds up quick.  The XSi has excellent image quality, more than sufficient resolution (12MP), and is feature-rich enough that only rarely do I feel I am missing any useful tools available on higher-end models.  Also consider that had I elected, say, to use a 40D or any other larger DSLR, it would have forced the spacing between the lenses even further apart.  As it is, with the Rebels butted up against each other, the inter-axial spacing is about 4.5".  The human eyes are, on average, 2.5" apart.  This means I'm already shooting mild hyperstereo, although for the vast majority of applications it is unnoticeable. 

To get the camera lenses as close together as possible, I elected the common stereo configuration you see in the photo above:  side-by-side, with the left camera flipped upside-down.  The lenses are slightly offset to the sides of the cameras, so this arrangement gets the lenses as close as they can be when the cameras are butted up against each other.  And, being interested in primarily landscape photography, I elected to take the spacing penalty of orienting the cameras for landscape format rather than vertical/portrait aspect ratio.

Synchronization

The only way to currently synch two SLR cameras together without opening up the camera enclosures and hand-wiring things by hand (good luck!) is to use Rob Crocket's SLR Shepherd.  This small electronic box (seen on the left side of the above image) wires into the SLR's hot shoe and shutter release jack, and synchrinizes the shutters for you.  I have found it to be both reliable and to have an easy, plug-and-play functionality.  I don't have a single instance of noticeable shutter timing misalignment (it advertises synch to within a few microseconds).  Mr. Crocket is also a nice guy, and repaired the cable jacks on my unit for a very nominal fee (be careful about applying side load to the cable plugs).

The only down side to this arrangement is that you must still manually match the focus and exposure of the two cameras.  For focus, I found that the two Canon XSi's had no trouble agreeing on focus adjustment 99% of the time with AF engaged.   Very, very rarely was any focus discrepancy noticeable.   When I was really concerned about it, I would either set both lenses to infinity (since I'm doing mostly landscape work) or adjust both lenses by hand.  Exposure is another story:  the cameras often disagree (if only by a little bit).  My SOP (standard operating procedure) for shooting was to use the left-hand camera in aperture priority mode to do the metering.  I would then have to actively adjust the right-hand camera (set to full manual mode) to match the shutter speed (assuming the apertures were already set).  Since 90% of my shots are done with a tripod, I use f10-f12 apertures for the vast majority of my shots, excepting only low-light or with moving objects.  Small apertures are preferred not only for most landscape photography, but especially for 3D stereo photography (where you want a wide depth-of-field to make sure that foreground, midground, and background objects are all in focus).

The Bracket

Being an aerospace engineer in my day job, it was natural to start by modeling up the bracket design in CAD, using Pro/Engineer.  It took a good bit of effort to take careful measurements of the cameras and ensure that the lens axes would be lined up vertically (using granite tables and height gauges).  I also ran FEM analysis to ensure that the bracket would be stiff enough to prevent the cameras from tilting (in the roll axis) with respect to each other due to bending in the bracket.  

I managed to scrounge up a few pieces of fairly sizeable scrap 6061 aluminum plate, so that almost all of the bracket (except the L bracket holding the SLR Shepherd, seen on the left, and the mounting block that attached to the tripod head, on the bottom of the assembly) could be machined with precision from a single piece.  No worrying about trying to fasten and align separate parts.  This also allowed me to design in flanges on both sides of the camera-mounting surfaces:  large flanges opposite the cameras to provide stiffening, and a very tiny flange on the camera side which aided in aligning the cameras parallel to each other (aligned in yaw) without blocking the LCD screens or buttons.

I also added some notches into the vertical web of the bracket (in the center of the assembly, in between the cameras) in order to accommodate the SLR Shepherd cables that run into the shutter release ports on the side of the cameras (see the photo below).  So the cameras are butted up against each other as much as they physically possibly can be.  I also added some slots into the bracket to accommodate standard camera straps.

Notice that the left-hand camera bracket mount is slotted to allow for a variable stereo-base.  This is handy for when you need to slide it over for hyperstereo applications (about a 9" stereo base max).  Winged thumb-screws and fender washers allow both cameras to attach and detach easily, or for the left-hand camera to be slid over.  You must take care to hold the left-hand camera when loosening the fastener so that it doesn't fall onto the ground!  Also note the mounting block on the bottom of the assembly that attaches the bracket to the tripod head.  It was sized to allow the tripod head release lever to clear the camera and hotshoe plug.

It is also worth mentioning that there is space next to the right-hand camera that allows you to rest your hand in between the camera and the vertical bracket that holds the SLR Shepherd.  This is necessary if you want to comfortably adjust the right-hand camera, especially the jog wheel.  It also allows removal of the SD memory card from the camera without unfastening the camera from the mount.  Both cameras must be unfastened, however, in order to remove the batteries.

My friend and co-worker Paul Sherman (who also happens to be a pretty kick-butt photographer himself) happened to be skilled in machining with a manual mill, and he was kind enough to lend a fair number of hours to machining the bracket with care and precision.   We, of course, improvised some of the design features as we went, but the vast majority of the part was made according to the modeled design. 

Mounting the SLR Shepherd

This was a bit tricky, since the SLR Shepherd doesn't have any mounting features on it.  At first, I just used some electrical tape to secure it to the vertical web of the bracket.  This worked OK, but wasn't a final solution.  So I took advantage of the existing holes that accommodated 4 small screws that held the SLR Shepherd plastic housing itself together.  I tapped my bracket for #2 machine screws (I had to special-order unusually long #2's from McMaster Carr) and drilled completely through the Shepherd's housing at these 4 points.  Since I work at an aerospace composite shop, I laid up and cured some thin carbon fiber strips on the radiused contours of the Shepherd, and countersank the strips to seat the machine screw heads.  Here is the result:

Hyperstereo Configuration

For many landscape shots it is vital to be able to shoot using moderate to extreme hyperstereo bases in order to perceive depth in large and/or distant objects, where the cameras are separated by anywhere from 1-100 feet.  This obviously exceeds the length of the SLR Shepherd cables, along with requiring two separate tripods for mounting.  For synchronization, I purchased two wireless RF shutter releases (which advertise a range of 100 feet) made by RPS Studio.  Just set the two receivers onto the same channel as one of the receivers, and voila!  This solution does not provide synchronization nearly as precise as the SLR Shepherd, but for most hyperstereo applications you only need synch within a second or so.

Lenses

The lenses I use most frequently are the wide angle Tokina 11-16mm f/2.8 lenses.   Wide angle lenses are normally the lenses of choice for landscape photographers, and since landscapes make up the bulk of my portfolio, I chose the Tokinas for their sharpness and overall image quality.  They are also constant f/2.8 aperture, so they are quite capable of shooting in low-light and indoor situations as well. However, in the vast majority of situations I shoot between f/10-f12, since these lenses are sharpest in that range, and because, as previously mentioned, you want to have as wide of a depth-of-field as possible for stereo photography.  2D photographers love their sexy shallow depth-of-field, but in a 3D image it just ruins the perception of depth (3D videos can get away with it under some circumstances, but still stereo images cannot).

Stereo photography in particular benefits from these wide angle focal lengths, as it increases the perception of depth and "solidness" of objects.  Conversely, you run into problems with telephoto lenses in that it creates a cardboard-cutout effect, exactly the same phenomenon that you have probably seen when using binoculars.

I also use the Sigma 17-50mm f2.8 lenses in situations where the Tokinas are simply too wide.  I still need a few good prime lenses, although I do not anticipate the need for any focal lengths longer than 50mm for stereo applications.  

The big down side is that one must purchase two lenses, a matching pair, for a stereo rig, so you end up paying double for your lenses compared to regular photographers.  This adds up quick.  It certainly forces one to limit the number of different lenses in one's arsenal.

Tripod

This is an area of preference, but I can highly recommend using the Manfrotto 190XB tripod and 322RC head.  The tripod and head have a high enough load rating (11 lbs.) that I can mount nearly any (2!) lenses onto the cameras without fear.  The tripod is light enough (4 lbs.) for applications involving hiking, and is compact enough when collapsed to hook onto my camera backpack comfortably.   The 3-segment telescoping legs w/ thumb locks allow it to deploy quickly (I consider this a vital feature, not merely a convenience).  Since it is aluminum construction it is also reasonably priced.  I do not consider the marginal weight savings of carbon-fiber tripods to be worth the extra expense.  You will spend $170 more for the carbon fiber model that shaves 1.2 lbs. off of your rig.

The 'joystick'-style head is a huge convenience:  it is simply a ball head with a triggered hand grip that allows you to easily and quickly point your camera.  It is quite sturdy and well-constructed, and allows for smooth, effortless 'aiming' so that you can compose your shot without even thinking about it .  It just works.  Go and buy it, already.  You'll thank me.

Future Upgrades

The only major element I'd like to improve on my rig is the lens linkage.  Currently, I allow the cameras to autofocus independently, and the zoom rings must also be matched manually.  As I noted above, the focus does not normally present a problem, but to provide a more exacting solution, I need to install a belt or geared system to link the two lenses together.  Same goes for the zoom rings.  The only thing I need to research is whether or not I can link the focus rings together and use AF.  If I slave one of the lenses (set in manual mode) to the other (set to autofocus), will it burn out the focus motor in the 'live' lens?  I am also exploring the possibility of using electronically-controlled servos (film guys use these to pull focus) to control the focus rings in a synchronous manner, although these solutions are generally cost-prohibitive.

A minor concern is the fastener for mounting the left-hand camera.  When loosening it in order to adjust the stereo base, the danger remains of the camera becoming completely unfastened and falling onto the ground.  I'll probably install a plate and jack-screws that can allow the camera to remain securely attached while sliding it over.

For the sake of aesthetics, I will admit that the bracket at present doesn't look all that pretty.  It is unfinished aluminum, marked up with tooling paths, black marker, and scribe lines.  That is simply due to the fact that I finished machining it the night before I departed on my U.K. trip!  I'll have to get it sent out to get a nice, black anodize finish.